Anal. Chem. 1982, 5 4 , 62R-83R (71) Parees, D. M.; Prescott, S. R. J. Chromafogr. W81, 205, 429-33. (72) Parker, R. T.; Freelander, R. S.; Dunlap, R. B. Anal. Chim. Acta 1980, 720, 1-17. (73) Pellzettl, E.; Pramauro, E. Anal. Chlm. Acta 1980, 717, 403-6. (74) Phllllps, J. B. Anal. Chem. 1980, 52,468 A-470 A, 472 A, 475 A, 478 A. (75) Poole, C. F.; Singhawangcha, S.; Hu, L. E. C.; Zlatkls, A. J. Chromatogr. 1979, 178, 495-503. (76) POOle, C. F.; Sye, W. F.; Slnghawangcha, S.; Hsu, F.; Zlatkls, A,; Arfiwldsson, A.; Vessman, J. J. Chromatogr. 1880, 799, 123-42. (77) Pretl, C.; Tosl, F. Anal. Chem. 1981, 5 3 , 46-51. (78) Puacz, W. Mikrochlm. Acta 1981, 2 , 155-62; Chem. Abstr. 1981, 9 5 , 543830. (79) Rackham, D. M. Spectrosc. Lett. 1980, 13, 321-7. (80) Ramakrlshna, R.; Slraj, P.; Sastry, C. S. Acta Clenc. Indica, [Ser.] Chem. 1980, 6 , 140-1; Chem. Absfr. 1981, 9 4 , 9 5 4 2 7 ~ . (81) Realini, P. A.; Burce, G. L. V I A , Varian Instrum. Appl. W7g, 13, 8; Chem. Abstr. 1980, 9 2 , 1349770. (62) Rhoades, J. W.; Hosenfeld, J. M.; Taylor, J. M.; Johnson, D. E. IARC Scl. Publ. 1080, 3 1 , 377-67; Chem. Abstr. 1981, 9 5 , 85705t. (83) Roeper, H.; Heyns, K. J. Chromatogr. 1980, 793, 381-96; Chem. Absfr. W80, 9 3 , 60614h. (84) Ruzicka, E.: Paleskova, M.; Jllek, J. A. Collecf. Czech. Chem. Common. 1880, 45, 1677-83. (85) Sakra, T.; Madle, K.; Vynikler, V. Sb. Ved. Pr., Vys. Sk. Chemlckfechnol. Pardubice 1079, 40, 49-65; Chem. Absfr. 1981, 9 4 , 1976211. (86) Sawicki, E.; Carnes, R. A. Mlkrochlm. Acta I S M , 602. (87) Sellg, W. Mlkrochlm. Acta 1980, 1 , 112-18. (88) Sellg, W. Mlkrochlm. Acta 1980, 2 , 133-44. (89) Shlmada, K.; Tanaka, M.; Nambara, T. Chem. Pharm. Bull. 1979, 2 7 , 2259-60. (90) Silbert, L. S.; Maxwell, R. J. Fatty Acids 1979, 403-25. Edited by Pyrde, E. H.; AOCS: Champaign, IL: Chem. Abstr. 1981, 9 4 , 46266n. (91) Skrypnlk, Yu. G.; Barabash, Yu. V.; Shevchuk, 1. A. Zavod. Lab. 1981, 47, 16-17; Chem. Abstr. 1981, 9 5 , 102167~. (92) Spassky, N.; Relx, M.; Sepulchre, M. 0.; Guette, J. P. Analusius 1980, 8, 130-7. Chem. Abstr. 1980, 9 3 , 60610d. (93) Takahashi, H.; Yosida, T.; Meguro, H. EunsekiKagaku 1981, 30, 33941; Chem. Abstr. 1981, 95, 543594. (94) Takaya, T.; Sakaklbara, S. Pepf. Chem. 1979, 77, 139-44. Chem. Abstr. 1980, 9 3 , 68144d.
(95) Takeuchl, T.; Horikawa, R.; Tanimura, T. Anal. Lett. 1980, 73, 603-9. (96) Tan, B.; Melius, P.; Kllgore, M. V. Anal. Chem. 1980, 5 2 , 602-4. (97) Tanizawa, K.; Hlrasawa. T.; Soda, K. Anal. Lett. 1980, 13, 645-54. (98) Tawa, R.; Hirose, S. Chem. Pharm. Bull. 1980, 2 8 , 2136-43. (99) Tawa, R.; Shimizu, S.; Hirose, S. Chem. Pharm. Bull. 1980, 2 8 , 541-5. (100) Tomkins, B. A.; Ostrum, V. H.; Ho, C. H. Anal. Lett. 1980, 73, 589-602. (101) Tyshchenko, N. G.; Kozhaeva, N. G.; Korobeinikova, G. A.; Disklna, D. E.; Kononyuk, B. N. Neftepererab. Neftekhim (Moscow) 1979, 22-3; Chem. Absfr. 1980, 9 2 , 44273~. (102) Tyson, J. F.; West, T. S. Talanta 1980, 2 7 , 335-42. (103) Van Roosmalen, P. B.; Purdham, J.; Drummond, I. Inf Arch, Occup. Environ. Hea/th 1981, 48, 159-63; Chem. Absfr. 1081, 9 5 , 102471h. (104) Vincent, W. J.; Ketcham, N. H. ACS Symp. Ser. 1980, 120; Chem. Abstr. 1980, 9 3 , 100775~. (105) Vohra, S. K.; Harrlngton, G. W. J. Chromafogr. Scl. 1080, 78, 379-83. (106) Walker, E. A.; Castegnaro, M. J. Chromafogr. 1980, 787, 229-31. (107) Ward, J. L.; Walden, G. L.; Wlnefordner, J. D. Talanta 1981, 2 8 , 201-6. (108) Wieboldt, R. C.; Hohne, 8. A.; Isenhour, T. L. Appl. Specfrosc. 1980, 3 4 , 7-14. (109) Wllllams, A.; Hill, S. V.; Ibrahlm, I.T. Anal. Eiochem. 1981, 174, 173-6. (110) Winkle, M. R.; Lanslnger, J. M.; Ronald, R. C. J. Chem. SOC.,Chem. Commun. 1880, 87-8. (111) Woitvnska. E. Pollmew 1979, 2 4 . 238-41; Chem. Abstr. _ (Warsaw) . 1980, 92, 595222. (112) Wright, V.; McGavraugh, G.; Phillips, R. Polymer 1980, 2 7 , 1167-70. (113) Yang, Y.; D'Sllva, A. P.; Fassel, V. A. Anal. Chem. 1981, 5 3 , 2107-9_ . (114) Yang, Y.; D'Sllva, A. P.; Fassel, V. A.; Iles, M. Anal. Chem. 1980, 5 2 , 1350-1. (115) Yoshimura, C.; Mlyamoto, M. Eunseki Kagaku l W 1 , 3 0 , 286-90; Chem. absfr. 1981, 9 5 , 54370a. (116) Yuki Gosei Kogyo Co., Ltd. Jpn Kokai Tokhyo Koho 80,110,995 (CI. GOIN33/52), 27 Aug 1980, Appl. 79/18,033, 19 Feb 1979; 4 pp. Chem. Absfr. 1981, 9 4 , 8 4 5 1 4 ~ . (117) Zaichenko, L. P.; Babel, V. G.; Evdoklmova, S. I.Issled. Ob/. Khim. Tekhnol. Prod. Pererab. Goryuch. Iskop. 1977, 3 57-8; Chem. Abstr. 1980, 9 3 , 215053a.
.
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Kinetic Determinations and Some Kinetic Aspects of Analytical Chemistry Horacio A. Mottola Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078
Harry B. Mark, Jr" Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 4522 1
The basic organization of the 1980 Review (1) has been retained for this report. Papers included in this review have been selected from reports which appeared in the literature since the 1980 review through approximately November 1981. Again catalytic determinations comprise the largest number of references, backing up the same trend established through the years. Coincidentally also again, as in the 1978-1980 period, roughly 10 times as many applications of catalysis have been published as applications of differential reaction rate methods. An interesting departure with respect to previous reviews is the increase in the number of contributions discussing kinetics in solvent extraction.
BOOKS AND REVIEWS Following the trend of an academic increased interest in kinetic determinations, the subject is being now frequently incorporated in textbooks, even for undergraduate training (2). Three rather specific areas of analytical chemistry (chromatography, flameless atomic absorption spectroscopy, 62 R
0003-2700/82/0354-62R$06.00/0
and continuous-flow analyses) were chosen to illustrate the role that kinetic principles play in the understanding of the fundamentals of these analytical approaches (3). Exposure of students in analytical courses to these concepts is considered to provide them with a better understanding of fundamentals behind analytical techniques, put kinetics in analytical chemistry in a better perspective, and improve the mind of the student on creative thinking. The first review on kinetic methods of analysis in the Chinese language, containing 387 references, was published in 1978 ( 4 ) . Kinetic methods in organic analysis have been reviewed by Antonovskii (5) and by Kreingol'd et al. (6). A review on catalvtic methods in the German language - - has been authored by Muller (7). Nikolelis and Hadjiioannou have recently discussed the analytical use of inhibition, activation, and promotion of metal-ion-catalyzed reactions in trace analyses (8). Some theoretical principles (with emphasis on enzymatic methods) for kinetic determinations have been reviewed by 0 1982 American Chemical Society
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Hwacb A. DMnola was ban In Buanos Aires. Arwntina. and r-hd undagraduate and graduate edduc~llonat me University of Buarxls Alres. He earned Ucanliate and Doctoral degrees tram h e University 01 Buanos A i m and did pedoctoral work wim Rofessa Ernest 8. Sandell st me University of Minnesota (Minnaapolis). He spent two years at the UnivsrsW 01 A r k m (Tucson) as a postdoctmal research tellow In Pratess a Henry Frsiser's research group. Alter teaehing fw two years at me Unkersity 01 me Pacific (Stockton. CA). he pined OSU in the fall 01 1967. Dr. MOltOla's research iB tentsts Include s M k on me role of kinetks In analytical chemlshy (including reaction reate determinationsl. analyacal sapratanr (ooivent e m a n i n and liqu~-iiqwchamatography). continuousRaw analysis. and anabtlcal application of photochromism. Harry 6. Mark, JI. Rofessar , Of Chemisby and Head 01 me Depanmenl 01 Chemisby. University 01 Cincinnati. received his B.A. Unkmity of Vkginla In degree from 1956 and his m.0. degree from Duke UnkarrW in 1960. He war B postdoctoral rtc search associate at the University of N& Carolina (wim C. N. Reilby) nom 1960 to 1962and at me Carnorm institute 01~ e ~ h nalagy (wim F. C. Anson) hom 1962 lo 1963. He was a member 01 the staff of me 4c D e p m n t 01 Chemistry at the Universny of Michigan horn 1963 to 1970. VWng Protesm 01 Chemism at me UniversR6 Llbre de Bruxelles. 1970, and Idned staff at the UnWersity of Cincinnati In 1970. His research Interests are in e i e c t r ~ chemistw. svTaca chemishv. kinetic memDds of anahsis. environmental
Bartl, Deeg, and Ziegenhorn (9). A review on automated reaction-rate methods of analysis has been published hy Malmstadt, Krottinger, and McCracken (10). The review is devoted to details on preparation and handling of reactants, measurement of chemical reactions by electronic devices, basic determinative approaches, data calculations and display, control systems for the different manipulations and operations involved in reaction-rate methodology, and a brief description of some commercially available automated systems. Discussion of a rapid sequential analyzer developed by the authors is also included. Kulys reports on the development of new analytical systems hased on hiocatalysis (11). The paper deals with the macrokinetic dependence of new amperometric systems and provides an overview on the hasis of experimental data collected mainly in the author's laboratory. Paralleling other increasing interest in kinetic aspects of solvent extraction, two reviews of different aspects of the topic have been published in the past 2 years. A detailed and critical discussion on the kinetics of the solvent extraction of metal ions has been presented by Danesi and Chiarizia (12). The review coven extracting reagents and experimental techniques and considers the contributions of both chemical reactions and transport processes to the rate of extraction. A review on chemical species exhibiting catalytic action on the rate of solvent extraction of metals has been prepared hy Inoue and Nakashio (13). This rate effect has been also called "kinetic synergism" hut IUPAC recommends adoption of the name catalysis to describe an enhancement in rate of extraction without significant effect on the value of the distribution ratio. Under the authorship of Ruzicka and Hansen there has been published the first monograph on "Flow Injection Analysis", FIA, the unsegmented version of continuous-flow sample processing systems (14). In page 25 of this monograph the authors recognize that 'It is the relatively slow mixingand its incompleteness-that makes the FIA method kinetic in both a 'physical" and a "chemical" sense." Relevant kinetic
aspects of this approach to sample processing have been reviewed by Mottola (3). A few applications of ligand exchange reactions have been developed for differential kinetic determinations in unsegmented continuous-flow systems. They have been reviewed by Espersen, Kagenow, and Jensen (15). Volume 4 of a series on "Modem Trace Analysis" published in the German Democratic Republic has been dedicated to catalytic methods in trace analysis (16). The monograph, authored by Muller, Otto, and Werner, is a well-halanced account of theoretical as well as practical aspects of the topic. K I N E T I C M E T H O D USING CATALYZED REACTIONS Table I gives an overview of ratalytic methods proposed for different chemical species. As in the past redox reactions and those catalyzed hy transition metal ions are the basis of the larger portion of the proposed methods. An increase in the report of applications to actual samples can he recorded as a healthy trend that balances in part the perhaps disproportionate numher of indicator systems. novel or revisited, that the literature records every year. Considering that the main attraction of ratalytic method is the low limits of detection afiordahle with simple instrumentation, limits uf d e tection are rited as reported by the authors. This however, needs a qualifying statement sinre there are ground, fur suspicion that different authors interpret and define the conrept differently. Several years ayo (see Anal. L'hrm. 1974, 46, 825 A J the International L'nion of Pure and Applied Chemistry, IIJPAC. assigned Project 3.5 to its Commission V.3 the preparation of guidelines for nomenclature and presentation of results in kinetic-based determinations. Such guidelines have apparently not heen puhlicized yet hut are overdue particularly when one cunsiders concepts surh as limit of detection and sensitivity which seem frequently confused. This makes difficult the possibility of meaningful critical comparis'ms. Novel and Unconventional Catalytic Determinations a n d Applications of Catalysis. A contribution of significanre in the mea of catalytir determinations has heen rerently reported hy Shnpilov (72). In rerognition that the immohilhation of protein material9 (e.g., enzymes, on silicate supports (e.&. rontrolled-pore glass) depends on the availahility of free aldehyde groups when the glutaraldehyde immubilization prwedure is applied, a highly sensitive and selertive kinetic method for determining such aldehyde residues is propused. The determination is hased on the aldehyde gruup cawlysis of the H,02oxidation of p-phcnylcnediamine in neutrnl ur acidic solutions. The limit of detection is reponed as 2 5 pg of aldehyde g of supporting material. Marrornoiecular flocculants such as polyacrylamide are receiving increased attention and use in industrial situations including food technology. The analysis of the polymeric mawrial for residual toxic monomers becomes then a relevant analytiral prohlem. A kinetic procedure fur determination of acrylamide in pulyacrylamide. proposed hy Klyachko and offers advantages over other analytiral proceSladuva (31, dures in me. The chemical hasis for the determination is the oxidation of oxalate hv permanganate with Mn((Ii as a ratalyst. The rate of this reaction is proportionally atfected by the amount of monomer present in the pulymer prtparation. The authors suggest use uf the fixed-concentratinn Ivaria. ble-time! procedure enabling the determinatinn of a little as 0.01% monomer in a polyacryamide sample. The catalytic effect of sulfide on the axideiodine indicator reaction has been employed as a field test to evaluate soil corrosiveness in pipeline installations. Tuuvinene et al. have rerently cautioned regarding the use of this test hecause of interferences from a variety oisulfur compounds (741. The catalytic action o i sulfide ions on the iodineazide reaction ha3 been uwd b y Kiha t t al. (751 for the enthalpimetric determination ofsulfide ions. By injection of the sample wlution into the reacting mixture 0 1 iodine and azide, sulfide was determined in the 0 . ~ 20.5 umol range with about :370relative error. Determination was hmtd un the measurement uf the height of the heat pulse. Each determinatim took I O min. The appruarh was used to determine sulfur in samples of copper metal. The same approach, after separation by thin-layer chromatography, permitted Kiha et al. (761 to determine thiourea and its N.alkyl derivatives (l,;(-ditnethyl. 1,3-diethyl, and ~~~~~~
ANALYTICAL CHEMISTRY, VOL. 54. NO. 5. APRIL 1982
63R
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Table I. Determination of Different Chemical Species by Primary Catalytic Effects element
indicator reaction
bromine and iodine [as halides]
rhodamine 6Zh + chloramine B, pH 2.8 in presence of acetone
chromium [as Cr(VI)]
tellurite
+ ferriin
o-dianisidine
cobalt
comments
+ H,02
1,4-dihydroxyanthraquinone+
H202(borate buffer) oxidation of 1-naphthylamine by diperoxyadipic acid; borate buffer pH 9.2 H , 0 2 + 24hiosemicarbazone of sodium 1,2-naphthoquinone4-sulfonate
autoxidation of phenyl 2pyridyl ketone in basic medium catechol + H202
quinalizarin + H,02
dissolved oxygen (PH 8.25)
+ SO-:
autoxidation of ammonium sulfite (pH 8.5)
copper
Bindschelder’s Green leuco base + H202
Variamine Blue B leuco base H20,
64R
ANALYTICAL CHEMISTRY, VOL. 54,
NO. 5,
+
APRIL 1982
induction period measured potentiometrically; bromide determined at as low a concentration as 0.00018 pg/mL; iodide, however, requires much higher concentrations for reliable determination simultaneous comparison method; chromium reported as having an “inductive” action; microgram amounts can be determined with errors below + l o % chromium(V1) extracted with methyl isobutyl ketone from hydrochloric acid solutions and the extract added directly to the reaction system; determination based on the method of tangents; up to 5% (v/v) of MIBK does not affect the rate of the uncatalyzed reaction; prior extraction improves selectivity; method was applied to the determination of Cr in human blood after mineralization; limit of detection, 30 ng/mL of blood limit of detection reported as 0.3 ng/mL limit of detection 0.04 pg/50 mL; linear calibration between 1 X and 2 x M Co; application, determination of Co in NaCl method of tangents; linear calibration curves in the range 5-50 ng/mL of Co(I1);photometric monitoring at 549 nm; a phosphate buffer of pH 6.3 provides the largest difference between rates of uncatalyzed and catalyzed reactions; Cu(I1) is a serious interference but can be masked with citrate without affecting the catalytic action of Co( 11) fluorometric monitoring; calibration curves analyzed by the method of tangents. fixed-time, and variable-time procedures; in general, the method of tangents provided larger concentration ranges amenable to determination and better precision; cobalt can be determined in fraction of ppm level kinetics of the reaction studied; simple and selective system capable of detecting 0.75 ng in a 0.20 mL sample; absorbance measurement at 365 nm and reaction run at pH 11.2 and 50 “C;each determination takes about half an hour by the recommended fixed-time procedure optimum conditions for the determination (calibration graphs: plots of hew vs. concn; value of kexp obtained from plots of log absorbance vs. time) are proposed; range of determination: 1 X l o e 8 to 1 X M ; mechanism proposed similar to that of enzyme-catalyzed reactions, involving two intermediate complexes, to account for the choice of optimal conditions cobalt determined in the 1 t o 7 X lo-’ M range by a variable-time procedure; dissolved oxygen monitored with a commercially available electrode oxygen sensor concentration range for determination: 2 x lo-’ to 2X M;progress of reaction followed by monitoring the heat of reaction in a flow path with sensitive thermocouples; the rate of oxidation is linearly related to Co concentration in the range reported above; maximum errors about 3% (see also ref 47) dye leuco base generated in situ by treating N,Ndimethyl-p-phenylenediaminewith N, N-dimethylaniline; use of commercially available leuco base resulted in high blanks; fixed-time determination at 30 “C with reaction for 1 0 min; quenched aliquot used t o measure absorbance at 725 nm; pH range, 7.0-7.4; the catalytic rate depends on buffer composition; working curves nonlinear (between 0 and 100 ng/50 mL); relative standard deviations about 1% variamine leuco base generated by in situ reaction of N,N-dimethylaniline and p-anisidine; commercially available Variamine Blue B base gave high blank readings; fixed-time measurement at 60 “C after reaction for 1 0 min; pH range, 3.8-4.2; photometric measurement (740 nm) on quenched aliquot; as M Cu(I1) determined with RSD little as
ref 17
18 19
20 21
22
23
24
25
26
27
28
29
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
_
_
_
_
~
~
Table I (Continued) element
indicator reaction
p-hydrazinobenzenesulfonic
acid
+ H,O,
Fe3+-thiocyanate+ S,O,21,3,5-benzenetriol + H,O,
autoxidation of the hydrazone or azine of di-2-pyridyl ketone and the hydrazone of phenyl 2-pyridyl ketone
N-@-methoxypheny1)-N',N'dimethyl-p-phenylenediamine (MDP) t H,O,
gold
autoxidation of the azine of di-2-pyridyl ketone
hafnium and zirconium
KI + H,O,
iodine [as iodide]
sodium 1,2-naphthoquinone4-sulfonate 2-thiosemicarbazone + H,O,
N-methyldiphenylamine-4sulfonic acid + V0,(in 2 M H,SO,) N-(p-methoxyphenyl )-N',N'-
dimethyl-p-phenylenediamine (MDP) + H,O,
lead
Pyrogallol Red
+ S,O,z-
comments between 1 and 2%;method applied to determination of Cu(I1) in tap water and compared with flameless atomic absorption measurements oxidation product coupled with m -phenylenediamine to form a yellow azo dye ( k m m 454 nm); fixedtime determination at 35 "C after 20 rnin reaction; pH range, 4.3-4.7; determination of up to 100 ng Cu(I1) with RSD of 2-3%; Cu(I1) determined in some river waters; results correlate satisfactorily with independent determinations by flameless atomic absorption spectrometry the presence of 30% ethanol in the reaction medium allowed detection down to lo-' pg/mL; the increase in reaction rate in other organic solvents was lower application to determination of Cu(I1) in blood serum at a rate of 20 samples/h; limit of detection, 1 ng/ mL; method has approximately the same reproducibility and accuracy as absorptiometric and atomic absorption determinations but better limit of detection and sensitivity reactions followed by measuring fluorescence; methods suffer from few interferences and are substantially faster than equilibrium methods based on the same chemistries (3-4 min vs. about 1.5 12); the autoxidation of di-2-pyridyl ketone hydrazone is the preferred indicator reaction of those proposed in this paper; calibration plots of initial rates by the method of the tangents the preferred data handling approach for the determination of 0.1-1 ppm of copper MDP prodoced in situ by oxidative coupling of p anisidine and N,N-dimethylaniline;ammonia and acetic acid are activators; fixed-time procedure (absorbance measured after 1 0 min reaction) at 60 "C and pH 3.8-4.1; method applied to determination of copper in tap water; as little as lo-' M copper easily determined initial measurements of rate of formation of a fluorescent product; rangepf determination, 0.050.25 ppm; accuracy reported as 2%;relatively few interferences zirconium determined at pH 1.5 and Hf at pH 1.9; amperometric monitoring of reaction rate provides lower detection levels (about 1.5 orders of magnitude) than spectrophotometric monitoring a rpd-violet intermediate product monitored at 549 nm in phosphate buffer, pH 6.0-6.8 monitored every 30 s for 5-10 min; reaction rate estimated from a plot of d (log absorbance)/dt vs. iodide concentration; limit of detection reported as 0.5 p g/mL; EDTA enhances selectively by masking the interference of Co(II), Cu(II), Cd(II), and Ni(I1) (see also ref 17) limit of detection 2 x g/mL; Pt, Rh, and Pd d o not interfere; photometric monitoring for 45 min and application of the method of tangents; 5 min suffices to ascertain initial rate MDP, an analogue of Variamine Blue B leuco base, produced in situ by reaction of p-anisidine and N, N-dimethylaniline; the blue oxidation product photometrically monitored at 735 nm at 50 "C and after 30 rnin of reaction time (pH 3.6-3.7); liiiear working curves obtained in presence of 1 , l O phenanthroline as activator and in the range of 0.00 2-0.0 14 pg/mL lead determined in moss from bark of an oak tree; calibration curves made by plotting rate (3, 5, or 1 0 min reaction time, pH 8.5) vs. Pb concentration are not linear but reproducible; comparison with atomic absorption determinations yields a correlation coefficient of 0.866 (95%confidence le, el); since several metal ions interfere, preliminary solvent extraction with dithizone into chloroform required; lead back-extracted into aq HNO, of pH 2-3 prior to the kinetic determination; deterininaANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
ref
30
31
32
33
34
35
36
37
38
39
40
65R
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Table I (Continued) element
indicator reaction
manganese(11) [as Mn(HCO,),l
indigosulfonates t H,O,
mercury
exchange reaction between mercury(I1) and copper diethyldithiocarbamate; displaced copper catalytically determined by its effect on the H,O, + hydroquinone indicator react ion arsenite t Sn(I1) (strong acidic medium with gelatin as stabilizer) hexacyanoferrate(11) t p nitrosodiphenylamine
pentacyano(ammin0) ferrate(I1) t 2,2'-bipyridine or 1,lO-phenanthroline
molybdenum(V1) and vanadium(V)
I- t Br0,- t €I+
nickel(I1) and cobalt( 11)
diphenylcarbazone t H,O, (borate buffer)
osmium [as Os(VIII)]
azine dyes (e.g., Neutral Red or Neutral Violet) t Br0,-
palladium
reduction of Toluidine Blue by hypophosphite in 1 M H,S04
reduction of phenosafranine by hypophosphite in acidic medium phosphorus [as P20,4-]
precipitation of calcium triphosphate
platinum(IV) 0, (atmospheric) +
di-2-pyridyl ketone
rhenium [as Re(VII)]
66R
Malachite Green + SnCl, or thiourea(pH1.3)
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
comments
ref
tion range 0.002-10 pg/mL limit of detection 2 x to 2 x lo-' M depending on the use of indigomono-, di-, or tetrasulfonate indirect determination; method of tangents used after reaction had been followed €or 5 min; method applied to analysis of stationary waters of electrical stations
41 42
l o - * g/mL
43
mercury catalyzes the ligand exchange with p-nitrosodiphenylamine acting as the incoming one; limit of detection 2 x l o - * M ; fixed-time method absorbance measured at 640 nm after 1 5 min of reaction ligand exchange reaction; fixed-time procedure (absorbance measurement 10 min after initiation of the reaction); limit of detection, 0.05 ppm; concentration range amenable to determination, 0-0.3 ppm; reproducibility, 3%; silver and Zn interfere but Cd does not; lJ0-phenanthroline gives blanks twice as high as 2,2'-bipyridine iodide concentration monitored by an ion-selective electrode (pH 1.9); determination based on plots of rates vs. concentration; molybdenum determined in VI, in the 0.1-1.5 pM range the range of 1.0-100 & method of tangents; organic solvents (ethanol, acetone, dimethyl sulfoxide, and dimethylformamide) used to increase the rate of catalyzed reaction; nickel determined at the pg/mL and Co in the pg/mL level; possibility of determining Ni in the presence of comparable quantities of Co discussed determination at the picogram level without interference from Ru; reaction run at 30 "C with close control of temperature; photometric monitoring in the visible region of the spectrum and application of the fixed-time kinetic approach (measurements at 4 and 10 min after initiation of reaction) no interference from Cu(11), Mn(II), phosphate, alkaline earth ions; chloride iodide, sulfide, EDTA, Pt(IV), and Mo(V1) act as inhibitors; bromide, sulfite activate the catalytic effect; at certain Au(III)/ Pd(I1) ratios an induction period observed, its length increases with Au(II1) concentration; mercury( 11) behaves similarly; after the induction period the Pd(I1)-catalyzed reduction is faster than in absence of Au(II1) or Hg(11) photometric monitoring at 525 nm; length of induction period related to Pd(I1) concentration; catalytic action of Pd(I1) allows determining it in the range 0.1-1.1 ppm with relative errors of 2.6% variable-time procedure based on measure of time needed to reach an absorbance of 0.04 at pH 7.07.5; determination of pyrophosphate in pyrophosphate salts; interference of large amounts of PO,3- eliminated by extraction Pt(1V) photochemically reduced to colloidal Pt(0) with polyhydridesiloxane as a solid reducing agent; colloidal Pt stabilized with poly(viny1 alcohol) or gum arabic; analysis time about 1-1.5 h fluorimetric monitoring (kex.359 n q A,, 435 nm); platinum catalyzes formation of an oxidation product with intense blue fluorescence; optimum pH, 2.6; initial rate (method of tangents) measurement recommended over fixed- and variable-time procedures; only Cu(I1) and Au(II1) interfere seriously tartaric acid acts as activator and helps suppress interference of M o if the latter is present at concentrations equal or larger than 0.01 M; rhenium in sulfide ores determined by the method
44
limit of detection 4 x
45
46
47
48
49
50
51
52
53
54
KINETIC ASPECTS
OF
ANALYTICAL CHEMISTRY
-
Table I (Continued) element
indicator reaction Malachite Green (PH 1.3)
ruthenium
+ SnC1,
tropaeol Lin 00 t IO4(pH less than 2)
ruthenium [as Ru(III)] and iridium [as Ir(III)] Ru(1V)
Direct Blue 6B + H,O,, pH 0.8-1.2 (iridium: IO4-, instead of H,O, and pH; 8.5-9.5) N-methyldiphenylamine-4sulfonic acid + V0,- in 2 M
selenium
picrate
silver
indigo carmine + S,0,2(0.004 M H,SO,)
sulfur [as S Z - ]
NaN,
%SO,
+ S2' (pH 10.1)
+ I,
+ I--ascorbic
tungsten
H,O,
vanadium (IV )
Bindschelder's green leuco base + Br0,-
vanadium(1V) and -(V)
acid
N-phenyl-p-phenylenediamine
+ N,N-dimethylaniline Br 0
+
comments of tangents; range of concentration amenable to determination, 0.02-0.05 pg/L photometric monitoring of the Malachite Green cation (600 nm); citric and tartaric acid act as activators lowering the limit of detection 1 and 2 orders of magnitude; limit of detection in absence of activator, 0.0019 pg/mL determination in ores, mattes, cakes, slimes, and sludges ruthenium determined in standard reference samples of Cu alloys, anodic Ni, slurries, slimes, and sludges % or higher levels; sample when present at dissolved with H,O,-HCl or HCl-HNO, or by fusion with Na,O, and dissolution of the melt with HCl concentration range determinable, 0.0001-0.001%
ref 55
56 57
58
(iridium: 0.0001-0.01%) limit of detection, 4 x IO-" g/mL; determination in Ru complexes and in carbon-supported Ru catalysts variable-timeprocedure; potentiometric monitoring with a picrate ion selective electrode and automatic recording of time required for the potential to change by a preselected voltage value; determination in the range of 3-30 pg of Se with average error of about 4% and RSD of about 2% kinetics of reaction discussed; silver determined in the 0.3-4.0 pg/mL range with an average error of 2%;calibration curve, plot of pseudo rate constant vs. concentration sulfide precipitated as ZnS, suspended in water; and then treated with sodium azide and iodide; fixed time procedure with absorptiometric monitoring (350 nm) 5 min after mixing; interference by 5 0 : eliminated with formaldehyde; the presence of Fe(II1) yields high results sulfide (as H,S) absorbed in an ammoniacal solution of zinc sulfate; absorbed sulfide was kept for 15 min with azide-iodine reagent (25 'C, pH 7.2) with occasional shaking and then excess iodine titrated with standard hydrazine sulfate solution and starch as indicator; sulfide determined with reference to calibration plots in the ranges lo-,M and 10-5-10 - 7 M determination of free sulfur in sulfur-containing polymers (e.g., poly(pheny1ene sulfide),; determination requires a preextraction step and is based on the titration of excess iodine with AsO,; about 4 h needed per determination, errors about 6% determination based on measurement of induction period (Landolt's effect) or a fixed-time procedure; best experimental conditions are reported; applied to the determination of W in mine water; equal amounts of Mo, V, or oxalic acid are reported as interfere; manganese(VII), Cr(VI), Re( VII), and Ge(1V) d o not interfere at a 100-foldexcess; sulfate, nitrate, and acetate do not interfere at a 500-fold excess photometric monitoring (725 nm) and application of fixed-time procedure; as low as 0.008 ng/mL determined; iron(II1) and Cu(I10 accelerate the reaction; applied to determination of V in river and tap waters implementation of the procedure described above in an unsegmented continuous flow system (flow injection analysis); determination rate, 60 samples/ h; vanadium determined in the range 0-80 ppb fixed-time (20 min) determination by monitoring photometrically (735 nm) formation of a green product; calibration graphs linear between 0 and 1.0 ng of V/mL; reaction run at 50 "C; vanadium determined in seawater ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
59 60
61
62
63
64
65
66
67
68
67R
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Table I (Continued) element
indicator reaction 1,2-phenylenediamine t BrO; also: 2,2,4-trimethyl-6hydroxy dihy dr oqu inoline + Br0,autoxidation of 4,8-diamino-
vanadium as V(V)
reactions run in acetate buffer of pH 3.2 containing 0.01 M Tiron;limit of detection, 5 x pg/mL; applications. determination in waters, brines, and high-purity compounds fluorometric monitoring of pink product; reaction exhibits an induction period; application of 1,5-dihydroxyanthraquinoneinduction period measurement, initial rate, and 2,6-disulfonate fixed- and variable-time procedures discussed; measurement of induction period provides better precision and competitive limit of detection; range for V determination, 0.04-0.05 ppm oxidation product (p-sulfobenzenediazonium ion) p-hydrazinobenzenesulfonic coupled with rn-phenylenediamine to form a acid t chlorate yellow azo dyestuff monitored at 454 WM; tartaric acid activates t h e oxidation; fixed-time procedure; 25 min reaction at 55 "C and pH 3.0; linear working curves of absorbance vs. vanadium concentration obtained in the range 0.008-0.04 Mg/mL; iron(II), iron(111), selenium(1V); copper(II), and molybdenum(V1) cause positive interference; aiuminum(II1) interferes negatively (see also ref 46)
1,3-di-n-butyl) by exploiting their catalytic effect on the iodine-azide reaction. An error of about 5% is reported for the range 0.05-0.5 pmol in 5-pL samples. The possibility of removing the metal (by means of a complexing agent) from a metalloenzyme to obtain the corresponding apoenzyme, whose reactivation would provide a very selective procedure for the determination of the prosthetic metal, was fiist pointed out by Townshend and Vaughan (77). Little attention has been paid to this ossibility and only a few methods utilizing the idea have !,en published. Donangelo and Chang (78) have exploited the approach for the determination of zinc in plasma and serum. Their procedure involves removal of zinc from Escherichia coli alkaline phosphatase with nitrilotriacetic acid. The apoenzyme is then incubated with a serum or plasma sample and some of the enzyme is reactivated by the zinc in the sample, the extent of reactivation being proportional to the available zinc. The recommended procedure takes slightly over 1 h per determination. An interesting twist to the gas chromatographic determination of metal ions has been provided by Ditzler and Gutknecht (79) by catalytic conversion to an easily analyzed species by GLC. The amount of derivative produced is related to the concentration of catalyst. Cuprous ion has been determined by its catalysis of conversion of an aromatic diazonium salt (prepared from p-toluidine) to a chlorinated derivative, p-chlorotoluene (80). At 21 "C the limit of detection (concentration of sample that gives a signal of twice the standard deviation of the blank) is reported as 0.25 ppm of Cu(1). Lowering the reaction temperature to almost 0 "C, lowers the limit of detection to 0.06 ppm. In both cases, however, the reaction must be run for 30 min. The fact that interference by Cu(I1) is small adds attraction to the proposed method. The presence of Fe(II), on the other hand, was found also to increase the production of p-chlorotoluene, the chromatographically measured species. The authors have also determined Fe(II1) by a similar procedure (80). In this case Fe(II1) catalyzes the reaction between anisole and HzOz(in presence of hydroquinone) producing o-methoxyphenol which is chromatographically determined. The detection limit is reported as 0.25 ppb with 10 min of reaction time. Of several metals tested, only Cu(I1) was found to interfere \seriously. The proposed method was applied to the determination of iron(II1) in river water, NBS orchard leaf standard samples, and vitamin tablets. The overall approach circumvents some of the problems presented by other routes to the GLC determination of traces of metallic ion species. Ultraviolet detection after HPLC is not sufficiently sensitive for plasma levels of thyroid hormones. A modification of the Sandell-Kolthoff reaction has been used for postcolumn detection of thyroxine (81). This reaction system, however, utilizes reagents considered troublesome for routine application because of their corrosive character. To circumvent this problem, Lankmayr et al. exploited the catalytic effect 68R
comments
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
ref
69
70
71
of iodide on the reaction of N,N'-tetramethyldiaminodiphenylmethane with chloramine-T (82,83). This postcolumn reaction system was used for detection in the HPLC determination of enantiomeric iodinated thyronines in blood serum. The color produced in the reaction is photometrically monitored a t 600 nm. The paper includes a brief discussion of mixing configurations of interest in postcolumn reactors. The detection limit for isomeric tetraiodothyronines in serum is reported to be about 3 nmol/L. Sample preparation for the determination of vanadium in petroleum oils is generally performed by dry or wet ashing, with the possibility of volatilization losses of metallic species and introduction of contaminants. The oxygen flask combustion method for sample preparation avoids these problems. It, however, requires a small sample size (less than 100 mg) incompatible with the lack of sensitivity of conventional determination procedures. Catalytic determination of vanadium using the gallic acid-bromate indicator reaction provides sufficient sensitivity even in small samples and has been coupled with the oxygen flask combustion method for the determination of vanadium in crude oil and residual fuel oils (84). The reaction was followed spectrophotometrically at 420 nm and its rate was graphically obtained from the slope of the linear plot of absorbance vs. time (15-40 min of reaction). The vanadium content was estimated from calibration graphs of reaction rate vs. known amounts of vanadium. Linear calibration plots were obtained in the 0-100 ng range. Catalytic determinations of metallic species based on chemical reactions other than electron transfer are scarce. Tabata and Tanaka (85) proposed a sensitive and selective determination of mercury(I1) based on its catalytic action on the complex formation of manganese(I1) with tetraphenylporphinesulfonate, TPPS. Metal ion incorporation into the porphyrin nucleus is assisted by a variety of metal ion species (e.g., Cd(II), Zn(II), Pb(I1)) but mercury shows the greatest effect. The proposed method, although time-consuming (1 h for mercury distillation at room temperature and 10-30 min for measuring the change in absorbance at 413 nm) is highly selective and permits the determination of mercury in nanogram amounts. An unusual application of metal ion catalysis has been reported by Tawa and Hirose (86). They have determined microamounts of primary or secondary amines in the presence of tertiary amines by a method involving the following steps: (1) formation of the copper(I1) bis(dithi0carbamate) complex from the primary or secondary amine (tertiary amines do not react), carbon disulfide, and excess Cu(I1); (2) extraction of the complex into chloroform and (3) determination of the remaining copper(I1) ions in the aqueous phase by following the rate of oxidation of pyrocatechol violet by HzOzwhich is catalyzed by Cu(I1). Reaction rates were determined by a simplified complementary tristimulus colorimetric method. Alekseeva et al. (87) compiled mathematical models for the iodide-peroxide reaction catalyzed by tungsten(VI), niobi-
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
um(V), tantalum(V), hafnium(IV), and zirconium(1V). A catalyst-iodide and a catalyst-peroxide intermediate were identified. With the aid of experimental data and a comparative calculation method they derived optimal conditions for the kinetic determiination of Nb(V) and Ta(V). Determination of organic species based on their catalytic action were found only occasionally in the literature. Ushakova (88) reports the determination of oxalic acid in the 10-5-104 M concentration range using dichromate-iodide in pH 2.5 phthalate buffer and absorbance monitoring at 582 nm in the presence of sitarch. Since in some other dichromate oxidations oxalic acid increases the rate of oxidation (89), the possibility of promotion instead of true catalysis cannot be ruled out. No interfering effect was observed from succinic, malonic, tartaric, and citric acid present at the same concentration levels as oxalic. Reaction of chromiuim(III) with excess EDTA provides the basis for the direct or indirect (back-titration) titration of millimolar and micrornolar quantities of this species. Determination is performed in the pH range of 3-5 and an excess of EDTA of about 10 times the chromium present (on a molar basis). The reaction is slow and even under these conditions a 30-min boiling is required. Both benzoate and o-chlorobenzoate exert a catalytic effect on the rate of complexation which has been investiigated by van der Linden and Ozinga (90). Benzoate at pH 4.0-4.5 and o-chlorobenzoate at pH 3.0-4.0 increase the reaction rate by a factor of 10-20; this allows shorter heating times and/or use of smaller excess of EDTA. Unsegmented continuous-flow analyses are increasingly attracting interest. A common characteristic of the procedures is the introduction of the sample by a discrete injection into a continuously flowing, unsegmented stream. The carrier stream transports the sample (and in most cases provides a reactant for a chemical reaction with the sought-for species) to the detection area in which the analytical signal is acquired. Detection occurs while the system is attaining equilibrium by a physical or chemical ]processor both; thus, these procedures generally termed “flour injection analyses” (91), are kineticbased determinations. Yamane and Fukawasa (92) have incorporated a catalytic determination developed previously (93) into a flow injection procedure for the determination of vanadium in nanogram quantities. The indicator reaction is the chromotropic acid 4,5-dihydroxynaphthalene-2,7-disulfonic acid-bromate system. Vanadium in the 0.3-4.8 ng (10-160 ppb) was determined at a rate of ca. 60 samples/h with a 2.0% relative standard deviation. A catalytic determination of copper in a flow injection (closed-loop configuration) system has been implemented by Ramasamy and Mottola (94). The procedure was applied to the determination of copper in human blood serum. The indicator reaction was that of iron(II1) with thiosulfate. The sought-forspecies (the copper catalyst) was removed from the recirculating reagent by controlled potential electrolysis after detection has taken place. Electrochemical removal of the catalyst was paralleledl by reoxidation of iron(I1) to iron(II1) permitting maintenance of a constant level of the monitored species (the red Fe(H[20)5SCN2+ complex). The reported method has a limit of detection of 0.25 ppm of copper(I1) and permits 325 determinations/h with a relative standard deviation of 2%. Basically, the same chemical system has been used by Weisz and Fritz for the continuous monitoring of sample streams (instead of single sample processing). Their catalytic determination of copper was intended to demonstrate the feasibility of unsegmented continuous-flow sample introduction and the “stat” monitoring approach (95). Their work is discussed in more detail in the “Miscellaneous”section of this review. The Sandell-Kolthoff reaction (As(III)-Ce(IV), catalyzed by I-) is continuously being revisited. Elverog and Carr, for instance, have implemented this chemical system in an unsegmented continuous-flow thermochemicalreactor/detecting system (96). A limit of detection reported as 0.15 ng of Iappears to represent the lowest for thermochemical detection realized a t present. The reactors were used under pulsed conditions; the resultant heat pulse being detected with a differential temperature measurement system. Some observations on the relationships between peak height and flow rate and between peak width and sample volume are part of the report.
Catalytic Determinations Based on Inhibition and Activation. Most (if not all) of the kinetic methods based on inhibition of catalyzed reactions exploit a reaction with the catalyst which is thus prevented from entering the catalytic cycle. A kinetic determination of magnesium proposed by Valcarcel et al. (97), however, seems to depend on the interaction of magnesium with the dye of the indicator reaction. Their method makes use of the aerial oxidation of 1,4-dihydroxyphthalimide dithiosemicarbazone which is catalyzed by manganese(I1). Calibration curves are constructed by recording the change in absorbance at 594 nm 8 min after initiation of the reaction and calculating the percent inhibition, which is plotted vs. the magnesium concentrations. Magnesium can be determined at the M level; the method has been applied to samples of natural waters. Schwing et al. (98) report on a detailed study of the promoting effect of titanium(II1) on the oxidation of iodide by H202 The proposed promoting mechanism involves a preequilibrium step between a monomeric and a dimeric form of titanium(III), followed by free radical formation in which the monomeric form reacts with HzOzto form the active radical OH which oxidizes the I-. The active titanium(II1) form is converted to the inactive titanium(IV), hence the classification of the effect as a promotion (89). Stopped-flow measurements in the presence of 0.08 M hydrochloric acid make it possible to determine titanium(II1) at the M level with less than 5% error. The half-life of the promoting effect under the specified conditions is estimated as 2-3 s. Auffarth and Klockow (99) described the determination of fluoride based on the inhibition of the zirconium-catalyzed reaction between perbromate and iodide. Addition of small amounts of ascorbic acid results in a Landolt-type reaction system, the length of the induction period of which is proportional to the fluoride content. Species such as A13+,Fe3+, (which form complexes with fluoride Ca2+,HZPO,, and Sod2or zirconium) seriously interfere. These interferences, hohever, are overcome by a microdiffusion technique utilizing hexamethyldisiloxanein the presence of AgZSO4to fix the C1that interferes with the diffusion. The paper gives detailed information for the diffusion process, collection of the liberated fluorine compound on a receptor lid coated with a layer of NaOH + Na2C03,and an ingenious setup for the catalytic microdetermination of fluoride. Results are presented for the determination of fluoride in geochemical materials, rain water, and aerosol filter samples. The inhibitory effect of mercury on the peroxidase catalysis of the H202oxidation of o-dianisidine has been utilized for the determination of mercury (100). The inhibition increases in the presence of thiourea and this allows a limit of detection pg/mL. The authors applied the method of about 1 X of tangents followingthe reaction for 2 min after thermostating the reagents for at least 4 h. Bismuth and cadmium also exert inhibitory action and thus can interfere. The oxidation of 2-amino-4-methylphenolwith perchlorate is very slow at pH below 4. Traces of vanadium(V), however, markedly increase the rate of oxidation. The oxidation product of 2-amino-4-methylphenol exhibits two ac polarographic waves at about -0.45 V and -1.3 V vs. SCE (pH 3.0); the first wave is of tensametric nature. The presence of hgands such as EDTA inhibit the catalytic effect of V(V) with a decrease in the tensametric wave proportional to the EDTA concentration. Such a decrease permits the determination of EDTA in the 10-7-10-6 M range with a reported relative error of 3% (101). Almost all metal ions forming stable complexes with EDTA and other complexing agents for V(V) interfere. Ascorbic acid enhances the catalytic effect of Cu(I1) on the HzOz oxidation of the 2-thiosemicarbazone of sodium 1,2naphthoquinone-4-sulfonate. Although ascorbic acid also increases the rate of the uncatalyzed reaction, Igov et al. (102) have used it to lower the limit of detection for Cu(I1) from 0.05 pg/mL (absence of ascorbic acid) to 0.25 ng mL (presence of ascorbic acid at pH 6.6). The absorbance o the red-violet oxidation product has been photometrically monitored every 30 s for the 5-10 min reaction time and application of the differential variant of the method of tangents gave linear calibration plots in the concentration range 0.8-5 ng/mL. Cobalt(I1) and I- catalyze the indicator reaction, while nickel(I1) inhibits it. In the presence of EDTA only the uncatalyzed reaction takes place.
d
ANALYTICAL CHEMISTRY, VOL. 54,
NO. 5,
APRIL 1982
69 R
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Although the chemistry of molybdenum(V1) and tungsten(V1) compounds is closely parallel and both catalyze the HzOz oxidation of iodide ions to iodine, their complexes with citric acid in acidic medium (pH 1.7) behave diametrically different. In the presence of citric acid, the catalytic effect of W(V1) is totally suppressed while that of Mo(V1) is slightly increased. This different behavior has been used for the catalytic determination of Mo(V1) and W(V1) in the presence of each other (103), since in the absence of citric acid the catalytic effect of both species is additive. By application of the method of tangents and monitoring (spectrophotometrically) the change in absorbance of the I starch complex, as little as 2 X lo-' M of either species can [e determined with a relative standard deviation reported as 0.08. The effect of some organic solvents on the catalytic activity of copper(II), when the hydroquinone-H20zindicator reaction is used, has been reported by Dolmanova et al. (104). The organic solvents tested were ethanol, acetone, dimethylformamide, and dimethyl sulfoxide. The nature of the effect (activation or inhibition) seems to depend on the relationship between the electron-donor properties of the modifying species and solvent. Selectivity in Cu(I1) determination is improved by use of a water-dimethyl sulfoxide medium (30% by volume of Me2SO)and 2,2'-dipyridyl as activator. The authors report the determination of copper in blood serum after protein removal. Limit of detection was (2.0 f 0.6) X loW4 pg/mL. Sekheta and Milovanovic (105) describe the determination of microgram quantities of gentisic and chromotropic acids by a kinetic method based on their inhibition of the molybdenum(V1) catalysis of the H202oxidation of Azorubin S. Gentisic acid forms a complex with Mo(V1) of lower catalytic activity than molybdenum itself. Chromotropic acid, on the other hand, forms a catalytically inactive complex. The rate of the reaction was followed photometrically at 482 nm and calibration curves were prepared by a differential variant of the method of tangents. The rate of oxidation of pyrocatechol violet by hydrogen peroxide, catalyzed by copper(II), is affected by the presence of adrenaline, noradrenaline, thyroxine, and 5-hydroxytryptophan. Adrenaline and noradrenaline, in subequivalent quantities with respect to copper(II), increase the rate of oxidation; thyroxine and 5-hydroxytryptophan act as inhibitors. Kinetic determination of these biologically active species, based on these rate-modifying effects, has been proposed by Milovanovic et al. (106). Absorbance monitoring at 482 nm from 1 to 5 min after initiating the reaction and use of the method of tangents permits determinations in the lo4 M level. Reported results show relative standard deviations within the range of 1-10% depending on the compound and concentration level. Tetracycline, oxytetracycline, methacycline, and demeclocycline, all antibiotics of the tetracycline group, have been determined by their inhibitory effect on the molybdenum(V1) catalysis of Azorubin oxidation by H20z(107). Apparently, the antibiotics form a 1:l complex with molybdenum(VI) with lesser catalytic action than free molybdenum(VI) catalyst. The determination of the tetracyclines made use of a differential variant of the method of tangents and covered the 18-160 ppm range with relative standard deviations of up to 8%. Very low concentrationsof tin(I1) inhibit the catalytic effect of Fe(II1) on the N,N-diethyl-a-phenylenediamine sulfateH202indicator reaction; this effect has been used for the determination of Sn(I1) with a reported limit of detection of 10 ng/mL (108). The reaction is carried out in acetate buffer (pH 5.5) and the formation of a red-purple oxidation product is photometrically monitored during the first 5-8 min of reaction. Application of the method of tangents yields linear calibration plots in the concentration range of 10-80 ng/mL. Tartrate, SCN-, citrate, and EDTA (in 1:l ratio to Sn(I1)) interfere. Nitrogen-containingdrugs showing psychoneurological and coromry-stimulatory effects were studied for their modifying effect on the hydroquinone-H202 reaction with Cu(I1) as catalyst and in the presence and absence of pyridine (109). Using the method of tangents and exploiting the inhibitory effect of the ligands (except for the drug Ch-91, which is an activator in absence of pyridine) some of the drugs were determined in protein-free animal blood plasma. Inhibition of the Cu(I1)-catalyzedoxidation of hydroquinone by H202(using pyridine or NHIF as activators) by several 70 R
ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
phosphorus-containingcomplexones has been reported (110). If 2,2'-dipyridyl is the activator, nitrilotrimethane phosphonic acid enhances the activating effect. The study also is concerned with inhibition (by the same family of compounds) of the Cr(II1) catalysis of the HzOzoxidation of o-dianisidine. As a result of these studies, the authors proposed the determination of phosphorus-containing complexones (concentration range 10-8-10-9 M) by the method of tangents and photometric monitoring (during 4 min) of the reaction progress. A kinetic study of the copper-catalyzed decomposition of HzO, in the presence of pyridine as an activator has been reported by Otto et al. (111). An analysis of equilibrium concentrations of all plausible species shows a 1:l complex between copper and pyridine as the active catalyst. This study has been extended to illustrate the optimization of kineticcatalytic methods by a numerical model and simplex treatment (112). Optimum values for the concentrations of Hz02, pyridine, and pH are extracted from response surfaces and simplexes. Experimental verifications of simulated reaction conditions indicated a limit of about 1.5 X lo-' M for the catalytic determination of copper under the best conditions.
Catalytic Titrants and Catalytic End-Point Indication.
Titrations based on a titration reaction, in which an added catalytic titrant reacts rapidly and stoichiometrically with the sought-after species, and an indicator reaction, which involves the monitored species and can occur at a noticeable rate only when an excess of titrant is present in the system, have received occasional attention. Practical application of catalytic indication of end points recorded in the literature involve mainly complexometric reactions as the main titration reactions although there are a few cases of acid-base titrations. Pantel and Weisz have recently demonstrated the applicability of this approach to end-point indication in the case of redox reactions as main titration reactions (113). A condition for such applicability is that the catalytic titrant must be involved in the same oxidation state in the titration reaction (which in turn must be considerably faster than the reaction used for indication of end point) and in the catalytic cycle of the indicator reaction. Chromium(V1) reacts very fast with ascorbic acid, faster than it catalyzes the H20zoxidation of dianisidine, and this permits direct titration of ascorbic acid as well as backtitration of vanadium(V),thallium(III), or cerium(1V) (113). A sensitive end-point indication of equilibrium titrations involving gaseous catalysts has been proposed (114). The first excess of titrant produces the evolution of HzS(g) which catalyzes the iodine-azide indicator reaction. The liberated HzS gas is transferred by a stream of nitrogen into a separate container in which the azide-iodine mixture is contained. The titrant (responsible for the H2S evolution) is sodium sulfide which reacts with several metal ions forming insoluble sulfides (precipitation titrations) or with permanganate by electron transfer (redox titration). Free iodine is proposed as another vaporizable catalyst for end-point indication, and a variation of the method for acid-base titrations is discussed. Amperometry and constant-current potentiometry have been evaluated by Gaal and Abramovic for end-point indication of some precipitation reactions with catalytic titrants (115). Silver(I), palladium(II), and mercury(I1) have been titrated with iodide ions (the catalytic titrant) and the end point indicated by the catalytic effect of iodide on the Ce(1V)-As(II1) reaction or the Ce(1V)-Sb(II1) reaction. Since the nature of the indicator electrode affects the results, the authors compared the performance of platinum, palladium, gold, graphite, and glassy carbon as electrode materials. Graphite indicator electrodes are somewhat better for constant-current potentiometric end-point indication, and graphite and glassy-carbon electrodes for amperometric indication. Milligram and fraction of milligram amounts of the metals were titrated with relative standard deviations of about 1%. The proposed method was applied to the determination of mercury in Unguentum Hydrargyri, a pharmaceutical ointment, after sample preparation. Titrations based on the reaction of EDTA with metal ions using manganese(I1)as catalytic titrant and the manganese(11)-catalyzed autoxidation of 1,4-dihydroxyphthalimidedithiosemicarbazoneas indicator reaction have been described by Valcarcel et al. (116). Calcium, magnesium, strontium, and barium are titrated in the 0.1-0.01 mg range in a simple, semiautomatic, back-titration procedure. The method was
KINETIC ASPECTS
applied to the determination of total hardness in natural waters and total calcium and magnesium in sterilized milk This catalytic end-point indication permits determinations of alkaline earth ions at concentrations lower than normally determinable by use of metallochromic indicators. Secondary aliphatic amines treated with 2-ethylhexaldehyde and carbon disulfide in subsequent steps produces (in the presence of silver ions) silver dialkyldithiocarbamate which can be extracted into chloroform. By stripping back the silver into aqueous solution (4M in HNOJ, and titrating the silver with iodide as a catalytic titrant and the As(II1)-Ce(1V) reaction as indicator, Kiba et al. (117)determined micromoles of diethylamine, dipropylamine, dibutylamine, ethylpropylamine, and ethylbutylamine. Catalytic Determinations Based on Heterogeneous Catalysis in Electrode Reactions. Since the previous review in this series (I), the number of papers reporting catalytic determinations involving heterogeneous electrochemical systems has quadrupled. The requirements (e.g., catalyst, substrate, ligand-activator, and solution composition) for the use of catalytic polarographic waves have been examined by Milyavskii (118).Milyavskii's considerations, based on personal experimental studies and other published information, is an attempt to formulate basic criteria to guide the development of analytical procedures based on catalytic polarographic waves. Milyavskii's undertaking was inspired by the fact that polarographic methods based on catalytic currents, despite high sensitivity and precision, are not widely used. The author believes that clear-cut recommendations on the use of catalytic currents and on the choice of syietems should point out the advantages of the approach and plromote its use. A polarographic catalytic wave has been observed durin the reduction of titanium(1V) in solutions containing E D T i and acetate when bromate ion is present (119). This wave can be used for the determination of titanium in the 5 X lo-' to 2 X M range, and the approach has been applied for the determination of titanium in thin films of titanium carbide. Several ions interfere; iodate, tellurite, and H O2are reported as serious interferences if present in 20-folci excess. Takata et al. (120)detected cobalt ions in the effluent of ion-exchange chromatographic columns by monitoring the catalytic effect of these ions on the oxidation current of tartrate at a carbon cloth electrode. The limit of detection is reported as 5 ng and the linear dynamic range as lo3. The detection technique was used in the analysis of coolant for boiling water reactors in which cobalt is the most important element to be monitored, because of the roduction of 6oCo. Ce(III), Cr(VI), Fe(II), Fe(III), Mn(II), PbFI), and Sn(I1) ions can also be detected by their catalytic action. The polarographic catalytic hydrogen current in the presence of cysteinyltyronine, cystein lglycine, and cysteinylphenylalanine have been studied gy Kadlecek and Kalous (121). The wave height of the Brdicka current is directly proportional to the peptide concentration at the M level. Uranium(II1) ions formed when uranyl ions are reduced (electrochemically) at a dropping mercury electrode in an acidic medium react with some electrochemically inactive or less active materials (e.g., nitrate ion, nitrite ion, oxalic acid) regenerating U(1V) and initiating a catalytic cycle. The catalytic cycle increases the height of the second polarographic wave for UOZ2+. Nikolaeva and Zhdanov (122)have also observed the catalytic wave in the presence of glyoxylic acid. Under optimum conditions, as little as 5 X lo4 M uranyl ions or 1 x lo4 M glyoxylic acid can be determined with relative standard deviations of about 2%. The nature of the polarographic catalytic currents of molybdenum(V1) in 0.05-2.5 M H2S04containing amygdalic acid and KC103 has been investigated and reported by Zaitsev et al. (123). Submicrogram quantities of iron (III),0.01-0.16 pg/mL, have been determined by monitoring the catalytic polarographic reduction of 'bromate in the presence of resacetophenone isoniazidhydrazone (124).The proposed method was applied to the determination of iron in maize leaves after ashing. An iron content of 0.09% was found, in good agreement with previously reported values. The current peak observed in the catalytic polarographic reduction of potassium bromate by molybdenum in the presence of resacetophenoneisoniazidhydrazone has also been
OF
ANALYTICAL CHEMISTRY
used for the determination of submicrogram amounts of Mo(V1) (125). The peak current linearly varied with the molybdenum concentration in the range 0.04-0.16 ppm. An indirect determination of water hardness based on a catalytic polarographicprocess has been proposed (126).The determination depends on the stoichiometric displacement of magnesium ion from its relatively weak complex with EDTA by the majority of other divalent ions. The liberated Mg2+ is determined by use of the catalytic wave that results when it is reduced at the dropping mercury electrode. The proposed approach is more rapid than other conventional methods of water hardness determination and measures the hardness resulting from several dissolved metals simultaneously. Results are presented for determination of hardness in several types of samples: standard hard water, artesian well water, surface well water, tap water, and seawater. The catalytic wave developed when 2-phenyl-5-mercapto1,3,4-oxadiazole,PMODA, is reduced in solutions of pH 4.00 containing cobalt(I1) has been examined by Averko-Antonovich and Gorokhovskaya (127).The wave corresponds to the catalytic reduction of hydrogen ions from a complex of elemental cobalt adsorbed on the electrode and can be used for the polarographic determination of low concentrations of cobalt(I1) and PMODA. Umland et al. report on the determination of gallium (128)and indium (129)by pulse polarography based on the measurement of the anion-induced adsorption wave in the presence of Alizarin S. Gallium can be determined at levels as low as 50 ppb by this ligand-caM level) requires talysis effect; the indium determination ( several separation steps to eliminate interferences by cadmium, aluminum, and other elements. The catalytic polarographic currents of chlorate and hydrogen ions have been examined in the presence of the tungsten(V1)complexes with pyrocatechol (130). The reduction of chlorate ion at -0.8 V (vs. SCE) is catalyzed by tungsten(II1) and permits a selective determination of microamounts of tungsten. Electrocatalytic methods for ditrite determination have received special attention recently. Seiler and Avery (131), for instance, have described a cyclic voltammetric method at a hanging mercuy drop electrode for determination of nitrite in the to 10- M range in basic solution. Use of a basic medium avoids complications resulting from the volatility of nitrous acid, the predominant species in acid medium. The method is based on the monitoring of an unusual, sharply rising reduction peak at -1.65 V vs. SCE observed in a positive-going scan. The catalytic mechanism is not understood a t present but depends on the presence of chromate in the supporting medium. Determination by this approach requires that all solutions be sparged with nitrogen for at least 20 min prior to use. Nitrate interferes but sensitivity for nitrite is roughly twice that for nitrate. Acetate concentrations as large M did not affect the detection of nitrite. Similarly no as effect was produced by the addition of 5 drops of a 1% solution of Triton x-100 to the cell solution. As little as 0.001% thallium in sulfide ores has been determined by means of an adsorption polarographic catalytic wave in alkaline supporting electrolyte containing KF and KI (132).Thallium was coprecipitated with Mn02 prior to determination. Antimony in soil, blood, water, and mineral samples has been determined by monitoring the adsorption catalytic wave of its complex with cupferron in 2 N HCl with 5% tartrate (133).The wave shows a peak potential at -0.25 V and permits determiningas little as 0.02 Mg/mL of antimony. Interferences were avoided by extracting the antimony complex prior to determination into C6H6-BuOAc from 40% H2S04-bromide solutions. Cyclic voltammetry has been used by Verplaetse et al. to study the catalytic influence of C1-, Br-, I-, and SCN- on the reduction of antimony(II1)at a mercury electrode (134).The reversibility of this reduction in the presence of the mentioned ions is of analytical interest.
Additional Studies and Applications Involving Primary Catalytic Effects. Electrochemicalsensing of oxygen decrease has been used in the catalytic determination of cobalt by means of sulfite oxidation (by dissolved oxygen) in weak basic medium as indicator reaction (135). Iron had been determined in cow milk, human skin, and guinea pig kidney, lung, and spleen by a catalytic procedure (136)based on the previously proposed indicator reaction ANALYTICAL CHEMISTRY, VOL. 54. NO. 5. APRIL 1982
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involving the p-phenetidine oxidation (by H,O,) in the presence of 1,lO-phenanthroline as activator. The activator, together with p-phenetidine, forms a 1:l:l ternary complex responsible for the increase of catalytic activity (137). The catalytic method requires heating at 40 “C for 30 min and was compared with a photometric procedure (which requires 30-40 min of standing at room temperature) and with a flame atomic absorption determination. Because of the low limit of detection and good sensitivity the catalytic procedure permits working with samples l/loth to 1/20ththe size of those required for flame atomic absorption procedures. Traces of copper(II), mercury(II), cobalt(II), and manganese(I1) exert catalytic effect on the aerial oxidation of di-2pyridyl ketone hydrazone (138). The product of such an oxidation exhibits an intense blue fluorescence in acidic solution and its monitoring has been used to develop determinations of mercury(I1) (80-320 ppb) and copper(I1) (0.4-1 ppb). Emission occurs at 435 nm after excitation at 349 nm. More than 2 h are required for intensity measurement at 18-22 “C. Relative standard deviations (11 samples) are reported as 2% for mercury and 4.5% for copper. The chemical nature of the fluorescent oxidation product was investigated by infrared and mass spectrometry (138). The reaction between periodate and HA SO^^- exhibits an induction period. Osmium, a catalyst of this reaction, shortens the induction period proportionally to its increasing concentration. Alekseeva et al. (139) proposed the determination of osmium based on this effect by monitoring iodine, one of the producta of the reaction, amperometricallywith a rotating platinum microelectrode. The limit of detection is reported g/mL. The method was applied to the determias 6 X nation of osmium in lean platinum-containing products. Water in NJV-dimethylformamidehas been determined by the method of tangents in the range from 100 ppm to 50% (140). The indicator reaction is the oxidation of pyrocatechol by chlorate, perchlorate, or metaperiodate. A t low concentrations periodate provides good oxidizing capabilities but if the water content is over 20% the reaction is too rapid and chlorate or perchlorate should be used. The role of water which participates in the equilibrium between species of the oxidizing agent (e.g., IO4- 2H20 H4106-)resembles a catalytic-type effect. Errors are about 4% a t high concentrations and about 10% at low water levels (the ppm range). The analytical properties of a new reagent, picolinaldehyde nicotinoylhydrazone(PANH), have been described by Luque Castro and Valcarcel (141). The aerial oxidation of PANH has been found to be catalyzed by traces of titanium(1V) and this property has been exploited by the authors to develop methods for microamount determination of titanium (60-800 ng/mL) by different approaches to catalytic determination: initial-rate, fixed-time, and variable-time methods. The fluorescent oxidation product (maximum fluorescenceat pH 6-8) is monitored by following its emission at 445 nm (excitation at 365 nm). The kinetics of the oxidation of azo dyes by chromic acid in the presence and absence of oxalic acid has been reported by Subba Rao et al. (142). The reaction is first order with respect to reactants and initiates vinyl polymerization, suggesting a free radical mechanism. The accelerating effect of oxalic acid is termed as catalytic and used to develop a method for the determination of microgram amounts of this species. Determination is by a fixed-time procedure using methyl orange as the azo compound and performing absorbance measurements 10 min after mixing. The reaction is run at 30 “C and in 0.3 M HC104. The catalytic effect of carbonate ions on the formation of chromium(II1)-EDTA complex was used for the determination of carbonate (143). A variable-time procedure with photometric monitoring (550 nm) of the time required to reach an absorbance of 0.3 at 26 “C was used. The mechanism of the copper-catalyzedH202oxidation of m-aminobenzoic acid and 1,3,5-benzenetriol(two indicator reactions used in catalytic determinations) has been discussed by Bontchev and Gantcheva (144). The hydrogen peroxide oxidation of sulfanilic acid catalyzed by copper(I1) has been used as indicator reaction for the determination of copper. The roducts of this reaction have been investigated by Otto andPWerner (145).
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The redox behavior of the vanadium(V)/vanadium(IV) couple has been studied by cyclic voltammetry at a carbon electrode and in acetonitrile as solvent by Otto and Werner (146). This study was part of their exploration for possible kinetic-catalytic methods in nonaqueoug media. The effect of other solvents with different donor abilities was part of the study; they conclude that: (a) the vanadium couple is promising as a redox catalyst in nonaqueous systems, and (b) for the activation of catalytic reactions, mixed solvents will have the same imr>ortancein nonaaueous media as organic - ligands do in aqueous media. The oxidation (by H202) of disodium 1,2-dihydroxybenzene-3.5-disulfonate(tiron) has been used as an indicator reaction for the determination of metal catalysts. The corresponding o-quinone has been proposed as the actual chemical species acting as indicator but Otto et al. (147) conclude that the tiron semiquinone radical is the indicator species. Their proposition is supported from experimental data collected by electrochemical, EPR spectroscopic, and visible spectrophotometric techniques. Vanadium pentaoxide has been used as a catalytic aid in the HN03 digestion (under pressure) of biological materials being analyzed for trace concentrations of mercury (148). I
APPLICATIONS OF CHEMILUMINESCENCE A moderate increase in the number of analytical applications of chemiluminescence has been observed in the past 2 years. Although sometimes these determinations are termed as “catalytic”,this classification is doubtful and in this review we have preferred to review them separately from unquestionably catalytic methods. The measurements utilized in these applications, however, are kinetic in nature and as such they deserve a place in this review. The feasibility of using the catalyzed chemiluminescence of luminol by reaction with electrogenerated hydrogen peroxide for the determination of traces of metal ions has been explored by Haapakka and Kankare (149). Electrolytic generation of one of the reagents eliminates the need of injection into the reaction zone of the use or flowing systems. Using glycine buffer (basic solution) allows determination of copper(I1) in the 10-5-10-7 M range (calibration plots: log peak height vs. log Cu2+). Glycine keeps the Cu(I1) in solution at rather high pH, yet the copper complex with glycine is coordinatively unsaturated, permitting the participation of copper in the enhancement of luminescence. Glycine has the additional attractive feature of forming coordinatively saturated complexes with Cd(II), Co(II), Cr(III), and Zn(I1) and thus considerably suppressing the interfering action of these ions. Even though Cu(I1) does not show a great effect on the total intensity of the electrochemiluminescence of luminol, the use of pulse excitation with alternate positive and negative pulses applied to the working electrode revealed a considerable increase in luminescence. Effects of pulse amplitude, pulse width, pH, luminol concentration, and interference by other metal ions are discussed. Burguera et al. have adapted monitoring the chemiluminescence of the luminol-H202 reaction for the determination in a nonsegmented continuousflow system (flow injection analysis) of zinc and cadmium. They used a small anionexchange column just downstream from the point of sample injection for the retention of zinc and cadmium as their chloro complexes. These metal ions were then released in a sequential manner. Quantitation was based on the inhibitory effect that zinc and cadmium exert on the cobalt enhancement of the chemiluminescence process. Some detailed studies of halide ion enhancement of the chromium(III), iron(II), and cobalt(I1) action on the luminol chemiluminescence have been reported by Chang and Patterson (151). A general mechanism for the halide effect is proposed and the feasibility of analytical applications of the “halide effect” is discussed. Terketskaya et al. (152) reported that chromium(III), manganese(II),iridium(IV),ruthenium(IV),and osmium(VII1) increase the chemiluminescent oxidation of p-chlorobenzoic acid 5-bromosalicylhydrazide by potassium periodate. The authors proposed a method for the determination of iridium in a background predominantly of rhodium, palladium, and platinum. The chemiluminescence from the reaction between bis(2,4,5-trichloro-6-carbopentoxyphenyl)oxalate with H202 in
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
the presence of triethylamine in t-Bu-water has been used for the indirect determination of uric acid (153). The HzOz produced in the uricase-catalyzedaerial oxidation of uric acid is detected by the chemiluminescent reaction and under dynamic conditions (about 20% of reaction completed) in a continuous flow system. Linear response to uric acid was observed in the range 1 to 4 X lo4 M but application to the determination of uric acid in serum samples failed to give reproducible results. Highly toxic alkylphosphates (e.g., diisopropylfluorophosphate (DFP), sarin, soman, and tabun) can be determined by their reaction with Hz02(or sodium perborate or sodium peroxodiphosphate) to produce a peroxophosphate (or peroxophosphonate) coupled with the oxidation of an amine by this product, in basic medium. A very sensitive reaction system is obtained by )useof luminol as the amine and monitoring the resulting clhemiluminescence. An improvement to this determination approach has been reported by Fritsche (154). The improvement is based on the use of chloride which is reported to strongly “promote”the chemiluminescence and EDTA, which suppresses background emission due to metal ion impurities. Detection limits are reported in the range of 0.025-0.5 ng/FL depending on the toxic nature of the alkylphosphate. “Non-toxic”alkylphosphates do not interfere. Measurements are baaied on injection of the sample after 4 min of mixing of reagents and preparing calibration graphs based on either the peak height of the enhanced signal or the area under the peak vs. the alkylphosphate concentration. Calibration graphs are reported to be linear over more than 3 decades of concentration. The time interval between injecting the sample and the maximum signal is characteristic for the individual alkylphosphates adding a possibility for qualitative identification. Luminol in alkaline medium seems specifically reactive to NO2,while other nitrogen-containing compounds (ammonia, organic nitrites and niitrates, NO) do not interfere. On the basis of this reaction, Maeda et al. (155)developed a chemiluminescent method for NO2 determination. A limit of determination of 50 ppt is reported and RSDs of 0.54% (3.8 ppm) and 4.6% (5.5 ppm) were obtained. Ozone, SOz, and C 0 2 interfere. The successful application of a commercial chemiluminescent NO, analyzer (Beckman Model 951, Beckman Instruments, Fullerton, CA) to the determination of oxides of nitrogen in cigarette smoke has been reported by Jenkins and Gill (156). Cox has proposed the determination for nitrate and nitrite by reducing them to nitric oxide and then measuring it by the chemiluminescencereaction with ozone (157). Limits of detection of 0.5 pg L for nitrite and 5 pg for nitrate for 20-mL and 5-mL aiamp es, respectively, are reported.
1
DIFFERENTIAL REACTION RATE METHODS No significant novel developments can be cited in the area of differential reaction rate methods for the simultaneous determination of two or more closely related species without prior separation. Perhlaps to be signaled out as unique is the time-resolved determination of sulfur compounds by emission spectroscopy described by Schubert et al. (158). Determination of sulfur compalunds in complex solid samples such as atmospheric particulates has been restricted in the past to analysis for total sulfur. This is not satisfactory since different forms of inorganic sulfur play different environmental roles. Individual determination of the four forms of routine interest (So,S2-, SO3”, S042-) commonly requires elaborate procedures which start with dissolution of the solid sample. A simultaneous determination off these species, based on different rates of molecular sulfur emission in a H2/N2 flame, has been proposed (158). The time interval (in seconds) between the introduction of a sample into the flame and the observation of a molecular sulfur emission maximum follows the order: sulfite: 1-5; elementall sulfur: 14-20; sulfide: 25-31; sulfate without air: 35; sulfate with air: 11-19. Relative detection 1; S2-, 456; S(O), 1200; and SOS2-, lo6. limits are: S042-, Determination is based on log-log plots of emission intensity and weight of sulfur in the sample. This time-resolved molecular emission deteirmination seems to be insensitive to differences in the associated cations. However, the simultaneous determination of these sulfur species is affected by the presence of air in the flame. The effect is complex since apparently a beneficial increase in the production of molecular
sulfur is offset by an increased production of sulfur oxides. In the presence of air, the sulfide response is completely eliminated, while elemental sulfur, sulfite, and sulfate show an increase in emission rate in the presence of air. This behavior can be exploited to eliminate an interfering sulfide peak or enhance a weak sulfate peak. A few more applications of time honored approaches to differential reaction rate methods, such as graphical extrapolations and the method of pro ortional equations, have been reported in the past 2 years. Etepanov et al. (159) have determined rate constants for the dissociation of complexes of the rare-earth elements (except promethium) with diethylene traiaminepentaaceticacid, DTPA, by photometric monitoring. Significant differences in the collected values of the rate proportionality constants point to the possibility of using DTPA for the differential determination of rare earth elements. Mixtures of transition metal ions (Co(II), Ni(II), Mn(II), Pb(II), Cu(I1)) have been analyzed by Mentasti using a differential rate method based on ligand substitution reactions (160). The complexes between the metal ion and 2-(4-sulfophenylazo)-1,8-dihydroxy-3,6-naphthalenedisulfonicacid, SPADNS, react with ethylenediaminetetraacetic acid, the entering ligand, at sufficiently different rates that simultaneous determination can be accomplished by monitoring the disappearance of the colored metal-SPADNS complex spectrophotometrically (600 nm) at pH 8.0-9.3. The metal ions can be determined at a lo4 M level with relative standard deviations ranging between 4 and 8%. Within experimental uncertainties the rates of ligand substitution were found independent of the entering ligand, indicating that if the conditional stability constant of the metal ion with the entering ligand is greater than that with the SPADNS ligand, the reaction rate primarily depends on bond-rupture steps rather than bond-forming ones. Simultaneous determination of manganese and zinc (0.04-2.5 pg in 50 mL) is possible based on the ligand substitution reaction between 1-(2-thiazolylazo)-2-naphthol metal chelates and EDTA (161). By use of the average from the accumulation of 10 runs obtained by stopped-flow mixing and photometric monitoring (590 nm), manganese and zinc can be determined in tap water. The same approach can be used for the simultaneous determination of cadmium(I1) and manganese (162). The substitution reaction monitored in this case is between the metal chelates of 1-(2-pyridylaz0)-2-naphtholand EDTA. Monitoring at 554 nm permitted the determination of cadmium and manganese in mining waste and treated water, by linear extrapolation of a kinetic run. Another application of differential reaction rate methodology to the flow-injection analysis, by Jensen et al. (163), proposes the simultaneous determination of strontium and magnesium (or calcium) ions by use of cryptand (2.2.2) in substitution reactions. Determinations are performed in the M level. This paper describes the implementation of a manifold that makes use of only one detector in contrast with the dual detection setup proposed in earlier papers by the same group. Simultaneousdeterminationsof phosphate and silicate have been reported by a procedure based on the different rate of heteropoly blue formation with a mixture of molybdenum(V)-molybdenum(V1) in 0.28 M perchloric acid (164). The reaction was carried out at 80 “C and the absorbance at 840 nm was measured every 5 min to obtain absorbance vs. time curves. Heteropoly blue formation due to phosphate is remarkably faster than that due to silicate which starts affecting the rate 3 min after mixing. Because of this difference, phosphate could be determined from the absorbance obtained by extrapolating the slope of the plot of absorbance vs. time to 3 min. Increasing amounts of phosphate depress the rate of heteropoly blue formation from silicate and new calibration plots for this latter species had to be prepared after the amount of phosphate present in the sample was established. The procedure was applied to the simultaneousdetermination of silicate and phosphate (ppm level) in synthetic mixtures. Either the single point kinetic procedure of Lee and Kolthoff or the logarithmic extrapolation procedure can be used for the selective, simultaneous determination of carbenecillin and its pro-drugs carindacellin and carfecillin (165). The method is based on the different rates of degradation of carbenicillin and its carboxy ester derivatives in acidic solution ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
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(0.1 M formate buffer of pH 3.00). The course of the reaction is followed by spectrophotometric monitoring using an imidazole assay. The method of proportional equations has been used by Bundgaard (166)in a differential kinetic method for the simultaneous determination of ampicillin and its prodrugs pivampicillin and bacampicillin. The reaction with an imidazole reagent to produce UV-absorbing penicillenic acid derivatives is the basis for determination. Measurements were performed at 5 minutes (optimum time) and 30 min after reaction. Analyses of binary mixtures of ampicillin and pivampicillin or bacampicillin hydrochloride in the 10-200 Mg/mL range are reported with a maximum relative uncertainty of 5%. Individual as well as binary mixtures of low-molecular weight alcohols have been determined by exploiting differences in the rates of oxidation by silver(I1) perchlorate (167). The rate of oxidation was followed spectrophotometrically by monitoring the silver(I1) absorption at 470 nm. Individual alcohols (e.g., methanol, ethanol, 1-propanol,and 2-pentanol) were determined from the values of rate proportionality constants. Binary mixtures (e.g., methanol-ethanol, 1-butanol-2-methy1-2-propano1, and cyclohexanol-cyclopentanol) were analyzed by application of a first-order integration or a single-point method. The authors suggest their method as an alternative to gas chromatography which is a competitive technique for high-molecular weight alcohols but is not as successful with low-molecular weight ones. The oxidation of N,N-dimethyl-p-phenylenediamine proceeds in two steps: first to the semiquinone of a stable free radical and subsequently to a very unstable quinonediimine. Under appropriate conditions the quinonediimine reacts with aromatic amines and produces intensely colored indamine dyes. Using as oxidant a mixture of K,Fe(CN)6 and K2Cr20,, Tawa and Hirose (168) determined mixtures of aniline and its derivatives by a differential reaction rate method. Necessary rate proportionality constants for application of the differential rate method were determined by simplified complementary tristimulus colorimetry and from the slope of the straight-line portion of plots of concentration of semiquinone vs. time. Phenols and carbonyl compounds cause serious interference. Application of simplified tristimulus colorimetry has also been proposed by Tawa et al. (169)in a method aimed at determining uric acid in serum without the removal of proteins. The method requires calculation of the absorbance a t 290 nm as a function of time by simplified tristimuIus colorimetry and computation of initial and final absorbances based on the kinetics of first-order reactions and using multiple linear regression analysis. The chemical detector system uses the uricase catalysis of the conversion of uric acid to allantoin. A differential determination of acetylsalicylic acid and paracetamol in mixtures with caffeine has been proposed by Elsayed and Ogbonnia (170). The basic reaction is the bromination of the species under determination by a mixture of bromide-bromate in acidic medium. The time needed for bromination was measured from initiation of the reaction until the presence of excess bromine visually detected by its decolorization of methyl orange. This time is directly proportional to the concentrationof brominated species The method requires independent estimation of caffeine and paracetamol. The method of proportional equations has been applied by Bundgaard in the development of a kinetic method for the determination of mecillinam, (GR)-G-(hexahydro-lH-azepin1-y1)methyleneaminopenicillanicacid, in the presence of its hydrolysis and epimerization products ( 171). The same procedure permits the quantitative simultaneous determination of mixtures of the (6s)and (6R)-mecillinamepimers. Chemical basis for the method is the different rates of reactions with a glycine reagent (pH 9.9,35 "C) and the monitoring (spectrophotometric, 330 nm) of a 4-aminomethyleneimidazol-5(4H)-one derivative. Absorbance measurements were performed at two different times (13 and 30 rnin of reaction) to set the simultaneous equations. The (6s)-epimer reacts more slowly than the (6R)-epimer and the kinetic information suggests the presence of an intermediate in the course of imidazole formation. The applicability of the kinetic method to assess the stability of mecillinam was also studied and reported. 74R
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The method of proportional equations has been applied to the simultaneous determination of betamethasone and its 17-valerate ester (172). This is a selective determination based on the oxidation of the steroid 21-hydroxygroup with methanolic cupric acetate. The resulting aldehyde function is subsequently condensed with 3-methylbenzothiazol-2-one hydrazone in alkaline medium forming a highly absorbing azine (Arnm = 394 nm) which is monitored.
KINETIC METHODS USING UNCATALYZED REACTIONS Gaytan et al. (173)reported the determination of selenite M) by a kinetic, noncatalytic, turbidiions ((0.8-3.8) X metric, fixed-time procedure. The method is based on reduction to elemental selenium by ascorbic acid, the absorbance being measured at 460 nm within 2.5 min after initiation of the reaction. Copper(I1) ions have been determined (0.02 pg/mL) with about 3% error by monitoring the ligand exchange between its complex with 2- (2-thiazolylazo)-5-dimethylaminophenol, THAM, and EDTA added to initiate the reaction (174). Stopped-flow mixing and spectrophotometric monitoring at 563 nm were used. The difference between the initial absorbance and the absorbance after 0.03 s of reaction provided the basis for Cu(I1) determination. Using the indicator reaction of molybdate reduction by ascorbic acid, Rudenko and Adanenko (175) have determined germanate. The determination is based on monitoring the formation of molybdogermanate blue at 25 "C and between 2 and 7 min after mixing the reactants. Several species interfered Sn(IV),Hg(II), Hg(I), Cr(III), Sb(V), Pb(II), W(VI), V(V), P(V). Gallium(II1)has been determined in the 0-4 Mg/mL range on the basis of its substitution for copper(I1) in the Cu-EDTA complex (pH 2.3) (1 76). The reaction was followed by monitoring the absorbance at 240 nm. Plots of A. - A , vs. gallium concentration were used as calibration plots (Ao = initial absorbance,A , = absorbance at infinity). The procedure was applied to the determination of gallium in bauxite after separating it as its chloro complex by solvent extraction with methyl isobutyl ketone. A method has been proposed for the determination of the ions Ni(II), Co(II), Ce(III1,Mn(II), Cu(II), Cu(I), Cr(III), U02?, CrOd2-,and Fe(CN)@-, based on the quenching of the triplet state of eosin (177). A kinetic method for determining the iron-reducing activity released from tomato roots has been proposed by Kojima and Bates (178). The method is based on measuring the initial rates at which an aliquot of nutrient solution reduces the iron(II1) in the Fe(II1)-nitrilotriacetate complex. The reduced iron reacts with bathophenanthrolinesulfonate and the rate of formation of the complex is followed photometrically at 535 nm. The kinetic approach may offer advantages over the equilibrium approach since it is faster, uses less nutrient, and may distinguish between medium and high concentrations of reducing agent(s). A kinetic spectrofluorometric method for the determination of vanadium at the ppm level has been described by Grases et al. (179). Chemically, the method is based on the oxidation of 1-amino-4-hydroxyanthraquinone by V(V) producing a fluorescent product A,, = 480 nm, A,, = 575 nm. The initial-rate (method of tangents) measurement approach is recommended over fixed or variable-time procedures since it provides a broader dynamic range of applicability (100-530 ppm) and is more accurate. Only Ce(1V) exerts a serious interferent effect. The method was applied to the determination of nonvolatile vanadium in crude petroleums and found to compare well with atomic absorption and colorimetric procedures. A kinetic fluorometric determination of iron(II1) and thallium(II1) based on the oxidation (by the metal ions) of 1,4-diamino-2,3-dihydroanthraquinone has been reported by Salinas et al. (180). The method of tangents, initial rate measurements, and fixed- and variable-time procedures are compared and the initial rate method is recommended for the determination of 0.05-0.4 ppm of the metallic species. A kinetic determination of uric acid in whole (undeproteinized) serum based on the rate of reduction of iron(II1) ion in the presence of 2,4,6-tripyridyl-s-triazinehas been proposed by Tabacco et al. (181). The change in absorbance (593 nm) between 1 and 3 min after reaction is initiated at
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
25 O C is directly proportional to the urate concentration. The procedure is reported to be free from significant interference from L-ascorbic acid, gentisic acid, bilirubin, hemoglobin, glucose, creatinine, acetylsalicylic acid, reduced glutathione, L-cysteine, and salicylate. Corticosteroidspecies in pharmaceutical skin preparations (methasone, /3-methasone, valerate, triamcinolone acetonide, and fluocinolone acetondde) were determined by a reaction rate method based on a modification of the blue tetrazolium reaction (182). Reaction of the corticosteroid species with blue tetrazolium in an alcoholic solution of a strong base produces a red formazan which can be monitored at 525 nm. A fixed-time procedure (meamrement at 30-70 s after mixing) was utilized which in conjunction with an automated system previously described (183) yielded relative standard deviations of 0.3-2%. The approach correlates well with the equilibrium determination based on the same chemistries which is widely accepted but time-consuming. Acetaminophen in pharmaceutical tablets has been determined by a kinetic method (184). The method is based on the monobromination of acetaminophen. The time for bromination, visually indicated by the decoloration of methyl orange, is directly proportional to the acetaminophen content. A kinetic procedure has been developed for the determination of 21-dehydrocorticosteroid contaminants of the corresponding corticosteroid preparations (185). The method is based on the reaction with 3-methylbenzothiazol-2-one hydrazone hydrochloride and disodium ecetate, recording of the change in absorbance with time at the wavelength of maximum absorbance for the corresponding steroid, and extrapolation of the recorded curve to time zero. The zero intercept represents the absorbance due to 21-dehydrocorticosteroid present iin the steroid sample. The detection limit is reported as about 0.02 % of 21-dehydrocorticosteroid for a corticosteroid sample concentration of 10 mg/mL. A very detailed study of the oxidation of thiamine by Hg2+ in basic solutions has been published by Ryan and Ingle (186). The oxidation results in the formation of fluorescent thiochrome A,, = 365 nm, A,, = 444 nm. Monitoring the fluorescence of the thiocllrome provides a highly selective, fast, and convenient kinetic determination of thiamine. Measurement of the change iin fluorescence signal over 16 s (initial rate method) yielded linear calibration plots from a detection M. The method was applied limit of 2 x lo4 M up to 1 X to the determination of thiamine in vitamin-mineral preparations (commercial and synthetic preparations). Fifteen different phenol species in methanol and acetic acid solutions have been determined by a fixed-time procedure based on the measuremLent of the yellow products resulting from the oxidation of the phenols by metaperiodate (187). The reactions were run at 50 "C for time periods ranging from 15 min to 2 h, depending on the species under determination. Attempts to apply the approach for differential rate determination in mixtures were unsuccessful. A variety of physiologically important chemical species form unstable complexes in the presence of molybdenum(V1)and hydrogen peroxide in carbonate-bicarbonate buffer solutions of pH 8.2. The rate of decomposition followed photometrically for 5 min at 350 nm allows determining serotonin, 5hydroxyindoleaceticacid, L-dopa, methyldopa, and carbidopa in microgram quantities by the differential form of the method of tangents (188). Gallic acid and rutin can be similarly determined by monitoring the decompositionof their colored complexes at 482 nm (189). The concentration level of the enzymatically active form of pyridoxal 5'-phos hate can indicate vitamin BBgroup deficiency. Hassan a n f Rechnitz (190) have proposed a simple, selective, and reliable method for the determination of this phosphate by monitorinq the rate of ammonia liberation (with an ammonia gas potentiometric sensor) from excess L-tryptophan by action of tryptophanase apoenzyme as a function of pyridoxal 5'-phosphate coenzyme. The procedure requires less time and affords a better competitive limit of detection M level) than otheir proposed methods. No interference wa8 observed in the presence of 100-fold amounts of other members of the Be vitamin group and their phosphate derivatives. Salicylic acid and nitrophenol have been determined at the lom4M concentration level by a kinetic procedure based on their reaction with a mixture of HBr03-HBr (191). Excess
bromine, after total bromination of the determined species is effected, was detected by its bleaching action on methyl orange. The time taken for completion of the bromination reaction is directly proportional to the salicylic acid and nitrophenol concentration in the reacting mixture. Maximum errors are reported as 7.3 and 3.8% for salicylic acid and the nitrophenol, respectively. The authors suggest the use of this method for the estimation of phenolic wastes from dyestuffs and other industrial wastes. A determination of kojic acid (a chemical species of increasing interest in the food industry) using stopped-flow mixing and absorptiometric monitoring at 525 nm has been proposed by Obata et al. (192). The colored, oxidized form of 2,6-dichlorophenolindophenolchanges to its colorless reduced form upon reaction with kojic acid. The observed second-order rate constant for the reaction is proportional to kojic acid concentration in the range 1-10 mmol/mL and serves as the basis for determination. The method was applied to the determination of kojic acid in fermentation media. The normal buret procedure for total protein determinations (equilibrium-based) requires 15-30 min for full color development. Law and Crouch (193) reported a two-point, fixed-timerate method, based on stopped-flowmixing, which allows determinations in 10 s with good precision and reproducibility (% RSD: 0.1 to 1.4). Determinations could be made in as short a time as 0.2 s by sacrificing some precision and reproducibility.
KINETICS I N SOLVENT EXTRACTION AND OTHER SEPARATION APPROACHES. Interest on different kinetic aspects of solvent extraction has shown such a marked increase that the authors of this review consider it appropriate to group and itemize these contributions under a separate heading. Continuing their studies on the kinetics and mechanisms of solvent extraction systems, Freiser et al. (194) report on a detailed study of the extraction of Cu(I1) with 2-hydroxy5-nonylbenzophenoneoxime (LIX65N), a reagent of the LIX family of reagents used in the hydrometallurgical extraction of copper. Their studies are directed toward answering important fundamental questions such as the change in reaction mechanism that may take place when the distribution constant of the extracting ligand increases. They postulate a mechanism which explains the experimentallyobserved dependence on metal ion, ligand, and pH and in which the rate-determining step is the addition of a second molecule of neutral ligand to the already formed 1:l complex. The extraction of copper by LIX65N has been observed to follow a second-order dependence in ligand contrary to the first-order ligand dependence observed in other systems. In preliminary work, the authors have observed a catalytic action of LIX63 (5,8diethyl-7-hydroxydodecan-6-one oxime) on the extraction of copper by LIX65N. In this work the authors used a highspeed stirring system described previously (195). By use of the same extraction system, Ohashi and Freiser (196) examined the kinetics of extraction of nickel(I1) complexed with a series of diarylthiocarbazones into chloroform. The temperature dependence of the rates of extraction were used to calculate the second-order rate constants for 1:l complex formation. The kinetics and the mechanism of complex formation were analyzed from the extraction data. Examination of activation parameters led the authors to conclude that the entropy of activation is the dominant factor in the kinetics of complex formations. The comparative effect of some Lewis acids such as strongly complexing metal ions on the kinetics of nickel dithizonate back-extraction has also been considered (197). Analytical implications for improved separation procedures are discussed. A common problem in the solvent extraction of cobalt chelates is the irreversible oxidation (by atmospheric oxygen) of Co(I1) to Co(II1). An investigation of the kinetics of cobalt oxidation during its solvent extraction with 8-hydroxyquinoline and Kelex 100 has been reported by Guesnet et al. (198). The paper presents data for the extraction from borate (pH 8.15) and acetate (pH 4.65) solutions as well as electrochemical observations of the Co(I1) oxidation. The oxidation rate during the solvent extraction decreases upon addition of acidic or donor compounds to the organic phase. The kinetics of extraction of metals by chelating agents and organic solvents has also received attention from Nakashio ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
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and co-workers. Their studies are directed to the engineer but their conclusions are of interest to the analytical chemist. Their prolific contributions to the subject cover the extraction kinetics of copper with benzoylacetone during drop formation (199), during drop rise (2001, in a stirred transfer cell (201), and with liquid surfactant membranes containing benzoylacetone as a mobile carrier (202-204). The same research group has reported on the effects of interfacial rate process for the extraction bf hydrochloric acid by long-chain alkylamines (205). During the study of the kinetics of extraction of Be(II), Ga(III), Fe(III),and Cr(II1) from aqueous perchlorate solutions with P-diketones, Sekine (206)observed that when the organic solvent is of polar nature, the rate of extraction is dependent on the perchlorate concentration in the aqueous phase. The dependence of the rate of metal chelate extraction on perchlorate is explained on the basis that: (1) the metal ions are first extracted as ion-pairs (with C104-) and (2) the uncharged metal chelate complex is subsequently formed in the organic phase. The partition of acetylacetone between CCl., and aqueous solutions as well as the tautomerization of this widely used reagent have been found to be catalyzed by metal ions. The rate-determining step is the enolization in the aqueous phase (207). This is not surprising since enolization has long been known to be acid catalyzed. Elias et al. disclosed a correlation between the polarity of binary solvent mixtures and some kinetic solvent effects (208). The correlation involves transition energies, ET (kcal/mol), and the natural logarithm of the rate constant for solventinduced ligand substitution. Values of ET were taken from a compilation of Reichardt (209). Their observations, of interest in kinetic methods based on ligand exchange reactions, point to hydrogen bond formation as facilitating the formation of the transition state. Byron et al. have a pair of papers of some general as well as of analytical interest (210,211). Their first report demonstrates that partition coefficients may be calculated from data resulting from the partitioning kinetics in a simple transfer cell when the chemical species of interest undergoes decomposition in the aqueous phase. They used as model system the 2-cyclohepten-1-one partitioning between liquid petrolatum and water or aqueous hydrochloric acid. The approach requires certain limiting conditions: (1) the solute should remain neutral or have a constant ionic form in the pH range under study, (2) the value for the partition constant (coefficient) must be sufficiently small to allow accurate estimation of the rate constants for the partition processes, (3) the solute must not undergo self-association, and (4)the rate constants for the decompositionprocess in the aqueous phase (forward and reverse) must be in the same order of magnitude as the rate constants for the partitioning processes, to prevent domination by either extraction or degradation. The method provides partition constants for some cases in which equilibrium experiments are not possible. The second paper applies the approach to one of those cases: the simultaneous partitioning and hydrolysis of amoxicillin and ampicillin with 2-methyl-1-propanol-aqueous hydrochloric acid as solvent pair. The increased interest on kinetic aspects of solvent extraction is in part the result of more emphasis on solvent extraction in hydrometallurgical processes, particularly toward the development of specific extractants. Since the chemistry involved parallels the chemistry familiar in metal ion separations in analytical chemistry, many of such observations recorded have analytical relevance. Typical examples are the studies of Fleming and Nicol on the kinetics of the extraction of copper and iron with 8-hydroxyquinoline initiated as a prelude to a study of the same extraction by the commercial extractant known as Kelex 100 (Ashland Chemical Co.). Their work has been prompted by the observation that Kelex 100 selectivity toward copper over iron(II1) is kinetic in nature (212) and 8-hydroxyquinolineis a parent compound of Kelex 100. In their first paper (213)they describe the aqueous-phase chemistry species involved of iron(II1) and 8-hydroxyquinoline, and the second paper in the series (214) discusses the extraction of iron(II1) with 8-hydroxyquinoline from aqueous solutions into toluene. The experimental work is based on the use of a stirred cell providing constant interfacial area. 76R
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Solvent extraction is not the only separation process for which kinetic studies have received attention. Kinetic data concerning binding and elution of zinc ion on ion exchange membranes have been studied by Matei et al. (215). By use of a diffusion theory for the liquid film formed at the solution-exchange membrane interface, the equations for degree of binding vs. time and degree of elution vs. time functions were deduced. The anion-exchange kinetics of uranium in sulfate solutions has been studied by Streat and Take1 (216). The sorption of to 4 X M) from acidic sulfate solutions uranium (1 X on strong-base anion-exchangeresin was found to be particle diffusion controlled. The kinetics of ion exchange of cobalt(I1) and nickel(I1) on a resin loaded with 5-sulfo-%hydroxyquinoline has been studied by Akaiwa et al. (217). The chelate-forming reaction in the resin matrix is considered to be rate determining. The separation of plutonium in microgram quantities with high degree of purity, from mixtures with uranium and the lighter transplutonium elements (e.g., Am and Cm) based on different rates for the dissociation of their complexes with cyclohexane 172-diaminetetraaceticacid, DCTA, has been reported (218). Paper electrophoresis has been proposed for the separation. The method was applied to the separation of plutonium from solutions of spent nuclear reactor fuels. The rate of dissociation of the rare-earth complexes with DCTA has also provided the basis for a kinetic method for the determination of the specific activity of radioactive preparations (219). The rate of extraction of gallium with “Kelex 100” is considerably increased if a high-molecular-weightcarboxylate and an alcohol are present in the system (220). A mechanism for extraction is proposed which takes into considerationthat the organic phase is a microemulsion.
MISCELLANEOUS KINETIC ASPECTS OF ANALYTICAL INTEREST In 1978, Mottola and Hanna (221) proposed a series reaction mechanism to describe transient signals in unsegmented-flow systems. A similar treatment has been applied by van Den Winkel and Mertens in connection with an automatic kinetic determination of iron(II1) and ascorbic acid (222). Their proposed method is based on the formation of a short-lived complex involving iron(III), acetate (present in the buffer medium), and ascorbate, which absorbs radiant energy of 550 nm. Since series reactions and processes play a relevant role in several aspects of analytical interest (3,171),the work of Lachmann et al. (223)is of interest. They presented a detailed discussion of the “formal integration” method for treating consecutivereactions with two linearly independent reaction steps (the first step being first order). The “formal integration” method requires absorbance-time measurements at two or more different wavelengths at which all components of the reaction system may absorb except the excess component. Molar absorptivities, however, do not need to be known. The method was applied to the reaction of pyridoxal with histidine in excess. Application of the common features of flow injection sample processing systems to an all-gas (sample-carrier)situation with the novelty of detection at a gas-solid interface has been demonstrated by Ramasamy et al. (224). Their paper dsmonstrated the determination of chlorine and bromine in air samples. The method is based on the following chemistry: Br,(g)
f
2NF(s)
2Br:NF(s) + As(II1)
-
-
2Br:NF(s)
(1)
2Br-(s) + As(V) + 2NF(s) (2)
in which NF stands for a-naphthoflavone (2-phenyl-4Hnaphthol[ 1,2-b]pyran-4-one). Equation 1 represents a gassolid reaction and eq 2 a subsequent solid-solid reaction. The overall chemistry, from a mechanistic viewpoint, can be envisioned as a series reaction process with Br:NF(s), the redbrown brominated naphthoflavone species, as the monitored species responsible for the transient signal whose height is directly proportional to the amount of bromine gas present in the injected sample. The series reaction process model described by Mottola and Hanna (221) fits t is case and the relative values for the rate proportionality constants of re-
h
KINETIC ASPECTS
"r
actions 1and 2 can be u. ide method development and optimization. For chlorine (IPd etermination, the reaction illustrated below precedes reactions 1 and 2: C12(g)+ 2Blr-(s)
-
Br2(g)
+ 2Cl-(s)
(3)
The reactions illustrated by eq 1and 3 are very fast relative to the reaction of eq 2. The latter, however, is fast enough to allow a rather large number of determinations per hour (as many as 100-120). Determination of traces of silicon by furnace atomic absorption spectrometry has poor sensitivity which is considered to result from the formation of silicon carbide. Muller-Vogt and Wend1 (225) have carried out kinetic experiments in both untreated and niobium-coated graphite tubes. These experiments (using scanning electron microanalysisof the tube surface) provide evidence for reduction of Na2SiOBduring the ashing cycle. This reaction with graphite was observed to begin at about 1200 "C: in uncoated and at 1000 "C in Nbcoated tubes. Neither carbide formation nor losses of Si were detected in either tube up to temperatures of 1650 "C. The rate of reduction increases and the activation energy decreases in Nb-coated tubes. To explain this it is suggested that niobium carbide, existing in large excess over Na2Si03,takes part in the reduction process, for example, by incorporation of oxygen into the Nb carbide lattice. Values for the rate constants for the rate of reduction and for the rate of S i c formation are presented, the latter occurring at temperatures higher than about 1700 "C. Boss and Hieftje fornnulated a mathematical model for the vertical acceleration of aerosol droplets in a flame or plasma (226). This model haia how been improved by Russo and Hiefte (227) by incorporation of a radially changing flame rise velocity. The improvement provides calculated velocities that show excellent correlation with experimentally observed velocities of an aerosol droplet in an air-acetylene flame. The rapid acceleration of the aerosol droplet toward the moving gas velocity has fundamental as well as practical implications in flame spectrometry. The application of fluorescence lifetimes (both measured and natural) to the qualitative identification of some drug series has been proposed (228). Control of environmental effects permits distinguishing structurally similar members of each drug group (namely, antidepressant drugs such as amitriptyline and proitriptyline and some benzimidazoles, barbiturates, and atabrines) through temporal resolution. Fluorescent lifetimes for amitriptyline and protriptyline, for example, are very similar in solvents such as 0.1 M HC1, water, acetonitrile, methanol, ethyl acetate, and chloroform; but in 1% NaOH/methanol they differ sufficiently for identification purposes. Tawa and Hirose have extended their studies on the application of a simplified complementary tristimulus colorimetry to kinetic situations with the determination of the rate constants of consecutive irreversible firstiorder reactions (229). As model systems they utilized the xanthine oxidase catalyzed oxidation of hypoxanthine to uric acid with xanthine as the unstable intermediate. The procedure is simple and allows direct analysis of kinetic data and estimation of the rate constants without restrictions on the initial concentrations of reactants. It requires, however, a knowledge of the concentrations of reactant, intermediate, and product for the graphical application of the procedure. A new nonchromatographic method of partial optical resolution predominantly (although not exclusively) based on kinetic resolution usinig an insoluble polymer-supported resolving agent has been proposed by Coisne and Pecher (230). An interesting property of this approach is that it can be used on a preparative scale. The described procedure provides an indirect way to obtain optically pure mandelic acid. The authors indicate that they are looking further into the principle of partial and stereoselective fixation of racemic compounds on chiral insoluble copolymers. Silylation is one of the most useful reactions in the contemporary practice of analytical chemistry. Benko and Mann (231) have recently reported on a comparative study of different silylating agents. A noticeable catalytic effect of methoxyamine was observed with all silylating agents studied. A 100% yield was observed using N-trimethylsilylimidazole in combination with the catalyst in an "on-column'' reaction.
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ANALYTICAL CHEMISTRY
The use of phase transfer catalysis as a route for synthetic reactions involving ionic species in aqueous phase and organic species in nonaqueous media is well documented in the field of organic chemistry. Sasson et al. (232) have described the application of phase transfer catalysis as a separation technique. They report the separation of carboxylate ions in aqueous solution by their selective esterification of butyl bromide in a two-phase system containing quaternary ammonium salts. This proved better in homogeneous reaction with dimethylformamide as solvent. Reactions in homogeneous systems are kinetically controlled and selectivity depends only on the nucleophilic character of the carboxylate anion(s). Selectivity in heterogeneous systems is controlled by the value of the extraction constant of the ion pairs involved. Campbell and Chang have reported on the use of catalysts for the determination of oxygen in organic compounds (233). Some metal-organic complexes (e.g., complexes such as Ni(11)-dimethylglyoxime, Ni(I1)-hydroxyquinoline, chloro(pyridine)Co(II)-8-hydroxyquinoline, and Fe(III)-8hydroxyquinoline) have been found to be very effective catalysts. Elemental sulfur in microgram amounts has been determined in metal samples (selenium, tellurium, arsenic, antimony), in vulcanized rubber (free sulfur), and in some metallic sulfides by a procedure based on sulfur induction of the reaction between sodium azide and iodine (234). Measurement of reacted iodine was performed when for all practical purposes deactivation of the inducer had occurred. Estimation of sulfur was based on the "induction factor" defined as the ratio of gram-atoms of reacted I2 (or N2 formed) to gram-atoms of sulfur empirically determined with standards. No reaction occurs between perbromate ions and lactic acid at a pH close to 5. The presence of iron(II), however, causes the oxidation of lactic acid via an induced chain reaction with the probable formation of an iron(1V) active intermediate, F e 0 2 t (235). Potentiometric monitoring, with a perbromate selective electrode, permits determining lactic acid in the 2 X lo4 to 1.5 X M range with relative errors and standard deviatigns of about 1-2%. The determination procedure requires measuring the potential of the system after about 1 / 2 h of reaction at 25 "C subtraction of the blank potential to obtain a E which, together with the slope of a standard curve of potential vs. log [BrO,-], permits a numerical calculation of the weight of lactic acid in the sample. A detailed kinetic study of the bromate oxidation of methyl orange has been priesented by Hasty et al. (236). At low bromide concentrations, the direct reaction between methyl orange and bromate slowly produces bromide, which is oxidized to bromine. This bromine causes the decolorization (oxidation) of met 1 orange. Such a behavior explains both the induction perio and autocatalysis observed in the system. Bromide, however, also retards the direct reaction between methyl orange and bromate, and this effect has been used for the determination of bromide to lo4 M) in an air-segmented continuous-flowanalysis system. The determination is based on the measurement of the extent of decolorization by a fiied-time approach under given experimental conditions affording 60-70% decolorization (236). Periodate oxidations are of interest in organic analysis and periodate is the oxidant in some indicator reactions used in catalytic determinations. Marques and Hasty have studied the kinetics of the periodate-iodide reaction by following the consumption of iodide with an iodide ion selective electrode (237). No rate dependence on hydrogen ion concentrationwas observed, contrary to earlier reports (238). The oxidation of ascorbic acid by dissolving oxygen in aqueous solutions is very sensitive to the presence of microamounts of heavy metal ions acting as catalysts. The system has been occasionally proposed as an indicator reaction for catalytic determinations, e.g., of copper ions. A detailed discussion and study (kinetic and mechanistic) of the Cu2+-ascorbic acid-02 system has been published by Shtamm et al. (239). The conditions leading to the formation of a-and P-molybdosilicic acid are relevant to silicate analysis that relies on this formation. Truesdall et al. (240) report studies aimed at finding the relationship between the rate of formation of the molybdosilicic acids and the concentrations of molybdate, hydrogen ions, and silicate. The effects of changes in mol-
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ybdate species upon the kinetics are discussed in detail. Basic triphenylmethane dyes such as Malachite Green, MG, or Crystal Violet, CV, have been used extensively in the development of catalytic determinations and as components of the indicator reaction. The chemical behavior of these species is also of interest for other analytical reasons since they have found application as redox indicators and as effective ionexchange sites in membrane type ion-selective electrodes. Kalinowski et al. (241) report on the kinetics of color fading for MG and CV in a series of aprotic solvents. Their spectrophotometric studies show that Beer’s law is not obeyed in solvents with dielectric constants below 10 (e.g., tetrahydrofuran, tributyl phosphate, dioxane, and l,&diethoxyethane) while in solvents with higher dielectric constants, Beer’s law is obeyed. Gutmann’s donor numbers (242),a measure of the nucleophilic properties of the solvents, correlate well with rate constamts for MG and CV fading. Their studies also lead to the following: (1) the stability of triphenylmethane dyes in solution is reduced by an increase in the Lewis basicity of the solvent, (2) a solvent molecule reacts principally with the central carbon atom of MG or CV, and (3) the MG cation is more sensitive to reaction with solvents than the CV cation because MG is a stronger Lewis acid. Gedeonov et al. (243)have discussed how to select o timal conditions for substoichiometric complex formation y use of a kinetic approach. From experimental kinetic data for the formation of complexes between cyclohexane 1,2-diaminetetraacetic acid and ethylenediamine-N,N,N’,N’-tetraacetic acid, EDTA, with europium and thallium, they calculated the time required for quantitative bonding of substoichiometric amounts of the ligand with different concentrations of the rare-earth ions. As an extension to their studies on catalysts for acylations by acetic anhydride, Connors et al. (244) studied the effect of substituents and solvents on the kinetics of the N-alkylimidazole-catalyzed reaction of this anhydride with isopropyl alcohol. Analytical acylations with acetic anhydride are used as standard procedures for the determination of amino and hydroxy groups. Of particular interest to analytical and solution chemists is the study on the kinetics and mechanism of trace metal chelation in seawater by Raspor et al. (245). The kinetics of reaction of Pb(II), Zn(II), and Cd(1I) with EDTA was followed M for these reactants at trace levels in the range lo-’ to in seawater and model solutions of its major salinity components. Monitoring involved differential pulse stripping voltammetry. The report highlights the pronounced specific influences of Ca(I1) on the kinetics when present in substantial excess. An extension to the theory of square-wave voltammetry including electrochemical systems complicated by electrode kinetics is presented by O’Dea, Osteryoung, and Osteryoung (246). The paper provides detailed discussion for first-order homogeneous reactions and centers on the following cases of analytical interest: (1)slow electron transfer, (2) preceding chemical reaction, (3) following chemical reaction, and (4) catalytic chemical reaction. Of analytical interest is a report by Arakawa et al. (247) on the catalytic property of rare earth metals-copper mixed oxides and their application for the sensing of gaseous species. Stopped-flow mixing is being reinvestigated to assess its potential as an approach for dedicated chemical analysis (248, 249) which could broaden its almost exclusive use as an instrumental approach for kinetic study of very fast reactions. The precision of reagent and sample measurement, efficiency of mixing, and the availability of faster reactions for method development are cited as advantages over other repetitive analytical approaches such as air-segmented continuous-flow analysis or unsegmented continuous-flow analysis. Holler, Enke, and Crouch (250) report the results of a critical study of temperature effects in three different stopped-flow mixing systems. The three mixing modules used were: a Durrum Model 110 stopped-flow spectrophotometer, a GCA McPherson Model EU 730-11 stopped-flow module, and a mixing module developed in the laboratory of the authors. The Durrum mixing unit exhibited the most stable temperature behavior in the 20-30 OC range. The EU 730-11 Model showed rather wide variations in temperature but separate thermostating of the drive syringes and modification of the push cycle resulted in substantial improvements. The cus-
%
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tom-made module exhibited large temperature variations from ambient ones. The potential of jets for fast mixing of liquid reactants has been recently explored by Davidovits and Chao (251). They describe a new technique for rapid kinetic studies in which jets produce mixing of two reactants in the time-frame of 5 FS.
The reaction between iron(II1) and thiocyanate has seen several analytical applications through the years and is frequently used as a test chemical system for evaluation of fast-reaction kinetic studies. Trimm et al. (252) have combined a stopped-flowtemperature-jump system and reevaluated the kinetics and equilibrium parameters for the reaction. The advantage of their approach is that the kinetics are determined by two different methods and the thermodynamic parameters by four different methods, thus providing a good check for mechanistic evaluations. So called “stat” methods are kinetically based in that they center on the measurement of the rate of delivery of a reactant (or sample) which is necessary to restore a certain parameter of a system to a preset value. Their consideration as kinetic procedures is independent of the chemistry involved in the respective methods since the dynamic component that qualifies them is a “rate” of signal restoration. A recent contribution by Weisz and Fritz (95) deserves special mention since it represents the implementation of the “stat” principle to continuous monitoring in an unsegmented-flow system. They describe a continuous-flow absorptiostat (absorbance monitoring) and a continuous-flow conductostat (conductance monitoring). Weisz and Fritz illustrate the application of their continuous-flow-“stat” system with the determination of copper levels (1 to about 10 pg/mL) based on its catalytic action on the Fe(III)-S202- indicator reaction. Photometric monitoring was based on the Fe(II1)-SCN- complexation. The rate of introductionof the copper-containingsample (by means of a regulated peristaltic pump) served as the basis for determination of the catalyst. The same reaction system was used to monitor iron(II1);in this case the rate of introduction of the iron(II1)-containing sample served as the basis for determination. Iron(II1) and iodide were also determined by monitoring the rate of sample introduction into a continuous-flow system (with all other reagents pumped at constant rate) containing SCN- (for iron(II1)) and cerium(1V) in 0.005 M H2S04(for iodide). Conductivity monitoring was illustrated with the titration of HC1 (in the narrow concentration range to 2.5 X M) with sodium hydroxide. The of 1 X pumping rate of sodium hydroxide solution was the parameter monitored in this case. The authors point out that by altering experimental conditions (e.g., pumping rate, concentrations, pH, etc.) the range of the determinations can be extended. This conceptualcombination of unsegmented-continuous-flow systems and “stat” technology should be of interest to those involved in process analyzed sampling systems (253). Krejci et al. (254) have described a procedure for the measurement of the exclusion volumes of packed chromatographic columns in liquid chromatography based on the detection of a suitable electrokinetic signal. The question of whether one can measure rate constants using chromatographic methods has been analyzed by Weiss (255). Using a theoretical model in which there can be a number of different binding sites, he concludes that “it would be unwise to use affinity chromatography to measure rate constants as suggested by Denizot an Delaage” (256). Grob and Leasure (257)presented a technique for the study of the kinetics of vapor-phase catalyzed reactions occurring in a gas chromatographic column. Their approach consisted in the use of a single GC column filled with an analytical packing and a small amount of catalyst in the injection port section of the column. Generally only first-order or pseudofirst-order reactions are amenable to quantitative study by this approach. The kinetics and activation parameters of ternary complex formation of N,N-disubstituted dithiocarbamate chelates of nickel(I1) and copper(I1)have been studied by high-performance liquid chromatography (258). The approach permits one to examine the kinetics of systems reaching equilibrium fast.
KINETIC ASPECTS OF ANALYTICAL CHEMISTRY
Glick et al. (259) warn of interference by ketones in the kinetic-spectrophotometric determination of creatinine by Jaffe-type methods. They point out that the assertion that “accurate determination of creatinine in the presence of acetoacetate should be feasible with any kinetic measurement” (260) is a dangerous generalization since not all kinetic creatinine methods are free of interference by acetoacetate. Adoption of a kinetic approach over an “end-point”approach, however, is recognized as improving the selectivity of the creatinine determination by Jaffe-ty e methods (261). Of interest to those using such a metho is the observation by Burnett et al. (262) that after about a week of storage, the phosphate-borate-sodium dodecyl sulfate buffer develops a flocculent precipitate which creates problems. The precipitate seems to be a compound of boron and oxygen, but is neither borate nor boric acid. The authors suggest the use of diethylamine to prevent the precipitate formation. The kinetic Jaffe method for creatinine received attention also from Bergman and Ohman (263). Usin isotope dilution-mass spectrometry as a reference methocf they found that sometimes the kinetic method gives high results. High results were most frequently obseirved in sera collected from kidneytransplant patients. Sudden false increases were also observed in samples from other patient populations but in a lesser frequency. Although no explanation is offered for it, the authors have observed that the use of detergent in the reaction mixture totally eliminatesthe interferent effect leading to high results. Two detergents have been evaluated: Triton X-100 and sodium dodecyl sulfate, SDS. The use of SDS permits higher concentrations of picrate in the reaction mixture (necessary to avoid interference from bilirubin) without precipitating protein. A main problem in the determination of reaction rate constants from microcalorimetry is correcting the output signal for the large thermal iinertia usually present. Sargent and Moeschler (264) reported a simple raphical method for extracting rate information (first-or er or pseudo-first-order reaction) using the LKB 10700 batch microcalorimeter. The minimum reaction timle constant that can be resolved is reported as about 1 s. Kelter and Carr’s discussion on the limitations of some statistical evaluations of kinetic data (265) has been examined by Ellerton and Mayne (266) who recommended that a test of hypothesis on the proportion of positive residuals be conducted before erforming the run test (normal variate test in Kelter and 8arr’s paper). Brady and Carr (2?67), in a theoretical study of the steady-state response of potentiometric enzyme probes, indicate that the linear response can be extended beyond the substrate concentration corresponding to the MichaelisMenten constant when the enzyme bonded to the membrane surrounding the sensor is sufficiently active. Their study was based on a model superposing Fick’s second law of diffusion and a chemical reaction following Michaelis-Menten kinetics. Meites’ research group at Clarkson College of Technology continues the application of constant-rate titrations in studies of chemical kinetics. As part of these efforts Voll and Meites have illustrated the constant-rate thermometric titration of an aqueous solution of ethyl glycolate with sodium hydroxide (268). Their data and application of nonlinear regression techniques allowed them to find both the percentage of free glycolic acid present as an impurity and the rate constant for the reaction of ethyl glycolate with hydroxide ion. A variable-timekinetic model has been used by Pardue and Fields to describe unsegmented-continuous-flow systems (flow injection analysis) with large dis ersion (use of mixing, gradient chambers in the manifoldr (269). A detailed mathematical treatment is presented in a subsequent paper (270). The equations, developed in elegant manner, project the variable-timemodel as 13uperiorto so-called “titration”models previously proposed for the same description. Bause and Patterson (271) have reported that halide ions enhance the Cr(II1) catalyzed oxidation of luminol by hydrogen peroxide. The resulting increases in the chemiluminescence intensity yields a significantly increased sensitive for trace Cr(II1) determination. Transport phenomena in flow injection analysis without chemical reaction has been studied by Reijn et al. (272). In has long been recognized that finite rates of the chemical reaction kinetics could be a significant factor in the practical dispersion, D, in flow injection analysis.
d”
d
Painton and Mattola (273) were the first to experimentally demonstrate the quantitative effects of slow reaction rates on D values and, hence, peak profiles. They also compare the effects of finite chemical rates using both straight and coil reactor tubes.
INSTRUMENTATION AND COMPUTERS In the past 2 years, there has been a significant number of papers dealing with instrumentation, automation, data handling, detectors, etc. which have been applied to kinetic measurement and/or analysis or which could be potentially applied to kinetics. As clinical analyzers extensive employ rate based reactions, two extensive, comprehensive and critical papers on “Instrument Justification, Selection and Technical Evaluation in Clinical Chemistry” by Gudzinowicz et al. (274) and Drescoll et al. (275) are of considerable interest. Part I (274) deals with “Instrument Justification and Selection” and Part I1 (275) is concerned with the “Technical Aspects of Instrument Evaluation”. There have been some novel detector systems devised for flowing systems that are applied or are applicable to kinetic measurements. Alexander and Seegopaul(276) have devised a rapid flow continuous analysis system using ion-selective electrode detection. They employed consecutive manual aspiration of sample and wash solutions into a flow manifold with air segmentationto reduce carry-over. They suggest that the concept of combining rapid flow with air segmentation gives an improved method for achieving rapid sampling with electrode detection. Dovichi and Hanis (277) have studied the experimental parameters affecting the performance of thermal lens calorimetry as a detector system in flowing systems. Fraticelli and Meyerhoff (278) have devised a novel tubular flow through membrane electrode detector for ammonium ions formed in a recipient internal buffer stream as the sample containing ammonia passes through a gas dialysis chamber. They have automated this system. The use of selective permeable thin membranes as a method to introduce the species of interest into the source of a mass spectrometer has been used in the studies of reaction kinetic by Colvo et al. (279). The study of the rates of surface controlled reactions is becoming increasingly important from both a practical and fundamental point of view. Kamin and Wilson (290) have devised a rotating ring-disk electrode method which has considerable potential for the study of surface catalysis as mass transport to the catalytic surface is well defined. The technique permits the in situ determination of kinetic parameters and characterization of the catalytic surface. Rapid scanning spectrophotometry is an important detection system for fast reaction studies. Retzlaff (281) has reported a computer program for the quantitative comparison of two absorbance spectra using the weighted least-squares method with a two-parameter model. Precision is improved and errors due to flicker and stray light are reject. Example spectra produced by a spectrophotometer using an optoelectronic imaging device are used to demonstrate the method. In the area of research in stopped-flowinstrumentation for the study of the rates of fast reactions, Stieg and Nieman (282) have reported a microcomputer-automated on-line reagent dilution system which significantly reduces the time requirements for reagent preparation and, hence, analyses time. Bonnell and Defreese (283) have automated a commercial stopped-flow spectrometer using two microcomputers. This parallel processing system is a simple and inexpensive method for overcoming the limitation associated with the low speed of microcomputers for measurement applications which also involve extensive computation such as reaction rate methods. Stewart (284) has examined the effects of refractive index gradients on the optical response of stopped-flow and temperature-jump apparatus. Also in the area of fast reaction rate instrumentation, Owens and Margerum (285) have published a comprehensivereview of pulsed-flow spectroscopy and covered the applicability and limitations in comparison with the other flow techniques. Owens et al. (286) have reported the design and construction of a pulsed-flow instrument. There have been several computerized (principally microcomputer) spectroscopic instruments reported that could be very useful in kinetic method development and instrumenANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982
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tation. Kaye et al. (287) have published a detailed description and the response of a microcomputer-controlled UV-Vis spectrophotometer. Nichols et al. (289) have reported a microcomputer interfaced spectrophotometer designed specifically for kinetic studies. Bush et al. (289) have designed a computer-controlled vacuum UV circular dichroism spectrometer with fourier digital data smoothing, Marino and Ingle (290) have devised a microcomputer automated and controlled intensified diode array system capable of acquiring a chemiluminescence spectrum from 200 to 840 nm in as little as 4 ms under direct memory control. Stieg and Nieman (291) have also developed a microcomputer controlled chemiluminescence research instrument which performs simple optimization of reagents and single-reagent continuous variations. The modernization by computer control of a commercial Gilford spectrophotometer has been published by Durham and Geren (292), and Efstathiou et al. designed a digital “Sample-Wash Timer” for the commercial Technion Sampler I1 (293). Automation of atomic spectroscopy techniques may allow these methods to be used as kinetic measurement instruments. Vigan and Wood (294) have constructed an automated system by interfacing an automatic sampler, a multichannelperistaltic pump and adjustable heating bath, and a gas-liquid separator to an atomic absorption spectrophotometer for the determination of ammonia. Jacinth0 et al. (295) have combined a flow injection system with an inductively coupled argon plasma atomic emission spectrometer. Fukamachi and Ishibashi (296) have developed a flow injection atomic absorption spectrometer using organic solvents. Also a computer-controlled inductively couple plasma double monochromator spectrometer has been reported by McCarthy et al. (299). Na and Niemczyk (298) have reported the design and construction of an inexpensive transient recorder used to obtain data from a stopped-flow spectrometer and Kat0 and Sugitani (299) have devised a microcomputer-assisted apparatus for the measurement of luminescence decay curves. An excellent discussion of the basics of computer experimentation techniques for the study of complex chemical systems can be found in the article by Frazer (300) and a specific application of graphical techniques for kinetic data analyses is given by Frazer and Brand (301). A microprocessor-based computer system for data acquisition and manipulations specifically designed for rapid kinetic studies has been reported by Holtzman (302),and Fasano and Nogar (303) have designed a useful multipurpose computer based data acquisition system. A method applicable to microcomputers for calculating the rate constants of competing first- and second-order reactions from data recorded at constant time intervals has been given by Bertrand et al. (304). A discussion of preliminary statistical analysis of kinetic data has been given by Ellerton and Mayne (305). Corrections (306) for equations in a paper on a microcomputer compatable method of resolving rate constants in mixed first- and second-orderkinetic rate laws (307) have been tested. Maeder and Gampp (308) have developed a method for spectrophotometric data reduction by eigenlvector analysis for equilibrium and kinetic studies. Electrochemicaltechniques employing net faradaic current with “tubular”electrodes have also been evaluated as detectors for flow systems. Mogehl and Johnson (309) have examined the amperometric response of tubular electrodes applied to flow injection determinations, and Baltensperger and Eggli (310) have reported the characteristics of an amperometric flow through detector with a renewable stationary mercury electrode. Chesney et al. (311)have evaluated Kel-F-graphite electrodes as detectors for continuousflow systems. Rubinson et al. (312) have developed a microliter volume thin-layer electrochemical detector for flow systems which uses a porous membrane separator.
ACKNOWLEDGMENT Research support (to H.A.M.) by the National Science Foundation, of which this review is a byproduct, is acknowledged here. The authors wish to dedicate this review to the memory of Professor Charles N. Reilley who was one of the innovators in the theory and application of kinetics to chemical analysis. His contributions to science and education are monumental, 80R
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Electron Micxoscopy John M. Cowley Department of Physics, Arizona State University, Tempe, Arizona 85287
In the 4 years since the previous review of the nonbiological aspects of electron microscopy, there have been important consolidations of the technical resources. Techniques previously mentioned as thle exploratory research of a few laboratories have become aclcepted, standardized, quantified, and widely applied. The necessary equipment is available commercially and comprehensive books and reviews provide adequate introductions for new users. There has been a lively growth in both the quantity and quality of electron microscopy applied to the materiahi sciences in industrial and academic environments. At the same time possibilities for important new advances are seen, arising in part from the ready availability of improved instrumentation, including minicomputers and field emission guns, and partly from the more complete understanding of the theoretical basis of electron diffraction and imaging.
INSTltUMENTATION Ultra-High-Resolution Microscopes. Since it has become clear that the high-resolutionmicroscopy of crystals can provide information of immediate and fundamental importance for many areas of solid-state science, there has been renewed emphasis on thie production of instruments for this purpose. The resolutiion has been improved and the requirement has been met that the best resolution should be attained using a goniometer stage which is necessary for aligning crystals into pireferred axial orientations. There are two competitive approaches to ultra high resolution. In Japan there are several high-resolution high-voltage (0.5-1.5 MV) electron microscopes which have shown crystal structure images which are directly interpretable for resolutions of about 2 A, i.e., the arrangement of atoms in the unit cell and, in favorable cases, in crystal defects can be inferred because the intensity distribution in the image is a direct, if nonlinear, representation of a projection of the distribution of electrostatic potential which has maxima at atom position. One such instrument has been made in England ( I ) . There are, as yet, none in American although one is on order from Japan for the Lawrence-Berkeley laboratory (2). On the other hand the more conventional 100-keV microscopes have been considerably improved and, especially when the voltage has been increased to 200 kV, directly interpretable images can be achieved with resolutions approaching 2.5 A. For these microscopes thLe detail visible in the images may be 0003-2700/82/0354-83R$06.00/0
much finer than this, down to the 1 A level, but such detail is not directly interpretable since the relationship of the intensity distribution to the specimen structure is greatly perturbed by lens aberrations. In principle it should be possible to interpret such fine detail by comparison with images computed from models of the structure or, in very favorable cases, by use of holographic or image processing techniques. In practice there are a few cases where these methods have been used with some degree of success, but the requirements for their successful application are normally so exacting that their wider use is inhibited. On the other hand the high-voltage, high-resolution microscopes do not usually show such fine detail since instabilities in the high voltage and other electrical supplies are more difficult to control and limit the image detail, whether directly interpretable or not, to the 2-A level. It remains to be seen whether the next major advances in high-resolution imaging will come as a result of engineering improvements to provide better stabilities for high-voltage machines or as a result of advances in the recording and processing of the data from the lower voltage machines allowing the better resolutions to be achieved on a routine basis by the more indirect methods.
Scanning Transmission Electron Microscopy (STEM) Instruments. The original STEM instruments of Crewe and associates (3)were designed and were mostly used for single atom imaging and for biological applications. The STEM instruments now produced commercially by VG Microscopes Ltd, England, are increasingly applied in the materials science areas. They are built, and often subsequently adapted further by the users, to allow for special modes which are particularly suitable for crystalline inorganic specimens. In spite of the notable successes in the imaging of isolated heavy atoms ( 4 ) ,STEM instruments are not usually competitive with the conventional TEM in producing high-quality, high-resolution micrographs, especially in the common bright-field modes. For imaging, their special virtue lies in the possibility of using special detector configurations to select any part of the diffraction pattern to form the image (5). However more rapid development is taking place in the analytical applications. For any position of the incident electron beam on the specimen the illuminated area gives a detectable microdiffraction pattern, an X-ray signal with the characteristic emission lines from the elements present, and 0 1982 American
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