Kinetic determinations and some kinetic aspects of ... - ACS Publications

Anal. Chem. 1984, 56, 96R-112R (410) Sky-Peck, H. H.; Joseph, B. J. Clin. Biochem. (Ottawa) 1981, 74, 126. (411) Rethfeld, H. Fresenlus‘ 2.Anal. Chem. lB82,3 7 0 , 127. (412) Scheubeck, E.; Joerrens, C. Siemens Forsch.-Entwlcklungsber. 1981, 70, 29. (413) Zee, J. A.; Szoghy, I. M.; Slmard, R. E.; Desmarais, M. Am. J . Enol. Vitic. 1981, 3 2 , 93. (414) Searle, E.; Thompson, C. M. Analyst (London) 1983, 708, 254. (415) Christensen, L. H.; Agerbo, A. Anal. Chem. 1981, 5 3 , 1786. (416) Bumbalova, A.; Havranek, E.; Harangozo, M. Radiochem. Radioanal. Lett. 1982. 5 4 . 367. (417) Wiesener, ‘W.; Schaefer, U. Zenffalbl. fhafm. Pharmakothef Lab. 1982, 727, 459. (418) Schramel, P.; Klose, 8. J.; Hasse, s. FfeSdUS’ 2. Anal. Chem. 1882, 370, 209.

(419) Schramel, P. Spectrochim. Acta, Parts 1983, 3 8 , 199. (420) Azlz, A.; Broekaert, J. A. C.; Leis, F. Spectrochim.Acta, Part E 1982, 3 7 , 369. (421) Black, M. S.; Thomas, M. 6.; Browner, R. F. Anal. Chem. 1981, 5 3 , 2224. (422) Tanner, J. T. J . Assoc. Off. Anal. Chem. 1982, 6 5 , 1488. (423) Long, S. E.; Snook, R. D. At. Spectrosc. 1982, 3 , 171. (424) Kuennen, R. W.; Wolnlk, K. A.; Frlcke, F. L.; Caruso, J. A. Anal. Chem. 1982, 5 4 , 2146. (425) Camerlynck, R.; Martens, R.; Verloo, M. Bull. SOC.Chim. Belg. 1982, 91. 677. (426) ‘Braetter, P.; Berthold, K. P.; Gardiner, P. E. Spectrochim. Acta, Part E 1983, 3 8 , 221. (427) Braetter, P.; Berthold, K. P.; Gardiner, P. E.; Gawllk, D.; Behne, D. J . Radioanal. Chem. 1982, 69, 159.

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 ofCincinnati, Cincinnati, Ohio 45221

This review retains, basically, the organizational structure of previous reviews ( I ) . The papers reviewed here have been selected from those that appeared since November 1981 and were received for the authors’ consideration through approximately November 15, 1983. Previous trends are paralleled in this review, with the determination of catalysts and methods based on catalytic reactions (other than enzymecatalyzed reactions) being the subject of the largest number of references included in this report. An event with international reverberationsneeds to be singled out here. Through September 27 to September 30,1983, in one of the oldest and most pjcturesque cities of Spain, the city of C6rdoba in Adalucia, over 150 participants from 17 different nations attended the First International Symposium on Kinetics in Analytical Chemistry. As active participants in this event, the authors of this review can testify to the success of this gathering, which indicates the maturity that kinetic methods and kinetic considerations have reached in the realm of analytical chemistry. Twenty three lectures (five of them plenary) and 53 poster presentations covered in a broad spectrum the main determinative approaches based on kinetics as well as many innovative and nontraditional aspects within the subject of the Symposium. The plenary lectures of this event will appear in the journal Quimica Analitica, the official publication of the Spanish Society of Analytical Chemistry.

for the design of experiments and analysis of kinetic data in analytical chemistry. Of equal interest are two chapters in Vol. 24 of the “ComprehensiveChemical Kinetics” series. The first of these chapters, authored by McDermid (4),discusses the study of fluorescence decay and in the other Come gives an excellent account of the use of computers in the analysis and simulation of reactions (5). A review, in Russian, on kinetic methods in analytical chemistry has been published by Dolmanova (6). Also in Russian, is a book on kinetic methods for the analysis of natural waters (7). A detailed review on the kinetics and mechanism of formation and reduction of heteropoly compounds in solution has been presented by Alimarin et al. (8).The review includes kinetic methods of analysis using heteropoly compounds. A brief account on reaction rate methods in Japanese with some emphasis on sensitivity and selectivity has been authored by Tanaka (9). Also in Japanese, Yonehara and Kawashima (10) reviewed kinetic methods based on spectrometric monitoring. The determination of nonmetals by kinetic procedures has been reviewed in the Russian literature by Ushakova and Dolmanova (11). Muller is the author of a review on catalytic methods other than those based on enzymes (12). This review updates previous reviews on the topic and focuses on the fundamentals of the approach, its sensitivity, selectivity, and applications. Kiss has reviewed the foundations, instrumentation, and applications of thermometric titrations with catalytic endpoint indication (13). Comparisons with other catalytic end-point indications, with noncatalytic end-point indication, and with direct catalytic determinations have been also considered in this review. A review of selected electrochemical methods based on catalytic waves and described in the literature in the People’s Republic of China has been included in an article by Wang. This rather special type of journal article overviews the state of the art of trace analysis in continental China (14). A useful and timely review on the determination of organic species by utilization of catalytic-based methods has been authored by Milovanovic (15).Special attention is given to the classification of the different effects that have been exploited for such determinations.

BOOKS AND REVIEWS A concise, excellently organized, and up-to-date monograph on kinetic methods in chemical analysis is part of volume XVIII in Wilson and Wilson’s “Comprehensive Analytical Chemistry” (2). The authors, Kopanica and Star&,have in 250 pages condensed in a clear and useful manner the information contained in 845 references. The proceedings of a satellite symposium on Design and Analysis of Enzymes and PharmacokineticExperiments [XIth International Congress of Biochemistry, July 14, 1979, Toronto, Canada] have been published in book form (3). This book contains useful information on kinetic data analysis (e.g., principles, properties, and methods of nonlinear regression, robust parameter estimation). Although the coverage is by no means exhaustive and applications are limited to biochemistry and pharmacology, the topics are of general interest 96 R

0003-2700/84/0356-96R$06.5OlO

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1984 American Chemical Society

KINETIC ASPECTS OF ANALYTICAL CHEMISTRY

lag bebind in rate with respect to the sample curve but wiII approach ita rate and a t a given time, t., the curves will ero88 (this time could be considered as an "end point" for the determination of catalvst). Bevond ~, t. the rate of reaction in the reference solution ekceeds that in the sample. At time 1, the following expression applies: ~

~~~

.

~~~~

~~~~~

~~

~~

I.

in which [C ISthe Concentration of catalyst in the unknown (eample) 80 ution .. and IC] is the concentration of catalyst in the standard solution added a t a constant rate, m. This equation permits direct calculation of the concentration of catalyst in the unknown sample without recurse to calibration plota. Pseudo-first-order conditions are assumed in the derivation of the equation. Alternatively, if dilution effects are negligible, a graphical procedure. plotting I , vs. [C]., may be used since the equation simplifies to

This novel appmaeh hm bean illustrated by the determination

Detmnimtiorm b a d on emienionof light (fluoresoenea and

luminescence methods) have received increased attention letely. Such increase in interest is r e f 4 in three reviews on kinetie-based methods in these arena. Ingle and Ryan (16) reviewed reaction rate methods with fluorescence monitoring. This paper includes a general review of kinetic methods. Fluorometric reaction-rate methods for the determination of inorganic species have been diseuased by Val&cel and Grases (17). Enzymatic and nonenzymatic methods as well as instrumental requirements are concisely treated. The fundamentals and biomedical ap lications of chemiluminescence and bioluminescence have teen reviewed by Seitz (18). In both approaches the variation of intensity with time provides the basis for their classification as kinetic methods. K I N E T I C METHODS FOR THE D E T E R M I N A T I O N OF C A T A L Y S T S Table I is a summary of methods propoaed for the determination of catalysts. Again, as in the past years, redox reactions and those catalyzed by transition-metal ions constitute the majority of the methods proposed. The ambiguity earlier pointed out regarding the we of the terms semitiuity and limit of detection remains in the literature and makes critical comparisons d i f f d t ; when cited such values represent figures reported by the authors. A relevant paper of interest in catalytic determination was published in 1980 (19). The new concept of 'limit of quantitation" is introduced and defmed as 10 times the standard deviation for the blank reading above the blank. An unconventional and imaginative approach to catalytic determinations was introduced by Mpez Cueto and Cueto Rejh during the First International Symposium on Kinetics in Analytical Chemistry (55). Their proposition centers on comparing the signal-time profile for the sample with that of a reference solution to which the catalyst (analyte) species is added a t a constant rate. Both signal-time profiles s t a r t at the same signal level at time zero. The reference curve w i l l

of iodide by the classic Sandell-Kolthoff reaction ampem. metrically, the limiting current of the cathodic wave of cerium(IV) being monitored with either a glassy-carbon rotating disk electrode or a platinum wire stationary electrode. The application of competitive reaction systems to catalytic determinations was explored by Klockow et al. in 1979 (56). The same authors have now applied their approach to the determination of sulfide by use of the iodine-azide reaction and the investigation of the microdiffusion of hydrogen sulfide from aqueous solutions (57). The classical Sandell-Kolthoff reaction (oxidation of arsenic(II1) by cerium(1V) catalyzed by iodide) is perhaps the most useful catalytic system at hand that continues to find applications. Jones et al. (58),for instance, have determined total iodine and iodateiodine in natural freshwaters by means of this reaction. The iodate was determined after any iodide was extracted into chloroform as an ion pair with tetraphenylarsonium cation. The procedure was implemented in an air-segmented continuous-flowsystem handling about 50 samples per hour for total iodine and 20 samples per hour for iodateiodine. Iodine-containing organic compounds in microgram amounts (e.g., 4-iodophenol, 2-iodo henol, and 4-iodo-NJVdimethylaniline),which catalp the !andell-Kolthof€reaction, have been determined by Pantel (59). Bioamperometric monitoring was used. Cyanide ion has been determined by a combination of homogeneous catalysis and gas chromatography (60). The basis for the procedure is the chromatographic measurement of methyl b e m t e produced by the cyanide-catalyed reaction between benzyl and methanol in basic aqueous solution. A reaction time of 15 or 30 min was allowed before measurement. A detection limit of 1ppb is reported and linear calibration was observed from 0.05 to 3 ppm cyanide. A 'stab" procedure with biamperometric monitoring of the iodine-iodide system is proposed by Pantel (61) for the d e termination of thioureas as catalysts of the iodine-azide indicator reaction. The thioureas listed can be determined in the nanomolar range in aqueom solutions and in 20% ethanolic solutions. At a pH of 9.64 and in 50% (v/v) watepacetonitrile solutions, w(-)-arabincse aeceleraten the molybdenum(VI)-H,O reaction (62). This effect has been used to develop a methoc! capable of determining w(-)-arabinose from 46 to 135 pg/mL with standard deviations lower than 10%. The procedure calls for monitoring the rate of color formation at 350 nm for about 5 min and application of the differential form of the method of tangents. m(+)-Glucose interferes when present in concentrations comparable to that of arabinose. On the basis of their studies of the molybdenum(V1)-H,O, reaction, the authors suggest that the formation of a catalytically active monocomplex of m(-)-arabinose and M O W )accounts for the effect. Sulfur-mntainingcompounds catalyze the reaction between sodium azide and iodine. This effect has been exploited by ANALYTICAL CHEMISTRY. VOL. 58. NO. 5. APRIL 1984

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Table I. Determination of Different Catalytic Species by Kinetic Methods Based on Primary Catalytic Effects element cadmium(II)

indicator reaction

comments

complexation of Mn(I1) with (Y,P,'Y, 6 rate of complex formation monitored spectrophotote tra(psu1fophenyl)porphine, metrically recording the disappearance of free H,TPPS, H,TPPS, in presence of ascorbic acid signal at 413 nm, pH 8.0, temp 25 "C; fixed-time procedure; measurement 30 min after start of reaction; determination at the lo-' M level; lead interferes but can be separated by coprecipitation chromium(II1) 3-methyl-2-benzothiazolehydrozone t monitoring of violet product (590 nm); nanogram amounts N,N-dimethylaniline t H,O, determined in the optimum pH range of 7.1 to 7.3 and [EDTA added as activator] at 45 "C; fixed-time procedure (50 rnin reaction); iron(I1) and iron(II1) are serious interferences and must be removed in advance cobalt salicylfluorona t H,O, in strongly photometric monitoring; fixed-time procedure; determinaalkaline medium tion of cobalt in pure solutions of its salts (limit of detection = 0.00001 pg/mL) and in blood Tiron + H,O, limit of detection 0.05 ng/mL (optimized from response surface studies); both differential and integral methods proposed for preparation of working curves mechanistic aspects of the indicator reaction and the cobalt catalysis reported in a separate paper 4-hydroxy-3-carboxyphenylazo-4' ,4" - monitoring at 533 nm; determination in the ( 3 to 27) x bi(phenylazo)-2-chromotropic acid pg/mL; 100-fold excess of Fe(I1) and Os(VII1) interfere f H,O, copper H,O, oxidative coupling of phenylene- fixed-time (15 rnin at 45 "C); nanogram amounts deterdiamine with N,N-dimethylaniline mined; application to tap and river waters 4,4' -dihydroxybenzophenone thiospectrophotometric monitoring at 415 nm; calibration semicarbazone t H,O, graphs linear in the range 10-90 ng of Cu/mL; a fixedtime method recommended; applied to determining copper in water samples; results compare well with those of atomic absorption determinations without need for preconcentration Pyrocatechol Violet + H,O, solutions containing 30% (v/v) acetonitrile permit the determination of copper in the range from 3.3 to 13.2 mg/m3,concentrations 1/300 of those measurable in pure aqueous solutions; acetonitrile also increases selectivity; 5-fold excess of manganese and cobalt toleratedOH- + 2-hydroxybenzaldehyde azine fluorimetric monitoring of the hydrolysis product [A,, = [PH 11.91 355 nm, A,, = 465 nm]; initial rate [method of the tangents1 method covers the 0.100 t o 0.500 unlmL range; proposed catalytic action: through co'iplexation, the positive charge of the metal ion modifies the electronic distribution in the ligand so as to facilitate OH- attack; when hydrazone product is in excess, Cu(I1) complexation with it terminates the catalytic cycle manganese Azorubin S t H,O, presence of 30% (v/v) acetone allows determining manganese in the range 0.44 to 1.76 mg/m3, one-tenth the concentrations measurable in pure aqueous solutions sulfanilic acid + 10,determination of submicrogram amount by the method of the tangents; kinetic and mechanistic interpretation provided limit of detection (in the presence of 2,2'-bipyridine as tiron t H,O, activator) 0.2 ng/mL (optimization by response surface studies); differential form of fixed-time procedure mechanistic aspects of indicator reaction and manganese catalysis are reported separately 2-hydroxybenzaldehyde thiosemifluorimetric monitoring (Aern = 440 nm); initial rate procarbazone + H,O, (basic medium) cedure recommended; linear calibration curves in the range 2 to 9 ng of Mn(II)/mL; interference from Fe(III), Pb(II), Zn(II), Ag(I), Sn(II),Sb(III),andEDTA iodine [as iodide] triphenylmethane dyes (e.g., Malachite method improvement by addition of acetone at pH 5.5 to Green) oxidation by chloramine R 6.1 and potentiometric monitoring; length of induction period proportional to iodide concentration NO,- + SCNphotomeric determination [monitoring of absorbance decrease at 450 nm; monitored species Fe-SCN complex(es)]; limit of detection 0.22 ng/mL; fixed-time measurement [20 min] ; application to determination of iodine in food products; method compares favorably with the classical Ce(1V)-As(II1) system iridium mercury(1) + cerium(1V) interference of ruthenium species avoided by treating with nitric acid to convert to inactive nitroso complexes iron H,O, coupling N-phenyl-p-phenylene- acetate activates the Fe(II1) catalysis; fixed-time deterdiamine with N,N-dimethylaniline mination (15 rnin at 40 "C); nanogram amounts determined; application t o water samples H,O, + I- (in acidic medium) consumption of iodide monitored by iodide ion selective electrode; rate of potential change (mV/min) during first 1 0 rnin of reaction linear in the range 5 to 160 pM 98R

ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

ref 20

21

22 23 24 25 26 27

28

29

28 35 23 24 36

30 31

32 33 34

KINETIC ASPECTS OF ANALYTICAL CHEMISTRY ~~~~

~~~

~

Table I (Continued) element molybdenum

indicator reaction photoassisted oxidation of 2-aminophenol

+ pyrogallol in acidic medium

Br0,-

ascorbic acid t H,O,

+ Io4-

osmium

AS0;-

ruthenium

diphenylamine t KIO, ( 3 M H,SO, 4.5 M H,SO,, or 9 M H2S04) Tropaeolin 00 t IO,-

,

Direct Blue 6 t H,O, (pH 0.8 to 1.2) silver

ethylenediamine (distilled before use) t

s,o,2-

potassium persulfate hydroxyquinoline

+ antipyrine-8'-

sulfur-containing Indigo Carmine t H,O, compounds tungsten K I + H,O,

vanadium

gallic acid t S,O,2-

2-aminophenol

+ C10,-

o-dianisidine t tert-butyl hydroperoxide in acetonitrile

tiron

yttrium zirconium

t S,O,2-

MOO,- + ethylhydrazine H,O, + I- (in acidic medium)

Perez Ruiz et al. to determine thiamine (vitamin B,)by a fixed time procedure (63).The catalyzed reactiop is quenched by adding an excess of arsenite a t a fixed time, and the amount of iodine reacted is calculated. The unreacted arsenite, determined bioamperometricall ,shows the amount of unreacted iodine at the fiied time. Cadration curves are prepared based on the consumed iodine as a function of thiamine in the sample. The concentration range amenable to determination is 5 X lo4 to 7.5 X M. Application to several vitamin preparations is reported.

comments

ref

sodium anthraquinone-2,6-disulfonateas activator; irradiation (at wavelengths larger than 380 nm) for 8 min; monitoring at 430 nm; application to alloy steels and nickel based alloys ; prior sample preparation needed use of pyrogallol and 0.005 to 0.15 M HClO, or H,SO, to improve selectivity and accuracy improved selectivity in presence of W by the use of ascorbic acid and by conducting reaction at pH 2.7-3.5 selectivity improved by adjusting the concentration ratio of the oxidizing and reducing agent in the indicator reaction; no interference from 50-fold amounts of Ru catalytic action ascribed to sulfate complexes; photometric monitoring at 530 nm; limit of detection 0.05 pg/mL reaction in acidic medium; method capable of determining Ru in materials containing more than 0.00002% Ru and without separation; application to industrial materials (e.g., stacked filler cake, converter matte, ore, solution of autoclaved and leached sludge) determination of 0.0005 to 0.01% Ru(II1) in platinum metal, PdCl,, PtCl, ,PtC1, , and RuC1, ; photometric monitoring reaction started with addition of peroxo disulfate; reaction at 30 "C followed by titrating the residual oxidant in aliquots at intervals; applicable concentration range; to 8.0 x l o w 6M; interference from Cu and Mn, 5.0 x zinc and Fe tolerated 2,2'-bipyridyl as activator; limit of detection: 5 X Mg/mL; relative errbr about *4%;determination of Ag in semiconductor materials determination of sulfide, thioacetamide, and thiosulfate by an absorptiostat technique in the micromolar range application to water analysis in presence of 10-fold excess vanadium; preconcentration by precipitation with methylene blue and tannin; vanadium does not coprecipitate; inhibiting effect of iron(II1) removed by ion exchange and molybdenum( VI) Tasked with ammonium oxalate; limit of detection, 0.002 pg/mL modifications recommended for the original method of Fishman and Skougstad (48); limit of detection and limit of determination 2 ng of V and 0.2 g/L; modifications produce improved precision and accuracy and faster analysis measurement of fluorescence intensity of oxidation product, 2-amino-3-phenoxazine;from 0.1 ppm to 5 ppm of vanadium(V) can be determined under optimum conditions, temperature 50 'C, reaction time 2 h, pH 2 ?: 0.2; iron(I1) and Mn(VI1) caused positive errors, and Cr(V1) and Mo(V1) negative errors if present in 40-fold (w/w) ratio to vanadium(V) limit of detection 0.7 ng/mL (optimized by a modified simplex method) vanadium determined in organic solvents of high purity (toluene, acetone, ethanol, methanol, and %propanol); fixed time procedure (10 min at 22 "C); mechanism and kinetics of the indicator reaction discussed in a separate paper fixed time method [absorbance measured at 364 nm after 1 5 min reaction]; best selectivity at pH 0-1, interference from Mo eliminated by masking with citrate; application, analysis of amalgams limit of detection, 0.3 pg/mL; several interferences consumption of iodide monitored by iodide-selective electrode; plot of rate of potential change (mV/min) from readings taken during first 10 min, linear in the range 1.0 to 12 pM

37

38 39 40 41 42

43 44

45 46 47

49

50

51 52

53

54 34

The organophosphorus compound phthalophos increases the rate of HzOzoxidation of 3,3'-dimethoxybenzidine (odianisidine). This effect has been used by Shapenova to determine phthalophos in natural waters in the presence of other organophosphorus compounds such as chlorophos, saiphos, anthio, and phosalone. The reaction rate at pH 11.7 is linearly related with the phthalophos concentration in the range of 0 to 0.20 p g / l l mL in water (64). The effect of several physiologically active nitrogen-containing compounds on the horseradish peroxidase catalyzed ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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oxidation of o-dianisidine by H O2 has been examined by Dolmanova et al. (65). Midazoie, 2-methylbenzimidazole, 14hydroxymethyl)benzimidazole, 1,$4-triazole, benzotriazole, 4-aminopyridine, 2-methylindole, and 5-methoxyindolewere considered. The paper reports equations for the corresponding calibration curves, determinable concentration ranges, and limits of detection. Reflecting a general trend in wet chemical analysis, unsegmented (continuous) flow sample processing of catalytic methods is increasingly being applied. Deguchi et al., for instance, report on the iodide determination by use of the Sandell-Kolthoff reaction and flow injection (66). The limit of detection of their procedure is reported as 1ppb, and the sample rate as 30 samples per hour. Iodide ion in rain water was thus found to be between 1and 2 ppb (barely above the limit of detection). The catalytic determination of molybdenum at the pg/L level has been implemented in a continuous-flow system (flow injection) by Zhao-Lun and Shu-Kun (67). The catalytic action of molybdenum on the oxidation of iodide by HzOzis the chemical basis for the determination. The procedure was applied to molybdenum determination in natural waters at a sampling rate of 90 per hour [200-pL samples].

KINETIC METHODS BASED ON INHIBITION OR ACTIVATION OF CATALYSIS The methods reviewed here involve interaction of a given chemical species with a catalyst so as to prevent it from entering the catalytic cycle (inhibition) or to produce a chemical species that reacts through a path of lower activation energy (activation). Most titrimetric methods using catalytic end point indication are based on inhibition but they are grouped separately in this review. Trace amounts of ascorbic acid in plant extracts have been determined by a kinetic procedure based on the measurement of an induction period by Celardin et al. (68). Two chemical systems proved suitable for this purpose. The first one uses fluorometric monitoring of the homovanillic acid-H202 indicator reaction catalyzed by horseradish peroxidase; the second one uses absorptiometric monitoring of the guaiacol-HzO, reaction (also catalyzed by horseradish peroxidase). Addition of ascorbic acid (probably by interference with the enzymatic reaction and destruction of an intermediate species of the normal oxidative path) quenches the reaction temporarily. The length of time needed for regaining the signal change with the time is directly proportional to the amount of ascorbic acid which mechanistically is acting as a transient inhibitor. Similarly the inhibition of horseradish peroxidase by bismuth(II1) has been used as the basis for a kinetic determination of bismuth, with a limit of detection of 2 X pg/mL (69). The indicator reaction used was o-dianisidineH20z. The method of tangents is recommended for the determination of bismuth in natural water and biological materials. Ramon et al. (70) report on the determination of theophylline in serum and in saliva by a method based on its inhibition of alkaline phosphatase. The method calls for extraction into a mixture of chloroform and 2-propanol (201, v/v) from phosphate buffer solutions of pH 7.4 prior to the measurement step. Phull and Nigam have determined microamounts of cysteine, thioglycolic acid, and thiosulfate by their inhibition of the mercury(I1) catalysis of the substitution reaction between p-nitrosodiphenylamine (p-NDA) and hexacyanoferrate(I1) (cyanide substitution) (71). The reactions were followed spectrophotometrically at 640 nm, the wavelength of maximum absorbance of the [Fe(CN),(p-NDA)I3- species. The silver(1)-catalyzed oxidation of sulfanilic acid by peroxodisdfate ions is inhibited by cysteine. By use of this effect (which is enhanced in the presence of 2,2’-bipyridyl) Alexiev and Angelova (72)performed the determination of cysteine in the 5 x IO-’ to 4 X lo+ M concentration range (minimum determinable concentration, 0.5 wM). The method is of potential application to samples of biological origin without prior separation (no interference from 20 typical amino acids tested). Comparison of the method of tangents and a fixed-time procedure (10 min reaction) showed the fixed-time approach to provide a larger dynamic range. Several metal ion species have been determined based on complex formation with 6-mercaptopurine (73-75). 6Mercaptopurineinduces the sodium azide-iodine reaction and 100 R


complexation results in inhibition. Platinum, gold, osmium, ruthenium, iron, cobalt, and nickel have been determined by a kinetic procedure based on this inhibition. Gold(II1) has an initial inhibitory effect on the reaction between Toluidine Blue and hypophosphite catalyzed by palladium(11). Such an effect has been used by SBnchezPedrefio et al. as the basis for the kinetic determination of gold(II1) at the microgram level (76). Increasing concentrations of gold increase the length of an induction period in a proportional manner. The induction period is a linear function of the [Au(III)]/[Pd(II)] ratio, within certain limits. In the presence of a fixed amount of gold(II1) the shortening of the induction period by increasing concentrationsof palladium(I1) makes it possible to determine this species in the 0.1 to 3 pg/mL range. The observed behavior is attributed by the authors to the formation of a pseudoalloy between gold(0) and palladium(0) formed by the hypophosphite reduction. Iron(II1) catalyzes the oxidation of 2-hydroxybenzaldehyde thiosemicarbazoneby Hz02in ammoniacal solution. Silver(1) inhibits the rocess, probably by forming a complex with the thiosemicarlazone. Such an inhibitory effect has been exploited by Moreno et al. (77) to determine silver in the 50-400 ng/mL concentration range using an indirect procedure. The reaction is monitored by following the fluorescence of the = 440 nm]. Calibration oxidation product [A, = 365 nm; hrn plots are prepared by plotting % inhibition of fluorescence vs. silver ion concentration. Oxalate, citrate, and fluoride ions inhibit the iron catalysis of the 2,4-diaminophenoloxidation by H202. On the basis of this effect, separate determinations of the inhibitors at the microgram level have been reported by Vasilikiotis et al. (78). The controlled anodic generation of copper(I1) and silver(1) as catalysts to be used in the determination of inhibitors, activators, and components of the indicator reaction in continuous-flow sample processing has been proposed by Weisz and Fritz (79). Copper(I1) was thus used in conjunction with the iron(II1)-thiosulfate reaction and the HzOz-hydroquinone reaction and silver(1) with the sulfanilic-peroxodisulfate reaction. Thiosulfate catalyzes the oxidation of indigo carmine by HzO * this catalytic effect is enhanced in the presence of i r o n h ) and aluminum(II1) while fluoride, when present, masks the enhancing effect of aluminum. Using these behaviors, Weisz et al. (46) have developed an absorptiostatic determination of iron at the nanogram level and of aluminum and fluoride at the microgram level. Sodium thiobarbitone promotes the oxidation of Pyrocatechol Violet by H20zcatalyzed by copper(I1). Using the method of tangents, MilovanoviE and Protolipac determined sodium thiobarbitone in the concentration range of 2 to 8 X lo4 M with about 7% relative standard deviation (80). The effect of sodium thiobarbitone is seen only during the first few minutes of reaction and soon the overall rate becomes the one in the presence of copper(I1) alone. This suggests the inactivation or decomposition of the promoter (81). The activating effect of 1,lO-phenanthrolineon the manganese(I1)-catalyzed oxidation of p-phenetidine by potassium periodate has been discussed by Alexiev and Mutaftchiev (82). Presence of the activating species increases the rate of the catalyzed reaction, lengthens the linear portion of the absorbance vs. time curves, and decreases the background reaction by a factor of 2. The enhancement of the vanadium(V)-catalyzed aerial oxidation of sodium 4,8-diamino-1,5-dihydroxyanthraquinone-2,6-disulfonate by iron(II1) has been proposed for the determination of vanadium(V1at the 1-10 ng/mL level (83). The fluorescent oxidation product [A,, = 524 nm; A, = 582 nm] is monitored 60 s after initiation of the reaction for initial rate measurements. Measurements based on the length of the induction period, however, seem slightly better in accuracy and precision. The method was applied to the determination of vanadium in crude oils after sample preparation. Zinc(I1) activates the manganese(I1) catalysis of the oxidation of 2-hydroxybenzaldehyde thiosemicarbazone by H2Op This effect has been exploited by ValcBrcel et al. (84) to determine zinc in nanogram amounts in milk samples using a kinetic method with fluorometric monitoring. Because of a larger dynamic range of application and better accuracy, the initial-rate method is recommended over a fixed- and a variable-time procedure. The rate of reaction is measured by


following the formation of the fluorescent oxidation product [A,, = 365 nm; A,, = 440 nm].

TITRIMETRIC METHODS WITH CATALYTIC END-POINT INDICATION The titrimetric procedures reviewed here involve the use of catalytic titrants, which react rapidly and stoichiometrically with the soughbafter species, and an indicator reaction, which involves the monitored species and which can occur at a noticeable rate only when an excess of titrant is present. Direct semiautomatic titration of microgram amounts of EDTA and EGTA with copper(I1)as catalytic titrant and the 4,4’-dihydroxybenzophenonethiosemicarbazone-H202 indicator reaction has been reported by Raya-Saro and PerezBendito (85). They also report the indirect determination of microgram amounts of copper(II), nickel(II), and manganese(I1) by use of the same chemical system. Several metal ions at the microgram level have been titrated with EDTA by Weisz and Schlipf using catalytic end-point indication (86). The novelty of this application is the titration setu described in the paper. The catalyst (or inhibitor) is titratezin a closed vessel with the inhibitor (or catalyst). Small portions of the solution are transported and mixed with the indicator solution in an external micromixing chamber (approximate volume 5 pL). This chamber has a capillary opening at its bottom which allows drops to fall on a moving plastic band. The end point is detected visually by color change in the drops. The setup, as recognized by the authors, can equally well be used with other titrimetric end-point approaches. Iodide and chloride were also titrated sequentially with standard silver nitrate by utilizing two different catalytic end-point indications. The time elapsed from the start of the titration until the first drop indicating a color change appears in the moving band is directly proportional to the analyte concentration and can be directly read from a scale mounted on the moving rubber band. The preparation of calibration graphs is recommended to avoid error due to the time interval between the completion of the reaction in the titration cell and the transfer to the moving band. Direct titration of some aminopolycarboxylic acids and indirect titration of copper(I1) and cobalt(I1) ions using catalytic end-point indication have been reported by Hadjiioannou et al. (87). The indicator reaction was periodatethiosulfate; copper(I1) was the catalytic titrant and a semiautomatic potentiometric indication was used. Titration of microgram amounts of antimony(II1) or arsenic(II1) by a catalytic end-point indication in which a gas is evolved at the end point and acts as a catalyst for the indicator reaction, has been proposed (88). The indicator reaction took place in a separate container. Bromate was used as titrant; at the end point, the bromine formed by redox reaction with the analyte in the sample was swept by nitrogen into a separate container in which, by reaction with iodide, iodine was formed. The liberated iodine was then swept into another container in which the components of the indicator reaction were located. The indicator reaction used was that of Sandell-Kolthoff [Ce(IV)-As(III)]. An alternative to this procedure involves the transfer of the liberated bromine to several vessels containing sulfite solution which reduces the bromine to bromide; the bromide catalyzes the permanganateiodine reaction. This elaborate alternative requires at least ten indicator test tubes which are changed every 30 s; after all these tubes have been used, hydrogen peroxide in 5 M H2S04is added to destroy the unconsumed sulfite and the permanganateiodide reagents are added. Although the approach allows use of redox reactions and noncatalytic titrants, the involved procedure seems to defeat the simplicity characteristic of catalytic end-point indication. The application of a chloramine T-selective electrode in the semiautomatic determination of silver and mercury, based on the inhibition of the iodide catalysis of the chloramine T-H202 indicator reaction, has been reported by Timotheou-Potamia et al. (89). Several pharmaceutical products containing mercury compounds as active species have been analyzed by using the Ce(1V)-As(II1) indicator reaction and iodide as catalytic titrant (90). Sample mineralization is required to free the mercury for titration. Some dithiocarbamates have been determined by catalytic end-point detection measuring the heat evolved at the end point as a result of the catalyzed reaction (91). The method

is based on the acid decomposition of the dithiocarbamate and collection of the carbon disulfide formed (sweeping with nitrogen) in a solution of ethylenediaminein 2-propanol. The dithiocarbamic acid formed was titrated with potassium hydroxide solution in the presence of acetone for end-point indication. Iodide in table salt has been determined by back-titration of an added excess of standard mercury(I1) solution with standard potassium iodide and catalytic endpoint indication by means of the classical Ce(1V)-As(II1) indicator reaction (92). Using as indicator reaction the HzOz oxidation of Acid Blue-45, an anthraquinone dye, Abe et al. developed a photometric titration method for manganese(I1)and some other metal ions with catalytic end-point indication (93). Absorbance measurements were at 596 nm. The method was applied to determine micromolar amounts of cobalt(II), nickel(II), copper(II),zinc(II), and cadmium(I1) by back-titration using an excess of EDTA and standard manganese(I1)solution as titrant. Microgram amounts of manganese(I1) were titrated directly with standard EDTA solutions. Jeyaraj and Greenhow (94) have evaluated some aspects of catalytic thermometric iodimetry by studying the effect of using different solvents for the sample, different vinyl esters as indicators, and alternatives to iodine as titrant. Earlier, Greenhow provided an account of the use of iodine in dimethylformamide for the titration of amines, amides, dithiocarbamates, and phosphorodithioates in nonaqueous solvents as part of a comprehensive review on thermometric-catalytic end-point indication (95). Catalytic end-point detection in thermometric titrimetry for use in the study of the mechanism and application of condensation and rearrangement reactions of mono- and difunctional carbonyl compounds has been discussed by Marrero-Ardila and Greenhow (96).

KINETIC DETERMINATIONS BASED ON ELECTRODE REACTIONS Interest on kinetic determinations in electrochemical systems, particularly those based on catalytic currents, remains at a moderate level. The greater availability of information regarding performance and understanding of the operation of chemically modified electrode surfaces may in the future increase interest in the area. During the First Symposium on Kinetics in Analytical Chemistry, for instance, the potentialities for application of catalytic reactions at surfacemodified polysdfur nitride, (SN),, electrodes to trace analysis were pointed out (97). One distinctive characteristic of these electrode surfaces is their behavior as very strong Lewis bases adsorbing strongly or bonding metal cations, in contrast to metal electrodes, which preferentially adsorb anions. In the presence of nitrate or nitrite, the polarographic reduction of the vanadium(1V) complex with CDTA exhibits a catalytic wave. If the concentration of complex is about five times that of nitrate or nitrite, the catalytic current is proportional to the oxidant anion and this permits the determination of these ions (98). The electrolytic process is performed in NH3-NH&l buffer of pH 8.6, and nitrite in mixture with nitrate can be determined without interference by using a complex concentration lower than 0.5 mM. Adrenaline in the 3 X lo4 to 7 x 10”. M concentrationrange has been determined by use of the catalytic polarographic wave obtained as a result of the adrenaline catalysis of the reduction of germanium(1V) at the DME in 1M HC104 (99). A detailed study of the electrochemical process resulted in the proposal of a mechanism characterized by the formation of a catalytic complex, HzGe03H-L[L = adrenaline], probably involving two coordination bonds being formed between H2GeO3H+and the o-diphenol system of adrenaline and the loss of a molecule of water. Formation of this complex from ligand adsorbed on the electrode surface is considered to be the rate-limiting step in accord with the generalized metalcomplex catalytic mechanism (100). Application to the determination of adrenaline in a solution for hypodermic injection is described. The titanium(1V)-EDTA complex undergoes cathodic reduction at a dropping mercury electrode and a catalytic wave is observed in the presence of bromate ion. Titanium in the to M range has been determined by means of this catalytic wave by Yamamoto et al. (101) using differential pulse polarography. ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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AC polarography was employed for the electrochemical study of europium(II1)in the presence of 1,lO-phenanthroline in a wide range of acidity (102). A catalytic prewave was observed which permits the determination of europium a t levels higher than 0.15 Fg/mL. The procedure was applied to the determination of europium in cathode luminophors based on yttrium oxysulfide. A nickel(I1) catalytic polarographic prewave in borate media is observed in the presence of some amino acids. Ldpez Fonseca and Arredondo (103) proposed the use of this prewave for the DC polarographic and differential pulse polarographic determination of methionine a t the M and lo4 M levels, respectively. Iodide and osmium are catalysts of the arsenic(II1)oxidation by cerium(1V). A stat-procedure (amperostatic) for the determination of iodide ((1 to 8) X lo4 M) and osmium ((4 to M) has been proposed by Jedrzejewski and Cies32) X ielski (104). With an excess of arsenic(II1) a constant concentration of cerium(1V) is maintained by anodic electrogeneration. The concentration of catalyst in the system is proportional to the current needed to generate the cerium(1V) at the same rate that it is chemically consumed. The determination error is equal or smaller than &3%. Ezerskaya et al. have studied the catalytic hydrogen reduction current in solutions of iridium(II1) complexes with furfural monoxime (acetate buffer, pH 4.6) (105). They developed a polarographic determination of micro- and nanogram amounts of iridium based on such catalytic current.

OTHER STUDIES OF INTEREST IN DETERMINATIONS INVOLVING CATALY $IS Otto and Lerchner (106) have compared spectrophotometric, calorimetric, and amperometric indication in connection with the catalytic determination of copper using the autodecomposition of HzOzas indicator reaction (in the presence of pyridine as activator). A knowledge of the kinetics of the uncatalyzed reaction is of relevance in the development of catalytic methods because such information is intimately related to the method limit of detection (107). On this premise, Vasilikiotis and Papadopoulos (108) report on the kinetics of Hz02oxidation of 2,4diaminophenol, an indicator reaction for some catalytic determinations. The effect of organic solvents on the catalytic oxidation of Azorubin S and PyrocatecholViolet by HzOzhas been studied by Milovanovic et al. (109). The Azorubin S-H20z indicator reaction is catalyzed by manganese(I1) and the presence of 30% (v/v) of acetone permits lowering the limit of detection by one order of magnitude. Use of the pyrocatechol-H2Oz reaction in the presence of 30% (v/v) acetonitrile lowers the limit of detection of copper 300 times with respect to pure aqueous solutions. In both cases the differential form of the method of tangents was used. Draper and Crosby described an interesting analytical application of catalysis. They applied the peroxidase-catalyzed oxidation of Leuco Crystal Violet to the detection of H20zand other peroxygen compounds on thin-layer chromatographic plates (110). Enzyme catalysis and the high molar absorptivity of Crystal Violet provide a detection system of improved sensitivity relative to other used methods. Polo Diez et al. have used iron(II1) to catalyze the reaction between Hz02y d permanganate in the potentiometric titration of HzOz with permanganate in the presence of excess fluoride ions (111). The fluoride complexes with and stabilizes the manganese(I11) species which otherwise may lead to low results.

APPLICATIONS OF LUMINESCENCk The measurement conditions employed in both chemi- and bioluminescence are kinetic in nature and as such are included in this review. Table I1 is a summary of selected methods for determination of a variety of species by these luminescence techniques. The use in an inert matrix of a sensitizing species which absorbs incoming light and transfers excitation energy to a luminescent analyte has been proposed by Seybold et al. (129). The approach was illustrated by using naphthalene as sensitizer in the determination of anthracene on filter paper as matrix. Chemiluminescence analyzers are commonly used for the determination of trace gaseous species in atmospheric measurements. Mehrabzadeh et al. (130) have considered the 102R


optimization of the response of these analyzers in terms of response equations for exponential dilution and plug flow conditions. Results of a study of the factors affecting the chemiluminescence intensity of the reaction between bis(2,4-dinitrophenyl) oxalate and HzOzhave been reported by Honda et al. (131). The reaction generates an intermediate which can react with a fluorophore and produce a fluorescent species. The scheme was used for the determination, in a flow injection system, of picomole quantities of dansylalanine. A detection limit in the femtomole level is reported for the determination. Until recently only iron(I1) was known to produce luminol chemiluminescence in the absence of hydrogen peroxide. Klopf and Nieman have recently reported, however, that copper(I1) and cobalt(I1) also cause luminescence under the same conditions (132). Manganese(II),on the other hand, was found to produce very little, if any, chemiluminescence in the absence of H2OP Other experimental conditions affecting luminol chemiluminescence in the presence and absence of HzOz and which significantly affect the process are discussed in this paper. Arsine in air or arsine generated from reduction of aqueous arsenic compounds produces chemiluminescence upon reaction with ozone and can be monitored by measurement of the emitted light. The reaction mechanism and kinetics of the arsine-ozone reaction and the pressure dependence of the chemiluminescent intensity have been studied by Fraser et al. (133). Ultrasonicallyinduced luminescence has been used by Yamada et al. (134) for the determination of water in methanol. The approach is unique in being a determination without use of reagents. Yamada and Suzuki (135)used sonic chemiluminescence observed when ultrasonic waves were propagated through an alkaline solution of luminol containing dissolved oxygen for selectively determining 8s little as 0.07 pg of cobalt by its enhancement of the sonically produced luminescence.

DIFFERENTIAL REACTION RATE METHODS A novel approach to the treatment of rate data for the simultaneous determination of different species has been developed by Olson and Shuman (136). The proposed nonlinear differential rate approach requires no prior knowledge of rate coefficients, initial concentrations, or number of components in the mixture. The method was applied to the study of Cu(I1) complexation by humic acid($. A “kinetic spectrum” results from application of the treatment with peak maxima correspoeding to rate coefficients and areas under the peak to initial concentrations. A ratio of at least 40 in rate coefficients is needed for complete resolution of two components. An interesting application of the method of proportional equations has been implemented by using a fluorescence quenching -approach for the determination of two or three components in solution (137). The method is based on the Stern-Valmer equation which describes dynamic quenching as follows: F o / F = 1 + K[Q] where (Fo/F) is the ratio of fluorescence intensities in the absence and in the presence of a quencher, [Q] is the quencher concentration, and K is a constant equal to the product of the decay time and the bimolecular rate coefficient for the quenching process. If several quenchers act simultaneously the over@ quenching process can be accounted for by adding additional terms to the Stern-Volmer equation F o / F = 1 + KitQiI + Kz[Q2I +

+ K,[Q,I

The method of proportional equations is implemented by use of n independent equations obtained by measuring the ratio of fluorescence intensities in the presence of n indicator species whose fluorescence is independently quenched by the quenchers to be determined. Applying this approach,Wolfbeis and Urbano have simultaneously determined chloride and bromide in organic materials and chloride, bromide, and iodide in a synthetic mixture (137). Quinine, acridine, and harman in sulfuric acid media served as indicators since their fluorescences are dynamically quenched by halides (except fluoride). Of interest to those contemplating or engaged in the application of differential reaction rate methods are the con-


Table 11. Selected Determinations Based on Chemi- and Bioluminescence determined species

ref

comments

determination in complex metabolite mixtures [reaction medium: NaClO-H,O,] ; limit of detection, 30 pmol/mL; results correlate well with those by HPLC determinations use of CY -hydroxysteroid dehydrogenase and a commercially available reagent bile acids containing low activity of NADH-FMN and high activity of bacterial luciferase ; limit of detection about 0.2 pmol/L specific determination of 12-CY -hydroxy bile acids; use of enzymes coimmobilized on Sepharose 4B; limit of detection, 4 pmol/0.5 mL of volume used in the measurement step; linear range, 4 to 2000 pmol of bile acid; good agreement with GLC determinations; applicable to determinations in serum of humans and of experimental animals certain reducing agents markedly intensify the chemiluminescence from reaction carbohydrates, ascorbic acid between luminol and periodate complexes of copper(11) electrochemical generation of luminescence from luminol at a rotating ring-disk cobalt( 11) in aqueous alkaline M; Cr(III), Cu(II), and EDTA interfere electrode; limit of detection, solutions mechanism of luminol electrogenerated chemiluminescence in aqueous alkaline solutions as produced with the rotating ring-disk electrode system discussed flavin adenine dinucleotide (FAD) determination based on the reactivation of apo-D-amino acid oxidase by FAD; in the Dresence of D-alanine. luminol. horseradish Deroxidase. and excess of the apoenzyme, FAD can be determined‘at the femtomole range’(bicarbonate buffer, pH 9.2, and 37 “C) flow system in which an enzyme solution (glucose oxidase) flows, under pressure, glucose through a microporous membrane allowing creation of a pH gradient in the flow cell; a 0.1 M potassium phthalate buffer carries the enzyme across the membrane and assures a pH of 5 close to the membrane wall, optimum for the enzyme catalyzed reaction; the sample is carried by a 0.5 M KOH solution containing luminol and a metal complex “catalyst”; the pH in the bulk of the solution and close t o the optical window for chemiluminescent measurement is about 11,optimal for the chemiluminescent reaction of luminol and H,O,; simple addition of economical amounts of enzymes are primary attractions of the approach ;application to the determination glucose in serum after deproteinization and comparison with more conventional methods (nonchemiluminescent) and other chemiluminescent systems for glucose measurement reported histidine enhances the effect of Mn(11) in luminol-H,O, chemiluminescence via histidine formation of a Mn(1II) complex with the Schift base resulting from the reaction of histidine with 5-sulfosalicylaldehyde automated flow systems utilizing coimmobilized enzymes (on Sepharose packed NADH, glucose 6-phosphate, into small flow cells); limits of detection in the picomole level; the enzyme primary bile acids, and ATP reactor usable for up t o 700 consecutive determinations preliminary report based on the highly selective electron transfer reaction between oxalate oxalate and the ruthenium( 111) complex with 2,2’-bipyridine;ruthenium complex electrochemically generated; results suggest sufficient selectivity for direct determination of oxalate in urine oxalate and urate microamount determination using continuous-flow processing and column reactors packed with immobilized oxidase and urate (two column system) on porous glass beads; chemiluminescent monitoring of H,O, by reaction with bis( 3,4,6-trichlorophenyl) oxalate in presence of 9,lO-diphenylanthracene and trimethylamine in a solution with freshly distilled dioxane as solvent; determination range, 2 X l o - * to 8 X l o + M protein use of the luminol-H,O, chemiluminescence; decrease of the Cu(I1) effect on the rate of light production by interaction with protein used as basis for the determination in a flow injection system; as little as 0.2 pg/sample can be determined at a rate of 30 samples/h; application to the determination of proteins in serum phosphorus gas-phase chemiluminescence with ozone after hydride generation by passing the sample mist (produced by ultrasonic nebulization) through an incandescent carbon tube; limit of detection, 8 ng of P/mL; phosphorus determined in organic phosphate esters; time per determination, 2 min H,S and other reduced sulfur chemiluminescent reaction with ozone; chemiluminescent intensity decreases in compounds the order CH,SH > CH,SCH, > H,S > thiophene; a commercially available, modified, ozone monitor used; the effect of oxides of nitrogen as increasing the signal is reported some organic-nitrogenreaction of luminol with persulfate and silver containing compounds various metabolites species determined: D-glucose, L-lactate, 6-P-gluconate,L-malate, L-alanine, Lglutamate, NAD, NADP; use of up to four coimmobilized enzymes on Sepharose 4B; peak light intensity of bioluminescence or rate of increase of emitted light level at 460 nm used as basis for determination; wide range of concentration amenable to determination lDmol to nmol ranee) .. . benzo[a]pyrene-7,8-dihydrodiol

\-

siderations on the adaptation of standard nonlinear regression algorithms for the treatment of simultaneous kinetic data presented by Mak and Lan ord (138)and application to the treatment of kinetic data or mixtures of free A13+ and the aluminum-citrate complex. According to the authors own

f

0

113

114 115

116 117 118

119

120

121 122 123

124

125

126

127

128 129

,

statement this work was performed as ‘‘an attempt to demonstrate that provided reliable initial estimates of the values of the parameters to be fitted are ascertainable, the kinetic data of a well resolved 2- or 3 component system where the respective rates observed are separated by nearly an order of ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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magnitude can be treated satisfactorily by any one of the common non-linear regression methods”. Binary mixtures of tertiary amines have been determined by Tawa et al. by measurement of pseudo-first-orderreaction rates corresponding to the decomposition of the ?r complexes with tetracyanoethylene (139). The determination approach is based on simplified tristimulus colorimetry. An advantage of the approach is that mixtures at any concentration levels can be analyzed provided large rate differences exist. The simultaneous determination of iron(II1) and cobalt(I1) exploiting the fact that complex formation of cobalt(I1) with pyridoxal thiosemicarbazone is substantially faster than the correspondingiron(II1) complex formation has been proposed by Ballesteros and Perez-Bendito (140). Both logarithmic extrapolation and single point determinations are reported and compared. Smaller errors and wider concentrationranges for determination of the metallic species in mixture (1-6 M level) are realized with the single-point method. Only simple mixing is required, making accessible the use of conventional spectrophotometric procedures for monitoring the course of the reactions. A differential reaction rate approach to the determination of chlorine and chloramines in aqueous solutions has been proposed by Gray and Workman (141). The method is based on the reaction of the oxalate of N,N-diethyl-p-phenylenediamine with free chlorine, monochloroamines, and dichloroamines in the presence of iodide ion as catalyst in acidic solution and utilizes stopped-flow mixing. After mixing, the free chlorine reacts too rapidly for its contribution to the rate to be recorded; its concentration can be estimated by the sudden change in absorbance at time “zero”. A multipoint computer analysis of the rate profile is used to estimate concentrations. A total of 256 data points are used in each run. Some considerations on the use of IR and NMR spectroscopy to study relatively slow reactions have been presented by Doerffel (142). The determination of compounds with similar properties is incidentally mentioned. A differential rate determination of phosphate and silicate using the method of proportional equations has been proposed by Kircher and Crouch (143). Time is used as the discriminatingvariable and stopped-flow mixing with photometric monitoring used to follow the rate of formation of the 12-heteropolymolybdates. Several approaches for the development of differential reaction rate determinations by means of flow injection analysis were proposed by Valciircel et al. (144) during the First International Symposium on Kinetics in Analytical Chemistry. Their procedure makes use of simple manifolds and a sin le detector which considerably simplifies the application y providing measurements at two different times. One of the proposed implementations calls for splitting the sample (after injection) into two channels of different geometric and flow characteristics; the other approach makes use of a large sample volume and a single transporting tube. Monitoring of the reaction at the two plug-reagent stream interfaces provides measurements at the two discriminating times to give the necessary data for solving the simultaneous equations.

KINETIC METHODS BASED ON UNCATALYZED REACTIONS The number of kinetic-based methods for determining alkaline earth ions is very limited because of their lack of participation in catalytic cycles. Laserna et al. have recently proposed a selective determination of magnesium in microgram amounts based on the reaction, in alkaline medium, with 2-fluoroaldehyde2-pyridylhydrazone(FAF”) (145). A t high pH values (- 12.5-13.5) miscellar magnesium hydroxide forms an intense green fluorescent species with FAPH. The fluorescence decreases with time and its monitoring at 520 nm (Aex = 375 nm) allows preparation of calibration curves of initial rate vs. magnesium concentration in the 0.36 to 1.22 pg/mL range. Determination is possible in the presence of 10-fold amounts of beryllium and calcium and 25-fold amounts of strontium and barium; but only 0.5 pg/mL of mercury(II), iron(III), chromium, and aluminum and 0.4 p g / d EDTA can be tolerated. A kinetiefluorimetric determination of cerium(lV)has been proposed based on the cerium oxidation of 4,8-diamino-1,5dihydroxyanthraquinone-2,6-disulfonate(146). Initial rate 104R


measurements permit determining cerium(1V) in the range of 0.02-0.37 pg/mL. The fluorescent oxidation product is monitored at 585 nm. Mercury(II),CrOZ-, and vanadium(V), at concentration levels comparable to that of cerium, are the only interfering species of 29 tested. Cerium(IV) at microgram per milliliter levels has been determined by a kinetic-fluorometric method based on the oxidation of 1-amino-4hydroxyanthroquinone(147). The yellow fluorescent product in acid medium was monitored for determination [A,, = 480 nm, A,, = 575 nm]. The initial reaction rate approach is recommended by the authors. An indirect fluorometric determination of iron, cobalt, nickel, and palladium in nanogram per milliliter concentrations has been reported making use of the complexing ability of the corres onding metal ions for benzyl 2-pyridyl ketone 2-pyridylhy&azone (148). Complexation decreases the amount of free ligand available for oxidation by bromate ion to a fluorescent product (Aex = 290 nm; A, = 362 nm). The same chemical system was also used by the authors for the direct reaction rate determination of bromate. Initial rate measurements were used in these studies. Flavonols emit yellow fluorescence and their gradual autoxidation to nonfluorescent products results in a decrease of fluorescence with time. A reaction rate determination of quercetin (3,3’,4‘,5,7-pentahydroxyflavone)and kaempferol (3,4’,5,74etrahydroxyflavone)in alkaline medium has been proposed based on monitoring the rate of fluorescencedecay [A, = 350 nm for both compounds; A,,(Quercetin) = 530 nm, A,,(kaempferol) = 510 nm] (149). In 1975 Jung et al. (150) reported a kinetic method for serum urea nitrogen based on the color reaction with 0phthalaldehyde and (1-naphthy1)ethylenediaminein moderately concentrated sulfuric acid. The absorbance of the blank, however, increases rapidly with time. Momose and Momose (151) replaced the reagent amine by N-(1-naphthyl)-N’-diethylenediamine and developed a method using dilute hydrochloric acid [incubationat 37 “C] in which there is no need to measure the reagent blank since its faint yellow color shows no absorbance change at the wavelength of measurement [520 nm]. Theirs is a two-point rate measurement of absorbance at 20 and 110 s after mixing. The corresponding absorbance changes give linear calibration curves passing through the origin and comprising the urea concentration range of 5 to 120 mg/L. The color reaction is not specific for urea (urea derivatives such as methylurea and citrulline, aromatic primary amines, and sulfa drugs interfere) but interfering substances are either absent or present at very low levels in serum. Thorium in uranium samples has been determined by following photometrically the rate of the exchange reaction between the Th complex with DCTA and Arsenazo I11 (152). The method requires preconcentration steps and the total time to g of Th/g of U could of analysis amounts to 8 h; be determined. Farasoglou and Karayanis report on the determination of ascorbic and dehydroascorbic acids in pharmaceutical preparations, human blood plasma, and urine (153). Their determination approach involves the reaction of the analytes with 2,6-dichlorophenolindophenoland stopped-flowmixing. A fixed-time (2 h) procedure has been proposed for the determination of phenol and di-, tri-, and tetrachlorophenols (154). Oxidation by metaperiodate ion in acidic medium provides the chemical basis for determination. The magnesium complex of cryptand(2.2.2) dissociates rapidly but the calcium complex with the same ligand shows a much lower rate of dissociation when potassium ions are used as scavengers. This chemical property has been exploited by Kagenow and Jensen in an unsegmented continuous-flow system with stopped-flow capabilities during detection to simultaneously determine magnesium and calcium (155). The magnesium totally released from the cryptand before reaching the detection cell and the calcium ion still exchanging at maximum peak height signal, are photometrically detected (575 nm) by using phthalein complexone as chromogenic reagent. As many as 80 determinations can be performed per hour. A microcomputer system for data acquisition and control is also described. A fixed-time, potentiometric, determination of creatinine in serum, based on the Jaff6 reaction, has been described by Diamandis and Hadjiioannou (156). The reaction was monitored with a picrate-selective electrode and the increase in potential, between 30 and 270 s after the


reaction was started, used for the construction of calibration curves. The method calls for adsorption of creatinine directly from acidified serum by strong cation exchange resins. The kinetic procedure (run at 37 "C) was applied to the eluted creatinine. The method is free from optical interferences that plague spectrophotometric methods and interestingly enough also free from interference from bilirubin. Pimenova et al. (157) applied a kinetic, accelerated-oxidation method for the study of the natural antioxidant content changes in sea buckthorn (Hippophue) oil. The method was applied to evaluate the stability of the oil. Kinetic curves of cumene oxidation in the presence of the oil samples indicated three types of antioxidants: a-and y-tocopherols and some other strong and mild antioxidants. The addition of Chromazurol S improves the determination of carbonates in aqueous solutions, alkali, and alkaline-earth metal halides, in methods based on measuring the rate of Cr(II1) complexation in weakly acidic medium (158). Pyridine-2-aldehyde2-pyridylhydrazone(PAPH) is slowly oxidized to a fluorescent product by bromate in acidic medium. An indirect determination of cobalt based on the decrease of fluorescence produced by cobalt complexation of PAPH as revealed by a dqcrease in the rate of oxidation has been developed by Garcia Shchez et al. (159). Detailed information on acidity effects and two procedures, one based on the method of tangents and the other on the length of induction period for the ligand oxidation, is presented. A fixed-time turbidimetric determination of serum lowdensity lipoprotein (LDL) has been proposed by Bart1 et al. (160). The turbidity results from the specific precipitation of the lipoprotein by heparin, calcium ions, EDTA, and lipase. An initial rate method for the determination of 0.03 to 0.15 pg/mL of vanadium with fluorimetric monitoring of the vahas been nadium oxidation of 1,3,5-triphenyl-A2-pyrazoline proposed by Grases et al. (161). The analytical use of some pyrazolines and isoxazolines as novel reagents for fluorimetric determinations is also discussed by the authors. Organic species that react with iodine and bromine have been determined, in the lo4 to lo4 M range, by Kreingol'd et al. (162). The bromine or iodine are chemically generated in situ by the H20zoxidation of the corresponding halide in acidic medium and o-phenylenediamineor o-toluidine is used as an indicator that reacts more slowly with Brz or I than the sought-for species. Species typically determine$ by measurement of the time delay ("pseudo induction period") are thiosalicylic acid, hydroquinone, thiuram E, phenylhydrazine, thiourea, 1-naphthol, phenol, and ascorbic acid. An interesting trend observed in the past 2 years is the increased application of kinetic measurements in quality control of real samples. A typical example of this is the application made by Fridman et al. (163),who measured the rate of titanium dissolution in fluoride ion containing baths used for acidic etchin and brightening of semifinished articles made of titanium ancfits alloys. The initial rate measurement is taken as indicative of the bath composition and is proposed for monitoring. Determination of nitrite in water samples by stopped-flow measurement has been proposed by Koupparis et al. (164). Both a single-point and a multipoint procedure are reported based on the diazotization of sulfanilamide. The product of this diazotization is coupled with (1-naphthy1)ethylenediamine dihydrochloride to yield a highly colored azo dye which is absorptiometrically monitored a t 540 nm. As many as 360 samples per hour can be processed in the concentration range of 0.025 to 2 ppm of nitrite nitrogen. The determination must be carried out promptly on fresh samples to prevent the bacterial conversion of nitrite to nitrate or ammonia. The detection and semiquantitative determination of 1piperidinocyclohexanecarbonitrile,PCC, in illicit samples of phencyclidine by a fixed-time procedure has been presented by Baker (165). The absorbance of quenched solutions at 555 nm after a 45-min reaction time constituted the basis of the procedure. The absorbance is due to the reaction product with p-nitrobenzaldehyde and o-dinitrobenzene in 2-methoxyethanol. Gossypol in cottonseed extracts has been determined by a fixed-time procedure (166). The determination involves the reaction of gossypol with 1,3,5-benzenetriol (phlorogeucinol); the formation of a product with maximum absorption at 550 nm is spectrophotometrically monitored. Absorbance mea-

surement is recommended after 1.5 min reaction and the determination range is reported as 1 X loe5 to 8 X lo-* M gossypol.

KINETICS IN SOLVENT EXTRACTION AND OTHER SEPARATION APPROACHES The number of references encountered in the literature in the last 2-year period is about half the number reviewed in the previous review in this series (1). It is noted, however, that one of the plenary lectures in the First International Symposium on Kinetics in Analytical Chemistry was dedicated to the kinetics of solvent extraction of metal chelates of analytical interest (167). An automated system for kinetic studies of solvent extraction has been presented by Watarai et al. (168). It provides a continuousmonitoring of the rate of extraction and very fast data analysis by use of a novel Teflon phase separator and on-line minicomputer interface. This apparatus, which allows the measurement of absorbance changes in the organic phase during high-speed stirring, has been used to study the effect of stirring on the distribution equilibria of some n-alkylsubstituted dithizones (methyl, ethyl, butyl, and hexyl) (169). Observation of a reversible decrease in the absorbance of the organic phase under stirring conditions has led the authors to conclude that the liquid-liquid interface plays a significant role in the equilibrium. This interface is pictured as of intermediate polarity with respect to both phases but more like water than the organic phase. A rapid return to the original absorbance value in the organic phase is observed if stirring is interrupted. The extent of absorbance change increases with pH. The size of the alkyl group and the nature of the organic solvent also affect the degree of reversible absorbance change. In a separate paper, Watari and Freiser (170) also report on the partition rates of the zinc and nickel chelates of ethyldithizone, butyldithizone, and hexyldithizone between chloroform and aqueous phase solvent pairs. Contrary to what has been proposed for extraction with dithizone (Le., the rate-determining step is the formation of the 1:l chelate in the bulk of the aqueous phase) the results for the substituted dithizones can only be explained by chelate formation at the interface. The concluding remark offered by the authors indicates that in highly hydrophobic, surface-active extractant systems, the stirring speed (which generates interfacial regions) enhances the rate of extraction. The kinetics and mechanism of the nickel(I1) back-extraction by copper(I1) competition for 8-mercaptoquinoline in chloroform [pH 3.7-5.8, ionic strength 0.1 M, and 25 "C] has been reported by Haraguchi and Freiser (171). The exchange reaction proceeds in the aqueous phase by formation of two intermediates (partially unbonded nickel 8mercaptoquinoline chelate and the dinuclear mixed metal complex). The kinetics and mechanism of Ni(I1) extraction into chloroform by 8-mercaptoquinoline have been studied [pH range 5.9-6.8; ionic strength of 0.1 M, and 25 "C] by the same authors (172). Rate data seem to show that the ratedetermining steps are the reactions of the metal ion with the neutral and the anionic form of the ligand operating concurrently in the aqueous phase. In the same l i e of studies, Haraguchi and Freiser (173) also report on the equilibrium and kinetics of nickel(I1) ion extraction into chloroform solutions of 7-dodecenyl-8-quinolinol (Kelex 100, HL) at 25 "C and 0.1 M ionic strength. The extracted species is NiL2and the rate of extraction is described by a two-term expression. Both terms are first order in Ni(I1) and HL concentrations. The first term is independent of hydrogen ion concentration but the second depends on the reciprocal of the hydrogen ion concentration. The rate-determining step is reported to be two concurrent reactions of Ni(I1) with HL and L-. The first reaction (involving HL) occurs in the aqueous phase; the second (involving L-) occurs in the interface. The formation of the mixed complex PdCl2Loxine, LIX (from PdC13- and 2-hydroxy-5-nonylbenzophenone 65N, L) in the aqueous phase describes the extraction kinetics of palladium into chloroform solution of L from acidic chloride media (174). Samad et al. report on the kinetics of iron(II1) chloride extraction with tri-n-butyl phosphate (175). The kinetics and mechanism of interfacial mass transfer of zinc(II), cobalt(II), and nickel(I1) ions in a bis(2-ethylhexy1)phosphoric acid, n-dodecane, KNOB,water system have been presented by ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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Cianetti and Danesi (176). Some considerations on the effect of washing on the kinetic stability of the Cu(I1) complex with 1-pyrrolidinecarbodithioate extracted into isobutyl methyl ketone from strongly acidic media have been offered by Takada (177). The extraction rate of gallium from aqueous alkaline solutions and into kerosene by 7-(1-vinyl-3,3,5,5tetramethylhexy1)-8-quinoliiolis enhanced when a water-in-oil microemulsion is used as organic phase (178). The catalytic effect of LIX63 (5,8-diethyl-7-hydroxydodecan-6-one oxime) on the extraction of copper by LIX65N (2-hydroxy-5-nonylbenzophenone oxime) was considered by Carter and Freiser in 1980 (179). More recently, Kojima and Miyauchi (180) provided another detailed study in which the same catalytic effect is explained according to a controlling monocomplex forming reaction in the aqueous phase. Contact ion exchange has not received much attention in analytical chemistry. Bunzi and Schultz (181),however, point out that it could be used to desorb ions of interest very selectively from other materials with ion exchange properties [e.g., from inorganic or organic synthetic or natural polymers]. Motivated b this and other potential applications, they have recently stu4ed the kinetics of differentially small conversions in contact isotopic ion exchange and in contact ion exchange processes for Cs+ vs. H+and Cs+ vs. Na+ between two resin beads lying one on top of the other in pure water. Interruption tests and determination of the rates as a function of the force with which the beads were pressed together confirmed the theoretical assumption that diffusion of ions across the aqueous film between beads is the rate-determining step. Ultrasound increases the rate of approach to ion-exchange equilibria involving conventional ion-exchange resins in comparison with undisturbed systems, but its effect is less than that of average mechanical stirring. This effect has been discussed in terms of a cavitation action on the film diffusion (182). The kinetics observed in the sorption of uranium(V1) on strongly acidic cation exchangers has been reported by Cabicar et al. (183).

MISCELLANEOUS KINETIC ASPECTS OF ANALYTICAL INTEREST Despite of the increasing number of studies devoted to the kinetics of sample evaporation in flame and nonflame atomic absorption spectrometry, several discrepancies prevent the adoption of a general mechanism for the process. Substantial differences are found in the reported values of the activation energy for the evaporation of ions of the same element as a function of temperature. These differences appear as a break or inflection on the Arrhenius plot. L’vov and Bayunov have developed a diffusion-macrokineticmodel which explains these breaks as well as the differences between activation energies for the low- and the high-temperature ran es (184). Their paper also considers the impact of the mo el in explaining changes in sensitivity and in the extent of matrix effects due to changes in structure and the state of the graphite surface as a result of aging. Their considerations apply to graphite furnace, nonflame atomic absorption and is based on taking into account (for a sin le bore) the diffusion transfer of a material in its condenset phase in the porous graphite volume. Two different evaporation regions are postulated: (1)an effusion-kinetic region in which the evaporation rate is characterized solely by the value of the rate constant for evaporation, and (2)a diffusion-kinetic region in which the evaporation rate is dictated also by the value of the diffusion coefficient of the material in its condensed phase. In a subsequent paper, L’vov et al. applied the same model to study volatilization of samples using atomizers manufactured from nonporous materials such as pyrolytic carbon or metals (185). Musil and Rubeska (186) have included interaction of free atoms with the walls of the observation zone as a third process to be considered in the mathematical model of free atom formation and dissipation in electrothermal atomization. Most of the theoretical treatments describing atomization in analytical flame spectroscopy neglect the diffusion and/or kinetic effects on the atom distribution in the flame. A more generalized theory considering both diffusion and reaction kinetics has been advanced by Li (187). His considerations are directed to the descri tion of chemical interference, due either to dissociation, cornfination, or ionization, on the atomic distribution from a vaporizing aerosol particle. In a subsequent paper the theory is further developed to study chemical

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interferences in atomization at various dissociation and vaporization rates (188). Transfer catalysis is finding applications in analytical chemistry;Faigle and Klockow, for instance, report its use for the gas chromatographic determination of traces of chloride, bromide, iodide, cyanide, thiocyanate, and nitrate in aqueous samples (189). Tetra-n-butylammonium sulfate (the phase transfer catalyst) is added to the sample and the mixture is freeze-dried. The n-decyl derivatives of the anions are then formed by reaction with n-decyl methanesulfonate in acetonitrile. Gas chromatography of the esters permits attaining limits of detection (bmol in 10 mL of sample) of: chloride, 0.2; bromide, 1.3; iodide, 3.6; cyanide, 2;thiocyanate, 1.9; and nitrate, 5.9. Results for river water and rainwater are compared with those obtained employing independent methods. The kinetics of chromatography has been revisited by Weiss (IN), who presented a reformulation of the kinetic equations describing transport in a chromatographic column. This reformulation allows describing adsorption at different types of sites. The treatment makes use of some recent developments in solid-phase physics and derives a more general transport equation. Conditions necessary to incorporate tailing or permanent asymmetry in an isolated peak are also considered. Wells (191)presented evidence showing a catalytically enhanced mechanism for the formation of InCl in a gas chromatographic dual flame photometric detector for halogenated organic compounds. The catalytic action of nickel, tantalum, and activated stainless steel is discussed. Of interest to postcolumn reaction for chromatographic detection and continuous-flow systems incorporating reactors with immobilized catalysts (packed bed configuration) are the mathematical modeling and considerations offered by Nondek et al. (192). For first-order reaction kinetics their modeling results in a simple relationship between reaction band broadening and reaction rate coefficient as well as capacity factors for reactants and products. A mathematical relationship for the concentrationson both sides of a separation membrane under flow injection analysis conditions, as a function of time, has been presented by van der Linden (193). Based on the tank-in-series model, the relationship was experimentally tested for the case of gasdiffusion membranes. A simple model for the absorbance-time curves obtained when a flow injection sample processing system is used for introducing samples into a flame atomic absorption spectrometer is discwed by Tyson and Idris (194). The discussion is centered around (1)limited dispersion (anal0 e of discrete nebulization), (2) medium dispersion (stangrd addition method), and (3) high dis ersion (production of concentration-time profiles for Cali ration purposes). A rationalization of certain inconsistencies previously found in the flow-injection determination of chromium(VI), using 1,5-diphenylcarbazideas chromogenic reagent, has been advanced by de Andrade et d. (195). Their rationale is extended to the improvement of sensitivity and is based on the consideration of chemical kinetic contributions to the overall dispersion. The maximum signal was obtained with acid concentrations of 0.80 M or larger. Wade has briefly reviewed some search techniques for the optimization of chemical systems and shows the optimization of a flow injection determination of isoprenaline as well as maximization of the sensitivity of the catalytic polarographic wave of the uranyl nitrate chemical system (196). Details of a multivariable optimization procedure (extension of the modified simplex method) as applied to flow injection analysis have been given by Betteridge et al. (297).The approach was applied to the development of a flow injection procedure for the spectrophotometric determination of isoprenaline at the microgram per milliliter level. The complexity of the system is noted and considered to be the result of the complex kinetics of unsegmented continuous flow sample processing (198). This complexity is depicted in the form of dispersion resulting from mass transfer and mixing during sample transport by the carrier stream to the detector. The occurrence of chemical reactions during the transport can alter diffusion and convection. A review of dispersion in fields which have much influenced the views of those concerned with dispersion in continuous-flowsample processing and the limited attention paid to chemical kinetics in such systems has been recently published (199).

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A contribution of significance to the fundamental aspects of minimization of errors in fiied-time reaction rate procedures has been provided by Holler et al. (200). By application of the theory of propagation of errors they demonstrate that instantaneous rate measurements at a time t = l/k, with k = first-order or pseudo-first-order coefficient, yield reaction rates that are essentially independent of changes in parameters affecting the value of the rate coefficient. They analyze in detail the effect of temperature and show that experimental data obtained for the oxidation of iodide by HzOz are in excellent agreement with theory. Choosing the measurement time close to t = l/k should considerably improve precision and accuracy in fixed-time procedures, even for chemical systems which do not follow strict first- or pseudo-first-order behavior. A phase-plane method for treatment of kinetic data and evaluation of kinetic parameters has been proposed recently (201). The method has been compared with the Guggenheim method and was found to be advantageous when data are noisy, have a large base line contribution, or come from region(s) other than the optimum fitting one. The method is usefully applied to automated instrumentation and to transient signals which cannot be replicated to optimize measurement parameters. The advanta e of using replicate measurements over averages of provifing quantitative and objective tests of postulated mechanisms in kinetic studies has been discussed by Phillips et al. (202). Application of enzyme-catalyzed reactions to chemical analysis has increased in the recent past and constitutes a large portion of everyday clinical chemistry. Substrate concentrations derived from rate measurements are limited by the value of the Michaelis-Menten constant, K,. Substrate concentrations amenable to determination have to be smaller than K , to avoid loss of sensitivity by departure from linearity of working curves. A data-processing method resulting in a linear range of substrate concentration that exceeds the value of K , has been presented by Hamilton and Pardue (203). Nonlinear regression is used to fit data for absorbance and rate vs. time, collected for about 80% reaction, to the rate expression derived from Michaelis-Menten kinetics. The determination of uric acid by following its oxidation catalyzed by uricase was used to evaluate the method. Calibration curves linear for substrate concentration up to 3.5 times K are reported. The kinetic results agree with equilibrium-base8 results and are not affected by temperature and inhibitor level (added to increase the value of K,). Multiple nonlinear regression has also been applied by Hamilton et al. (204) to the determination of serum triglycerides utilizing a commercially available enzyme reagent system. Absorbance values collected at 3-9 intervals (between 20 and 305 s reaction time) were fitted to a pseudo-firsborder model and the fit was used to compute the expected absorbance at equilibrium. The computed equilibrium absorbance values were linearly related to triglyceride concentrations. The use of immobilized enzymes in analytical chemistry is receiving continuous attention. Mercer and O'Driscoll(205) have developed a mathematical model to explain and predict various phenomena associated with multiple immobilized enzyme combinations. Their work centered on collecting experimental data from the coentrapment of invertase and glucose oxidase within the same gel particle [poly(2hydroxyethyl methacrylate)gel]. Their kinetic model provided suitable prediction of experimental results and computer calculations were inexpensive and fast. Incidentally, the same dual enzyme system was used analytically in soluble preparations (206). In a subsequent paper, Mercer and O'Driscoll (207) examined the model's use to predict the results of changes of relevant system parameters. Additionally, they computer evaluated hypothetical changes of some gel-related parameters. Several multipoint kinetic procedures were evaluated by Lin and Pardue for the determination of theophylline by prosthetic group label immunoassay (208). Multipoint measurements are made during the kinetic phase of an antibodylantigen reaction and are used to compute the concentration of the species of interest. While the conventional single-point measurement approach yielded a nonlinear calibration plot, the several multipoint regression data processing methods yielded linear calibration plots for theophylline concentrations in the 0 to 40 mg/L range. Nonlinear re-

gression has also been shown advantageous in the determination of the forward rate coefficients of the chemical step regenerating the catalyst [following the electron transfer process at the electrode surface] from substantially overlapped linear-sweep voltammograms in pseudo-first-order, kinetically controlled, electrocatalytic reactions (209). Using simulated reactions, Efstathiou et al. (210) developed an approach to the calibration of ammonia gas sensing electrodes under dynamic conditions. The procedure helps to overcome the relatively slow response of this type of sensors. Kinetic effects that result in time-dependent selectivities in halide membrane ion-selective electrodes were reported and discussed by Morf (211). Several inconsistencies and some relevant comments concerningresponse time in electrochemical cells containing ion-selective electrodes have been pointed out by Pungor and Umezawa (212). A single potential step chronoabsorptiometric (SPS/CA) approach to the characterization of irreversible heterogeneous electron transfer reactions was proposed in 1979 (213). This has been used for the determination of the kinetic parameters of several biological species at different optically transparent electrode surfaces. The extension of this SPS CA approach to heterogeneous electron transfer systems wit some degree of electrochemical reversibility has been reported recently (214). Validation of the method was illustrated by determining electron transfer kinetic parameters for the oxidation of ferrocyanide at platinum optically transparent electrodes (phosphate buffers, pH 7.00). Bishop and Cofre have determined the charge-transfer kinetic parameters at glassy carbon for the generation of chlorine and evaluated the generation current efficiencies (215). Electrogenerated chlorine has found application in coulometric determinations of a variety of organic as well as inorganic species. Kinetic Parameters and current efficiencies for manganese(II1)production from manganese(I1)have also been examined by the same authors (216). Three electrode materials were included in the study: platinum, gold, and glassy carbon; the authors conclude that gold is the most adequate electrode material for manganese(II1) generation. Manganese(II1) is also of analytical interest in coulometric titrimetry. The use of single coulostatic decay curves to obtain concentration and Ellz information in the 20-100 ms time range has been considered by Reiss and Neiman (217). Because of the time scale accessible by the approach the authors suggest its use in liquid chromatographic detection and in flow injection analysis. The procedure developed by Nicholson some years ago (218) for the determination of standard heterogeneous rate coefficients by cyclic voltammetry at 25 "C has been now extended for temperature-dependentmeasurementsby Schmitz and van der Linden (219). A simple method to extract rate coefficients of a chemical reaction followed by electron transfer and of electron transfer followed by a chemical reaction using mechanical square wave polarography has been introduced (220). The idea was tested by studying the dissociation of the antimony(II1)-EDTA complex, the dissociation of monochloroacetic acid, and the benzidine rearrangement of hydroazobenzene resulting from electrode reduction of azobenzene. A kinetic model for the electrodeposition of heavy metal ions in solution a t parts per million and parts per billion concentration levels is proposed by Sioda (221). The formation of a metal monolayer and simultaneous deposition and dissolution constitute basic assumptions for development of the mathematical relationships describing the model. Silicon is frequently determined by formation of the colored 12-molybdosilicates. Kircher and Crouch (222) have recently discussed in a detailed manner the formation (from a kinetic-mechanistic viewpoint) of the 0-12-molybdosilicate in "03, HZS04, and HC104at 25 OC, 1.0M ionic strength, and in the pH range 1.2 to 1.8. From their study based on stopped-flow mixing and photometric monitoring, relevant considerations for the reaction-rate determination of silicon are derived (1)a reagent molybdate solution acidified with HzS04 would provide the best sensitivity, (2) there is need to control the ionic strength of samples and standards, and (3) reproducible analytical concentrations of acid and molybdate between samples and silicon standards are needed. Their studies also consider the mechanism of decomposition of the p-12-

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molybdosilicate in basic medium. A detailed kinetic study, aided by stopped-flow mixing, of the formation of 12-molybdophosphate in perchloric and nitric acid media has been performed and reported by Kircher and Crouch (223). Comments regarding implications for reaction rate determination of phosphate are part of this paper. The stoichiometry and kinetics of the Jaff6 reaction (widely used for the determination of creatinine in biological fluids) have been studied by Diamandis and Hadjiioannou (224). A picrate ion selective electrode was used as monitoring tool. Commonly inculcated from undergraduate training is the requirement that chemical reactions used in titrimetric procedures must be fast in reaching equilibrium. A titrimetric approach applicable to rather slow reactions h been proposed by Efstathiou and Hadjiioannou (225). Under computer control the end point is detected by monitoring the rate after addition of different volumes of titrant and when a preselected kinetic condition is fulfilled. The dynamic situation generated by a titration carried out under conditions in which the rate of the titration reaction is compared with the rate of titrant addition defines what is called kinetic titrimetry. A handful of papers have appeared in the literature since the early 1960s. More recently Rehm and Bright (226) have solved the equations describing a first-order kinetic titration by reducing them to a single nonlinear ordinary differential equation. The reaction between permanganate and oxalate ion can be termed as a “classical” analytical reaction which has been applied in titrimetric and colorimetric procedures and ancillary analytical steps through many years and is still in use. Manganese(I1) ions, one of the reaction products, exert a promoting effect during the earlier stages of the reaction. Koupparis and Karayannis have recently reexamined the kinetics of this reaction by stopped-flow spectrophotometry and have proposed methods for the determination of manganese(I1) and oxalic acid (227). These reaction rate methods are implemented by monitoring at 525 nm the rate of permanganate ion disappearance and relatin the rate changes to the initial concentration of determinet species. A method for the interpretation of direct titrations of natural waters with trace metals was presented by RuiiE (228). The method can be used effectively only if the concentration of the free trace metal is determined under equilibrium conditions. Consequently, RuiiE and NikoloviE (229)presented some theoretical predictions on the influence of the kinetics of complex formation on such titration curves. A rapid, easy, and inexpensive kinetic determination of fibrinogen has been proposed by Denegri and Prencipe (230). The blocking of light transmission by the fibrin clot is measured by a fixed-timeprocedure in a centrifugal analyzer. No interference from fibrinogen degradation products was observed even at concentrations as high as 600 mg/L. A unique use of light in kinetic based methodology was proposed by Mattusch et al. during the First Symposium on Kinetics in Analytical Chemistry (231). The unique use in question is catalytic activation by light and was illustrated with three examples: (1)direct photoactivation (photochemical acceleration of HzOzdecompositionin presence of copper as catalyst), (2) indirect photoactivation (photosensitized autooxidation of sulfite, which allows the determination of manganese, at parts-per-billion levels), and (3) direct photoactivation of catalyst (application tQ the tris(oxalatQ)iron(III) system in the redox reaction between methyl orange and bromate ion). The analytical payoff of this activating effect is better selectivity and increased sensitivity. The advantageous characteristics of lanthanide chelates of europium and terbium over conventional fluorescent probes for use with ordinary fluorometers are discussed by Soini and Kojola (232). They describe the design and operation of a suitable fluorometer and promise in a future publication to report on the preparation and use of the lanthanide chelates. An approach of interest to geoanalyticalchemists has been reported by Rischak (233) who has developed an apparatus for differentiation of different carbonates (e.g., calcite, dolomite, and ankerite) in rocks based on the rate of COz(g) evolution upon reaction with 20% (volume) HC1. The use of simple ketones in aqueous solutions is a situation encountered in analytical methodology and a knowledge of the pK, of the ketone may be useful. Guthie et al. have proposed a kinetic method for determining directly the pK,s 108 R


of ketones in aqueous solutions which eliminates the uncertainties of available indirect methods (234). Antimony(II1) is a serious interference of the Cr(V1) oxidation of ferroin. This effect was exploited by Yonehara et al. (235) for the determination of antimony(II1) in marine sediments in the 0.4-10 pg/mL range. The method involves monitoring the decrease of ferroin at 510 nm, preparing plots of log absorbance vs. time, estimating the intercept value at time zero, and preparing calibration plots of absorbance at intercept vs. antimony concentration. In a search for a coupling system suitable for the determination of substrates or enzymes in biological fluids in alkaline media, Karayanis and Siskos (236)studied the kinetics of the reaction of dichlorophenolindophenol with NADH in the presence of phenazine methosulfate. Their initial rate, stopped-flow studies in 0.10 M glycine buffer of pH 9.00, indicates the possibility of determining NADH in micromolar concentrations with an average error of 3 ‘70. A comparison of five different, commercially available, test kits for the kinetic determination of creatinine has been reported by Hoeflmayr (237). The method involves the use of picric acid and is a modified Jaff6 method without deproteinization. All kits have acceptable reproducibility and accuracy in the normal range, but two gave unacceptable results in the pathological range. Even though thermodynamic considerations favor the precipitation of CuS in the presence of excess Sz-and moderate concentrations of EDTA, a kinetic inhibition of such precipitation by EDTA has been reported (238). In an effort to learn why precipitate-basedion-selective electrodes do not behave ideally at low concentrations, Lindner et al. (239) have examined the dynamic behavior with the so-called actively step method. They report in the lower concentration range the response time increases and is no longer flow rate dependent. They conclude that the rate of electrode response becomes limited by some other processes slower than film diffusion.

INSTRUMENTATION AND COMPUTERS For the first time in at least 6 years, there has been a very significant increase in the number of papers dealing directly with instrumentation, automation, data handling, detectors, etc., which are designed specifically for kinetic measurement and/or analysis. In the past most instrumental advances were for fast kinetic methods, fast computer data acquisition, or clinical analyzers. Most of this instrumental research had little impact on analysis because of experimental complexity and expense of these techniques. However, with the introduction of the flow injection method (FIA) of making kinetic measurements, which are relatively simple and inexpensive instrumentally, the rate of development of research devoted to the developmentof inexpensive and practical kinetic methods of analysis based on FIA has been explosive. This was dramatically evident in the number of papers devoted to FIA at the recent First International Symposium on Kinetics in Analytical Chemistry (mentioned above in the introduction). Because of the tremendous practical applicability of FIA to chemical analysis, instrumentally oriented analytical chemists have responded rapidly to the demand for new hardware. Were this demand enough alone, simultaneously high performance liquid chromatography (HPLC) analysis has also had an exponential applicability and has inspired a demand for liquid flow system instrumentation development, especially with respect to detection systems. Papers on HPLC instrumentation, although not directed to kinetic methods in general and FIA specifically, are cited below generally without differentiation as the instrumentation can be adapted directly to kinetic analysis. Of course, there are numerous other publications devoted to instrumentation and automation designed for kinetic methods of analysis and/or kinetic measurement applicable to kinetic methods of analysis which are listed here, also. It should be pointed out here that there are several papers which contain instrumental development but which are cited elsewhere in this review because the major thrust of the paper was either a discussion of a specific analysis or kinetic method. With respect to FIA, there are two “Reports”in the A-pages of this Journal that are excellent overviews of the concepts, rinciples, theory of flow systems, instrumentation, applicahlity, and limitations, etc. The material presented in these “Reports” is not an applications review but is completely

KINETIC ASPECTS

comprehensive with respect to basic concepts, methodology, and instrumentations. Furthermore, the references are comprehensive and current with respect to the above aspects of FIA through date of Publication (early 1983). One of the authors of this review (H.B.M., Jr.) uses these two reports as the “textbook” on FIA in a graduate analytical course in Kinetic Methods of Analysis. There is no need to review these reports here, as there is no way that we could improve their discussion and evaluation of FIA here. The readers should go directly to the papers and the references cited. The first “Report” by Stewart (240) presents the state of the art (as of the end of 1981) and nature of each modular unit of an FIA instrument. It is written as one would write a textbook chapter on the subject. The basic point is given in the conclusion, “FIA... is a kinetic measurement system in which the system is normally not completely mixed and radial and axial sample concentration profiles are time-dependence functions”. This, of course, is the critical point that makes FIA techniques so practical with respect to all other fast mixing techniques for analysis. The second “Report” by Ruzicka (241) traces FIA “From test tube to integrated microconduits”. The historical discussion is brief but very interesting and the major trust of this paper is the latest developments in instrumentation and theory. As pointed out in the report, the recent development of the so-called integrated microconduitunits is a real giant step forward in the development of FIA as a practical and easily employed method of analysis and will make all present FIA instrumentation obsolete. The important point is that “the integrated microconduit design of the flow manifold is a system of integrated conduits situated in a permanent ri id and planar structure” (241,242). There is an extensive fiiscussion of the design of integrated microconduits and detectors compatible with them plus recent references in this report. Also included is an excellent analysis of the hydrodynamics and concentration gradients of the flow system. Of special interest is the discussion of the applicability and limitations of FIA in the next generation of design of “autoanalyzers”. Specific recent research papers on instrumental developments in FIA and flow detection systems not referenced in the above two reports (240, 241) are discussed below. A very sophisticated combination continuous flow FIA system which is computer automated has been designed by Hooley and Dessy (243). This system provides convenient reagent conservation, utilizes an inexpensive multiplier detector head colorimeter system,and contains a pump-free,dual flow-detector flow controller transport system coupled to multiple LE /photodiode colorimeter heads. The s ecial feature of this unit is that all elements use computer fielaback control for stability and application purposes. Stewart and Rosenfeld (244) have reported the design of a small mixing chamber for FIA systems that is applicable to colorimetric, fluorometric, conductometric, and flame emission detectors. Ogata, Taguchi, and Imarari (245) have developed and tested a unique phase separator for FIA solvent extractions systems. Fossey and Cantwell (246) have characterizeda FIA apparatus utilizing a membrane phase separator and a constant pressure pump. The effect of variations of extraction coil length, sample injection volume, and flow rates are given. Rossi et al. have examined the optimization of a FIA system for multiple solvent extractor (247). The effects of solvent segmentation devices, extraction coils, and phase separators are considered. A microsample filtering device for HPLC or FIA has been described by Gardner and Vanderploeq (248). One of the major areas of instrumental research in FIA and other flow analysis systems has been in the area of detector development. Harris (249) has demonstrated the advantages and limitations of a laser-induced fluorescence detector for FIA. The advantages include small volume sampling,lowjsk of contamination, and a large dynamic range for a given expenditure of time, reagents, and ultratrace levels of analysis. Electroanalytical methods of detection have received a great deal of attention. Simpson and Holler (250) have developed a potentiometric detection system utilizing microelectrodes for determining proton concentration. What should prove to be a very valuable electrochemical detector system has been developed by Caudill et al. (251) which is an amperometric cell containing an electrode consisting of 100 small disks constructed from carbon fibers. What is important is that this detector has improved signal to noise ratios because the

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ANALYTICAL CHEMISTRY

response is independent of flow rate in the system. Wang and Frelha (252) have demonstrated that selective electrode adsorption can be used advantageously as a preconcentration step for the desired analyte in the presence of 102-foldexcess of a similar nonadsorbed species with a similar redox potential. They used differential pulse polarography to determine chloropromazine in nanogram amounts in urine with no sample pretreatment. The response of this system was characterized with respect to preconcentrationperiod, solution flow rate, and carry-over effects. Caudill et al. have developed rapid scanning electrochemical detector for channel-type flow cells using lassy carbon and carbon fibers as working electrode materials f253). Wang (254) has designed a unique electroanalytical flow detector by modifying a dental oral irragating appliance. Kafil and Huber (255) have developed a nickel oxide electrode amperometric detector for organic species in nonaqueous FIA systems where an oxidative process is necessary. A flow-through cell detector employing differential pulse anodic stripping voltammetry which has very low detection limits has been designed by Martens and Johansson (256). Selective electrodes have been employed as detectors in flow-through cells in a number of papers. On-line monitoring with ion-selective electrodes in high volume flowthrough cells has been developed by Bond et al. (257). Limitations of linear response in FIA with ion-selective electrodes have been examined by Trojanowicz and Matuszewski (258). The use of a lipid bilayer membrane electrode as an FIA detector has been described by Thompson and Krull (259). Hanekamp and deJong (260) have made a theoretical comparison of the performance of several flow-through electrochemical detectors. Several flow-through electrochemical detector performances and designs have been discussed by Eg ers et al. (261),Wehmeyer et al. (262),Krull et al. (263), an8 Kissin er (264). The advantages of the use of multiple electrode letection systems is presented in a “report” by Roston et al. (265). A comparison study of the performances of some types of liquid phase flow-through detectors has been made by Poppe (266). FIA with inductively coupled plasma (ICP) atomic emission detection has been developed by Zagatto et al. (267) and an FIA atomic absorption unit has been described by Nord and Karlberg (268). An FIA based calibration method for atomic adsorption has been developed by Tyson et al. (269).

A spectrophotometric series differential detection systems has been given by Leach et al. (270)for the trace determination of metals. The effect of sample refractive index on the analysis where the absorbance and refractive index responses are distinguishable by their respective peak shapes were examined. Vogt et al. have desi ned an optically discriminating flowthrough cuvette (2717. Lyons and Faulkner (272) have developed a ray-tracing algorithm for the evaluation of various fluorescence flow cell designs. They show that certain cell designs optimize the detection responses with respect to light scattered by reflection and refraction from cell walls. A nanoliter-volume flow-through fluorometer detector has been published by Vurek (273). A potentially useful nuclear magnetic resonance (NMR) flow probe design has been developed by Haw et al. (274). Although this is obviously a very expensive detector system for flow systems, it could prove to be valuable for organic systems (such as fuel systems used in the paper) which are not amenable to spectrophotometric, fluorometric, electroanalytical, or other conventional transducers. It is important to note that 10-pgdetection limits were obtained as NMR is not usually considered to be a sensitive analytical method. With respect to instrument development, Efstathiou et al. (275) have devised a low-cost variable-time ratemeter for reaction rate measurements. Simpson et al. (276) have developed a very unique microdroplet mixing system for rapid reaction kinetic measurement with Raman spectrometric detection. Enzyme reactors for unse mented FIA have been designed by Johansson et al. (277) ancfOlsson and Ogren (278) have studied the design of packed-bed enzyme reactors. Patton et al. (279) have developed an electronic bubble gate for colorimetric air segmented continuous flow analysis. Gradient techniques and hydrodynamic injection techniques in FIA have been discussed by Olsen et al. (280) and Ruzicka and Hansen (281). A catalytic-kinetic absorptiostat has been designed by Weisz et al. (282). ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984

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Gerhardt and Adams (283, 284) have used FIA for the determination of diffusion coefficients. The applicability of phase-plane and Guggenheim methods for the treatment of kinetic data have been examined by Bacon and Demas (285). Haemmerli et al. (286) have devised a FIA system for the determination of response times of microelectrodes. A stopped-flow instrument for reaction rate measurement with immobilized enzymes has been constructed by Thompson and Crouch (283, and Wang and Dewald (288) have built a rapid stopped-flow voltammetry system which employs potential scan capabilities. Ruzic and Nikalic have carried out a theoretical study of the effects of kinetics on direct titration curves (289). Lemke and Hieftje (290)have developed a direct computer-controlled pH-stat. A microprocessor-controlled FIA instrument has been built by Strandberg and Thelander

(291).

With respect to theoretical developments in kinetic measuremenb, Geren and Mdett (292)have developed a nonlinear regression analysis program in BASIC for microcomputers and modified spectrophotometers for use in enzyme substrate kinetic analysis. A very useful study of the minimization of flow-associated noise in electroanalytical detectors in flow systems has been described by Weber (293). Experimental verification of the signal processing techniques is given. A detailed study of the effect of noise in the reaction monitor and of reaction condition reproducibility on the precision of rate measurements has been carried out by Ingle et al. (294). An approach for the optimization of FIA has been proposed by Reijn et al. (295)and Angelova and Holy (296) have studied the optimal speed as a function of system performances on continuous flow methods. With respect to computer advances in chemical instrumentation and automation which have direct applicability to kinetic methods of analysis, Frazer et al. have described how a simulation of chemical reactions can aid in the design of experimentation (297). The point was illustrated for the study of enzymatic kinetic reactions. A comprehensive discussion of the design of operating computer systems for the chemical laboratory has been given by Dessy (298). An in-depth discussion of “Electronic Instrumentation and Microcomputer’s Principles and Practice” for the microcircuit age has been given in a report by Enke et al. (299).

ACKNOWLEDGMENT Research support by the National Science Foundation, of which this review is a byproduct, is acknowledged here. LITERATURE CITED

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