Cereal Sci. Today, 15, 140 (1970); Anal. Abatr., 20, 4295 (1970). (537) Walker, R. I., Shipman, W. H., J . Chromatogr., 50, 157 (1970). (538) Wall, R. A., Anal. Biochem., 35, 203 (1970). (539) Wall, R. A,, J. Chromatogr., 60, 195 (1971). (540) Walter, C. W., Korkisch, J., Mikrcchim. Acta, 1971, 81, 137, 158, 181, 194. (541) Watanabe, H., Bunseki Kagaku, 19, 1658 (1970). (542) Webster, P. V., Wilson, J. N., Franks, M. C., J. Znst. Petrol., 56, 50 (1970). (543) Wenzel, E., Oppolzer, R., Schuster, D., Mikrochim. Acta, 1971, 680. (544) Wernet, J., Wahl, K., Fresenius’ Z. Anal. Chem., 251, 373.(1970). (545) Wetterquist, H., Whte, T., Scand. J. Clin. Lab. Invest., 25, 325 (1970); Anal. Abstr., 20, 4146 (1970). (546) White, S. H., Loken, M. R., Shields, C. E., Clin. Chem., 16, 861 (1970). (547) Widtmann, V., Hutn. Listy, 25, 733 (1970); Anal. Abstr., 21, 2601 (1971).
(548) Winkler, R., Sansoni, B., Starke, K., Radiochim. Acta, 15, 65 (1971). (549) Wolf, W. J., Thomas, B. W., J . Chromatogr., 56, 281 (1971). (549a) Wolford, J. C., Dean, J. A., Goldstein, G., ibid., 62, 148 (1971). (550) Wong, K. M., Anal. Chim. Ada, 56, 355 (1971). , (551) Woodroofe, G. L., Munro, J. D., Analyst, 95, 153 (1970). (552) Wu, C. M., McCready, R. M., J. Chromatogr., 57, 424 (1971). (553) Yaguchi, M., Perry, M. B., Can. J. Biochem., 48, 386 (1970). (554) Yasuda, K., J. Chromatogr., 60, 144 (1971). (555) Ying-Mao Chen, Fu-Pa0 TSBO,J. Chinese Chem. SOC., (Taipa), 17, 81 (1970). (556) Yoshikawa, Y., Yamasaki, K., Kagaku No Ryoiki, 25, 164 (1971); C.A., 74, 1 3 0 7 7 8 ~(1971). (557) Yoshio, M., Waki, H., Ishibashi, N., J. Znorg. Nucl. Chem., 32, 1365 (1970).
(558) Zagorodnyeya, A. N., Lebeder, U. D., Ponomarev, V. D., C.A., 74, 1307% (1971). Tr. Znst. Met. Obogashch., Akad. hauk. Kaz. SSR, 1970, 97. (559) Zaharescu, T., Rev. Roum. Chim., 16, 775 (1971). (560) Ziegler, M., Ziegeler, L., Winkler, H., Mikrochim. Acta, 1970, 1312. Zhitenev, V. A., Zh. Prikl. Khim., 42, 1699 (1969). (561) Zhukov, A. I., Kazantsev, E. I., Zhitenev, V. A., Zh. Prikl. Khim., 42, 1699 (1969). (562) Zima, S., Giacintov, P., J. Rad& anal. Chem., 7, 19 (1971). (563) Zlatkis, A., Buening, W., Bayer, E., ANAL.CHEM.,42, 1201 (1970). (564) Zsinka, L., Szirtes, L., Radiochem. Radioanal. Lett., 2, 257 (1969). (565) Zsinka, L., Szirtes, L., Radiokhimiya, 12, 774 (1970). (566) Zsinka, L., Szirtes, L., Acta Chim. Acad. SCi. Hung.,69, 249 (1971).
Ion Selective Electrodes, Potentiometry, and Potentiometric Titrations Richard P. Buck, Kenan laboratory o f Chemistry, University o f North Carolina at Chapel Hill, Chapel Hill, N.C.
T
HE LITERATURE COVERED in this review includes recent developments in the area of potentiometry which were published since the last review by Toren and Buck (944). The final issue of Chemical Abstracts consulted was Vol. 75, No. 22, November 29, 1971. The format of this review is altered in comparison with earlier reviews by a clear emphasis on ion selective electrodes and their applications in all fields of applied science. This comprehensive survey has required deletion of some topics previously reviewed : nonaqueous titrations, equilibria in aqueous and nonaqueous solvents by potentiometric methods, electrodes and cell systems pertaining primarily to batteries, fuel cells and electrochemical synthesis, and potentiometry in molten salts. A few exceptions are included in the final section. We have, however, retained a section on standard potentials in nonaqueous and mixed solvents together with a summary of fundamental papers on potentiometry in mixed solvents and the principles of membrane potentials. Most potentiometric application papers are listed in tables according to the type of electrode used. BOOKS, REVIEWS, AND SURVEYS
The selective ion electrode field is one of the most active and flourishing branches of potentiometry. While there were nearly two hundred fundamental 270 R
and application papers referred to in the previous review, the number has now risen to at least 500, including glass electrode developments and applications. The number of review articles and books has increased abruptly as well. Although already out-of-date in this rapidly advancing field, the chief source book and compilation is that edited by Durst, “Ion-Selective Electrodes” (231). A new book by Moody and Thomas “Selective Ion Sensitive Electrodes” (640) has just appeared. A chapter by Buck (128) in “Physical Methods of Chemistry” emphasizes selective electrode principles. Theoretical principles are treated comprehensively by Eisenman (243) while compositions and performance are described by Ross (808). Recent general reviews of potential response theory and experimental characteristics of glass, solid-state, synthetic solid ion exchange, liquid ion exchange, and neutral carrier membrane electrode systems are by Cammann (140-142), Covington (179), Durst (232), Florence (282), Liteanu (666) in Rumanian, Pearson (738), Simon (884, 888) and Taubinger (988). N onglass electrodes are also reviewed by Ishibashi (409) in Japanese and Moody et al. (639, 641). Shorter reviews are by B a g (34) and de Carvalho (369, 370), Karasek (447) and Ray (782). Applications emphasizing techniques and thermodynamic
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
27574
measurements are by Durst (229), Rechnitz (784, 786), Oehme (696) in German, and Butler (133). Although heterogeneous membranes of the Pungor type are included in these reviews, specific reviews by Covington (1 77) and Pungor and Toth (773) are recommended. Applications of selective electrodes in various fields are : industrial processes (67, 661, 662), electroplating (284), toxicology and industrial hygiene (128), water and air pollution (17, 146, 691, 796, 797, 889, 1026). Sensitizing of ion selective electrodes for measurement of materials via an intermediate chemical reaction product was achieved long ago in the development of the well known p C 0 ~electrode. This, together with immobilized substrate electrodes, e.g., enzyme or liquid ion exchangers, have been reviewed by Huang (396) in Japanese, Guilbault (334), Mueller (649),and Rechnitz (787). Glass electrodes for pH and pM measurements have been reviewed by Truesdell (963) and Galster (297) while the principles of pH measurement and some commercial pH meters are reviewed by Woudsma (1022) in Dutch. Other pH reviews are: sterilizable pH electrodes (662), pH control in industry (746),and pH electrodes in fermentation (401). Bates has reviewed pH measurements in nonaqueous and mixed solvents (48)*
General reviews of potentiometry including most of the topics of this paper are by Lee (518), Dubbeling (113), and a part of the book “Electrometric Methods” edited by Browning (117). Other books on titrimetry and titrimetric instruments are by Wagner and Hull (987) and Kantere, Kazakov, and Kulakov (445), the latter in Russian. Reviews on redos titrations and complex formation are by Erdey (151,151) and Musha (655) in Japanese. Automatic titrators are reviewed by Muto (660) in Japanese and Strafelda and Dolezal (910) in Czech. A review in German on complexometric titrations including potentiometric techniques is by Kraft (494). Electroanalytical techniques including potentiometry are reviewed by Harrar (361), Kelley (463), and Furdy (777). The latter emphasizes clinical analyses while Gnanasekaran and Narasimham review applications in fertilizer analysis (315). Electrometric methods for determination of sulfur a t low levels were reviewed by Kamaeva et al. (443). Source papers on the electrochemistry of ion exchange resins (316) and liquid ion exchangers (761) have been noted. Two reviews on the properties of solid electrodes (384) and solid-state cells (966) are relevant to this field. TITRATION CURVE ANALYSIS AND EQUILIBRIUM CALCULATIONS
Linearization of logarithmic sigmoid potentiometric titration curves before and after equivalence points was achieved by Gran some 20 years ago. It is the renewed interest in titrimetry monitored by selective electrodes that has brought Gran’s methods (321, 313) into common use (184). Coupled with data acquisition computers, the linearization schemes have produced new automation techniques, described in a later section, and improved rapidity and accuracy in determining equivalence points. Data analysis of linearized potentiometric curves i 3 also a powerful method for detecting and interpreting complexation and precipitation equilibria. Cormos and Marusciac have explored the linearization for strong and weak monoprotic acids and bases (173) and the polyprotic general case ( 1 7 1 ) . Experimental results treated by their methods give an equivalent volume scatter less than 0.05 ml. Sources of error and confidence limits were investigated by the same workers (172) and by Jandera and coworkers (426). Errors introduced into the linearization scheme by incomplete reaction, number of data points, and concentration of constituents were discussed by Rosenthal, Jones, and Megargle (804). An iterative curve fitting technique for redox titration curves has been explored by Marmasse (687). Choi (166) has
given a general treatment of the rate of potential change in the vicinity of the equivalence point in terms of the stoichiometric coefficients of each half cell. The position of the inflection relative to the equivalence point depends in general on reagent concentration. Conditions are given. Wolf (1018) has reviewed the Fortuin nomograph method for predicting equivalence points, but reduces the nomograph to two simple equations. Wijnne (1011) describes equations and graphical nddition of elementary curves to generate overall titration curves. Parameter values needed to produce precise successive titrations were summarized by Crisan (182) and by Hamann (360). Hannema et al. (356) give conditions for sharp potentiometric back-titrations. Properties of second derivative titration curves for strong acid-base titrations were discussed in detail by Meites and Meites (615). An analog computer has been used to perform the second derivative of potential us. volume calculation and to indicate the end point (306). A logarithmic diagram relating titration error and electrode potential for cases of noncoincidence of the inflection end point and the equivaience point has been suggested (400). Interfering ions in sample solutions or in titrant cause errors in potentiometric (850) and chelometric (849) titrations using imperfectly selective electrodes. Schultz (849, 850) has shown how Gran’s linearization plots can be used to advantage to minimize errors by reliance on points early in the titration where interference is least effective. Carr (147) has given a similar error analysis for precipitation titrations monitored by ion selective electrodes. Finally Chernova et al. (163) propose a statistical treatment of titrimetric results to distinguish between systematic chemical effects and random scatter. Dobrynina et aZ. ($13) suggest integration under pH-metric curves for various titration conditions as a means of calculating concentrations of species a t any point in a complexometric titration. The method was used to establish the formation constant of samarium ditartrate. Of course Bjerrum’s method of competing equilibria has already been the topic of extensive computer programming. A new Russian program for sequential formation constant determinations has appeared (677). An earlier English program SCOGS was corrected and improved by its author (837). A new program rRAVE for complex equilibria using Gran’s linearization of potentiometric titration data has been given by Ulmgren and Wahlberg (969). This program smooths data which are later subjected to LETAGROP. Rossotti et al. have reviewed various computer programs in this field (810). They cover simple non-
statistical programs, least-squares treatments, and complicated nonlinear search techniques. The same group (811) use an ALGOL procedure REFBAH to interpret Cu(I1) thiodicarboxylate species. Another iterative program in Fortran IV for pK values is by Blumenson et al. (88). Some other aspects of potentiometry have been treated theoretically. Mandel (581) has given an elegant treatment of the variation of pK’s with degree of titration for weak polyacids; and calculated titration curves are compared with experimental curves by Nagasawa (661). The oversimplification of the Henderson-Hasselbach equation in describing whole blood pH has been discussed by Linden and Norman (563). -4new and generalized three-parameter theory of buffer solutions and systems has been derived by Crisan (181). Fundamental papers on the use of potentiometric titrations in nonaqueous media to characterize acid-base properties of solvent systems and to select optimum conditions for acid-base titrations are by Bykova (136, 137) and Kozlenko (492). Redox and complexation titrations in anhydrous media are mentioned by Shul’man and Larionov (879). An interesting correlation of E o values for complexes of d1o ions on the basis of Eo us. E o diagrams has been found by Finkelstein and Hancock (172) and used to predict stabilities of complexes. I n case elementary logarithmic conversion is difficult, CFSTI has available conversion tables for pH to hydrogen ion activities (174). PROBLEM AREAS OF MODERN POTENTIOMETRY
I n a particularly lucid paper by Popovych (758), the linked problems of ionic soivation, medium effect, and junction potentials in nonaqueous and mixed solvents are presented. While a salt medium effect, Le., the difference in the free energy of the standard states in two solvents is measurable, thermodynamically inaccessible single ion medium effects are necessary for establishment of universal scales of activities (particularly acidities) in different solvents (113). Existing methods for the separation of single ion medium effects are analyzed in detail. Strehlow and Schneider have also reviewed these topics (911). Bykova and Petrov (135) use Ismailov’s procedure to establish acidity scales for the common alcohol, amine, nitrile, and sulfoxide solvents. Kalidas (441) reproduces the NBS system for establishment of relative pH scales. Kundu et al. (510) using the Cs/Cs+ electrode in glycol-water solvents conclude that Strehlow’s extrathermodynamic assumptions do not apply in this case. The same sssumption, uiz., potential constancy for large size univalent reference couples in all
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
271 R
solvents, was applied to the ferrocene reference electrode to determine acidity scale in and autoprotolysis of glacial acetic acid (228) and solvation of silver ion in ether solvents (194). The Feakins group has studied transport and free energies of solvation in MeOH-H,O (266-268). Acidity scales, buffers, and halide reference electrode behavior have been investigated in MeOH-H20 (628, 732, 911), EtOH-H20 (664, 829-831), methoxyethanol-H20 (792), iso-PrOHH20 (643, 861), l~t-BuOH-H20 (643, (813), EtOH-benzene-H2O (IO),glycolic solvents (609, 827), DMSO-H20 (26, 473,474) , and methylpyrrolidone-tetramethylene sulfoneH20 (633). Free energies of transfer of KC1 between water and EtOH, dioxane and a c e t o n e H20 mixtures have been measured by De Ligny’s group (66). The mercurymercurous acetate electrode has been thoroughly characterized in dioxanewater mixtures (46, 46). Extensive measurements of acidity and dissociation constants of 26 acids and bases in D M F have been reported by DemangeGuerin (193). An intriguing example of the measurement of “real” potentials (and the “real” activity) is reported by Zaslavskii, Rybkin, and Vedmedenko (1089) who established an acidity scale in ammonia solutions and measured junction potentials against KC1 solutions. Closely related to the medium effect is the problem of junction potential theory and measurement of junctions between ionic solutions in different miscible solvents. Gaboriaud (296) proposes an a priori calculation for the case in which both solutions are saturated with the same salt. Van Veen and coworkers (964) show experimentally the consequences of the liquid junction potentials on the determination of pH and pK’s of acids in mixed solvents. Zegzhda et al. (1033) measured junction potentials between an aqueous saturated calomel electrode and acetic acid solutions while Jakuszewski et al. (&3) examined junction p.d.’s a t aqueousalcohol interfaces. Liquid junction potentials in entirely aqueous systems still>receive theoretical and experimental attention. I n a series of papers, Douheret (218-220) determined junction potentials by comparing computed potentials (known activities) with actual glass membrane potentials os. calomel reference in junction cell configurations. Rock (798) reviews the design of membrane and amalgam electrode cells to avoid liquid junctions. Ryazanov (892, 823) using his activity coefficient us. concentration theory, computes junction potentials including varying activity coefficient terms. The interesting case of diffusion potentials between reacting aqueous NaOH and aqueous HC1 is taken up by Murgulescu (663). Hickman (374) using perturba272R
ANALYTICAL CHEMISTRY, VOL.
tion methods demonstrates adequacy of quasielectroneutrality and derives the first order correction to .the Henderson equation. This work merely confirms these facts derived in the last decade by L. Bass, D. Hafemann, J. Newman, and others. A similar calculation using Poisson’s equation rather than quasi-electroneutrality has been performed numerically for aqueous and fused salt junctions by Okada and Kawamura (706). Measurements of junction potentials a t high ionic strengths for perchlorates have been made by Barcza et al. (41) and Ryazantseva et al. (824). Junctions p.d.’s have been measured for colloidal systems (66’7) and molten salts (619, 707). One of the outstanding contributions of the “Scandinavian School” of complex ion chemists has been the general use and success of inert high ionic strength aqueous media for equilibrium measure ments. In solutions typically 3M in ionic strength, variation of complexed and complexing species is believed to be accompanied by minor changes in activity coefficients. Ginstrup (314) has studied the scope of the method by measuring EMF’s of hydrogen electrodesilvel-silver halide electrode junctionless cells a t 3M in Ntt+, H+, C101-, C1-, or Br- from 25 to 60 “C. Standard EMF’S follow a simple linear equation which is an extension of Harned’s rule. Liquid junctions introduce uncertainties which are discussed. Ohtaki and Biedermann (704) also investigated these cells, cells with ’ glass electrodes, and junction cells at 3M perchlorate. Proton medium effects and junction p.d.’s are invoked to explain noneonstant cell EMF’s. Divalent and trivalent cations were included in this study. hctivity coefficients in constant ionic strength perchloric acid-sodium or lithium perchlorate mixtures were determined at 25 “C by Gelsema and coworkers (306). Butler, Synnott, and Huston used alnalgam, selective glass, and liquid ionexchange electrodes to measure activity coefficients in multicomponent salt solutions (134). Standard EMF’S and temperature coefficients of Ag/AgX couples and activity coefficients for hydrohalic acids and some salts were determined (622, 666, 892). Activity coefficients of H F in water from 0 to 35 “C were reported (362). Neff (674) suggests a new form of the Debye-Huckel limiting law to relate activity coefficients of one ionic species to the sum of activities rather than to the ionic strengths. Solutesolven t interactions have been deduced electrochemically in this biennium. Determination of relative base strengths in acetic acid-acetic anhydride was reported by Kolling and Cooper (486). Acid-base strengths in N,N-dimethylformamide were deduced potentiometrically using glass electrodes by Kolthoff et aZ. (487). Potentiometric
44, NO. 5,
APRIL 1972
titrations were used to establish a relative acidity scale in propylene carbonate (978). EMF measurements of cells without liquid junctions were used to study the ionization and to measure the thermodynamic constants of tris(hydroxy methy1)acetic acid and related acids in MeOH-H20 (60). A most important pair of compounds, tetraphenylarsonium and tetraphenylphosphonium tetraphenylborates were studied by NMR in water and other solvents (166). Use of distribution equilibria of these compounds between phases lias been proposed as a means of obtaining medium effects by splitting the free energies of transfer. Coetzee and Sharpe find specific solvation effects and suggest caution in using these compounds as references for extrathermodynamic procedures. NONGLASS SOLID STATE ELECTRODES AND APPLICATIONS
The lanthanum fluoride electrode has created the greatest impact in this field as judged by total numbers of fundamental and applied publications. Vesely (972) studied the responses of crystals doped successively with five rare earths. While the cell resistance, dominated by the crystal resistance decreases with increasing dopant, the potentiometric response is unaffected. This result is not surprising since intrinsic thermally generated defects in pure LaFz can be large enough in number and mobility to satisfy the only requirement that the crystal impedance be much less than the measuring circuit impedance. (Alternatively even a few ppm of dopant leads to a significant number of extrinsic defects as can be shown by simple calculation (123). Baumann (69) has shown that fluoride electrodes respond ideally to free fluoride ion activities 1-3 decades below the apparent solubility limit, 10-6M. Test of this point requires establishment of known low fluoride levels by making use of common ion effects. This effect is predicted whenever the exchange rate of a component ion (F- in this case) exceeds the dissolution rate of the crystal. The latter is determined by the slowest ion exchange rate, dissolution of Laa+ in this case. These statements are based on a general theory of coupled ion transfers across immiscible interfaces (124). Response has also been studied in 1M NaCl by Warner (991) and no detectable interfetence from C1- is found at least above 10-4M F-. A microelectrode has been designed and its response characterized by Durst (830). Sample volumes in excess of only 2 pl are required. A LaFs membrane electrode can be converted to a cation sensitive electrode via the common ion effect. Farren (262) has prepared nonporous LaFrCaFz (lO-M% CaF2) membranes responsive to calcium activities. Since the solubility product
of CaFs exceeds that of LaFa, fluoride activity detected by the fluoride electrode will be determined by the excess calcium activity provided the equilibrium between CaFz and solution is rapid, at least at the electrode surface. Application of this principle was done earlier by Frant and Ross (286) in corinection with mixed sulfide electrodes. Kubota (607) reports that LaFa membranes are unatrected by ydose rates of less than 15,000 rads/min. Applications of fluoride selective electrodes are summarized in Table I.
Understanding of the electrical and electrochemical properties of solid membrane electrodes is limited by too few fundamental papers on the characterization of these systems. Brand and Fkchnitz (106) have done an important survey study of the impedance us. frequency response of the LaFs and silver halide crystal or pressed pellet electrodes. The results are difficult t~ interpret even using the comprehensive recent theory of MacDonald (672). Another particularly knotty problem is the process by which solid metal-salt
contacts operate with so-called "all solid state" electrode systems which eliminate the inner electrolyte filling solution. This problem was enunciated some years ago (I%?), and it WBS suggested that solid-solid surface ion exchange or electron exchange (in cases such as Ag/AgX electrodes where measurable electronic conductivity in the salt exists) could lead to stable interfacial potentials. Vesely and Jindra (973) found stable potentials with Stockbarger (or zone melted) single or block crystals with Ag metal contacts
Applications of Fluoride Ion-Selective Electrodes Condition and comments La(NO& solutions into 60% Iso-PrOH-HtO Carboxylato buffers interfere by formation of mixed precipitates. Chelating ligands: lactate, malonate, citrate, and acetylacetonate attack the electrode and distort the response La(II1) and Th(IV) in unbuffered media are compared. End-point potential drift'due to ppt. -agingis noted. Th(IV) irI recommended for F- levels less than 1mM Method for plants, soils, fertilizers, bones, water, and urine F- freed from S i F P a t pH 8.4-10.0 Potentiom. a t H 5.7 5.9 Uses std. O.1M NaF, pH 5 buffer and mixed EtOk-Hn6 solvent. Std. dev. 1-2% Diphenylcarbamoyl fluoride inactivates enzyme sites liberating one F-/site
Table 1.
Application Titration of fluoride Titration of fluoride Titration of fluoride Potentiom. of FPotentiom. of F- and SiF6*Titration of Al(II1) L-Chymotrypsin indirect potentiom. Activity coeficients in NaClNaF mixtures Stabilities of F- com lexes of Cd, Mg, Ni, Zn, %(I), Ag Ionization constant of aq. H F a t 25 OC Stability constants at 25 "C for HF, HF*-, s cies StaKit constants at 25 "C of H% and HFI- in 1 4 M NaC)1O4 Stability constants at 5, 25, and 45 "C. of H F and T h F I - 4 species Stabilities of thorium fluoride species Solubility of species in Pb(II)-F- system Stabi!ities of fluoride species of Mg, Ca, Sr, and Ba Stabilit constants of MgF+ and 8 a F + Stability constant of MgF+ Stabilities of mixed species BFz(0H)r- and BF(0H)s Stability constants of fluoroborate species BF.(OH)& ,,Stabilities of Nb(V)-OH-F- species Potentiom. of F- in geological materials Potentiom. of F- In geological materials Potentiom. of F- in geological materials Potentiom. of F- in geoiogical materiais Potentiom, of F- in geological materials Potentiom. of F- in geoological materials Potent.iom of F- in soils Potentiom of F- in soils Potentiom. of F- in natural waters Potentiom. of F- in natural waters
Uses fluoride, Na-selec. glass, and Ag/AgCl electrodes
pH 5-6. Not applicable to Hg(I1) or Cu(I1) Best value 5.85
+ 0.03 X
a t infinite dilution
Potentiom. of F- in NHINOI extrapolated to infinite dilution Quinonehydroquinone eiectrode Standard enthalpies and entropies also Accumulated constants found. No evidence found for polynuclear or protonated species
Uses 0.1-1.OM NaCl electrolyte Used glass and fluoride electrodes 1M NaNO, a t 25 and 35 OC 3M KCl at 25 "C
Uses N&Os fusion Uses null point potentiometry
Uses a bismuth-tungstatevanadate flux Uses carbonatenitrate flux Uses pyrohydrolytic separation Extracted with acetateEDTA buffer mixture Uses HC10, distillation to separate fluoride Uses known addition (spiking) technique, high ionic strength buffer and complexing agent TJse 8-OH quinoline extraction to remove metals
(455) (Continued)
~~
ANALYTICAL CHEMISTRY, VOL.
44, NO. 5, APRIL 1972
273R
~~
To ble I. Applications of Fluoride Ion-Selective Electrodes (Continued) Condition and comments Application Cooke, ’Lim, Palin, Amer. Std., and LaF, electrode methods interwmpared. (32) Potentiom. of P- in potable Last is preferred. waters General procedure including addition of const. ionic strength buffer, EDTA, (33) Potentiom. of F- in fluoriand citrate to free bound fluoride dated water supplies Fksults of 60 samples are given. (34) Potentiom of F- in Austrian water Computer treatment of titration data is included. (35) Standard addition titration of F- in sea water Via aqueous extraction (36) Potentiom. of F- in plants Via oxygen bomb combustion (37) Potentiom. of F- in plants Via distillation (38) Potentiom. of F - in plants Via alkaline digestion (39) Potentiom. of F-in biological materials Via ashing, fusion, and distillation (40) Potentiom. of F- in vegetation and gases Via fusion (41) Potentiom. of F- in vegetation and gases Both continuous and batch methods-use Gran plots (42) Potentiom. of F- in air (43) Potentiom. of F- in air Ashing and extraction (44) Potentiom. of F- in preservatives and treated wood. Uses closed flask combustions (45) Potentiom. of F- in organic and organometallic compounds (46) Potentiom. of F- in organic Uses hydrolysis and organometallic compounds (47) Potentiom. of F- in urine ’
(970) (84,891 (119, 826, 743, 869, 967)
Using fusion methods
on the inner side. Metal contacts other than lanthanum to doped LaF3 did not give stable potentials. Farren and Staunton (263, 266) have patents on solid contact electrodes including metals different from those in the salts. Relative merits are not disclosed. Closely related are the Rueicka-Lamm “Selectrodes” which are hydrophobized carbon rods on to which are dried (baked) silver halides, silver sulfide, and other heavy metal sulfides to produce selective electrodes (819, 880). The writer feels that these electrodes function by development of a n interfacial potential which, by virtue of solid state equilibria, is communicated by the inner potential, possibly via electrons, to the carbon. These electrodes then have the same processes a t work as do the other “all solid state” cases. Only one paper has been published on the redox couple response of membranes with significant ionic and electronic conductivities (836), but these membranes of CuS and VzOa are not typical of those membranes used for ion selective electrodes, because of the high electronic component. The silver sulfide membrane electrode of Frant and Ross (287), although already much studied and used ( M I ) , was evaluated by Mirna (626). Applications of the commercially available 274R
0
966)
(896, 890)
(48) Potentiom. of F- in serum (49) Potentiom. of F-in beverages (50) Potentiom. of F- in plaque
and tooth deposits (51) Potentiom. of F- in difficultly soluble phosphates, and in the presence of phosphate species (52) Potentiom. of F- in insoluble aluminum, calcium, and silicon compounds
(168, 678,
sulfide pellet electrodes are summarized in Table 11-A. When CuS, CdS or PbS is mixed homogeneously into AgzS to form pressed pellet membranes, electrodes responsive to activities of Cu2+, Cd*+, and Pb2+ are formed, presumably via a common ion effect as in electrodes of the second and third kinds (123). A German patent (286) was issued to Frant and Ross covering these and the AgSCNAgzS electrode. Applications are contained in Table 11-B. Hirata e l al. (382) find a hot pressed mix of PbS, Cu&, and AgzS gives a more rapidly responding lead-sensitive electrode. They have also used a CUZSceramic electrode for C U ~ +activities and find a useful analytical range of 10” to 10-lM Cu2+ (383). Silver and copper(I1) activities can also be m e a sured using pressed pellet metal iontetracyanoquinodimethan-radical salts or the salts in a plastic binder (869). Response is Nernstian over 6 decades of activity. A number of papers have appeared on topics relevant to the silver sulfide and silver halide membranes or the equivalent electrodes of the second type using silver/silver salt electrodes. Onoue et al. (713) have prepared ill-defined solid mixtures of silver and silver chloride by
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5 , APRIL 1972
chemical and electrochemical reductions for use as reversible Ag/AgCl electrodes. I n two important papers, Watson and Yee report on the rate of conversion of AgCl into AgI by spontaneous reaction of aqueous I- a t the AgCl surface (999, 1000). Potentials of the nonequilibrium Ag/AgCl/I- system us. time and the apparent exchange rate were measured. An unfortunately erroneous paper on the potential distribution in cells with electrodes of the second kind by Hoffman (386) which neglected consideration of the interfacial p.d.’s a t AgCl and AgBr-aqueous C1- or Brinterfaces provoked a series of polemic articles by Kroeger (603),de Bethune (190), and Hoffman (387). The entire problem deserves better formulation via proper application of the electrochemical potential concept. Another well conceived experiment measured the rate of AgBr reaction with dissolved hydroquinone using a blocked concentration cell arrangement (834). Transient behavior of the blocked cell, Pt/AgBr/Ag, was used to calculate the apparent double layer capacity (1006). Hull and Pilla (397) using transient methods also, measured the applied voltage range over which the C/AgI/Ag cell is blocked and shows pure double layer (capacitive) behavior. On the other hand, Pt/
rotating disk of AgosI t o study the PO% sible interfacial proceases Ag+(soln.) + Ag+(elec.) and Ag+(soln.) e- + Ag. From impedance measurements they have shown t h a t the dominant process is ion exchange (872) and mobile speciea in the solid are Ag+ vacancies. An allsolid-state electrode sandwich cell containing the same electrolyte between silver contacts shows current-voltage curves interpretable in terms of reversible surface concentrations of silver
AgI/Ag passes Faradaic current and behaves nearly like a reversible electrode. The Newcastle-upon-Tyne group (94) has succeeded in measuring the exchange current density a t the Ag/Ag,RbIb solid electrolyte interface with various forms of Ag. Scrosati (868) gives a review of electrochemistry of cells using this electrolyte. Owens et al. (717) report on high conductivity electrolytm CabNI-AgI, and Shirokov and coworkers (87‘3) use a
+
interstitials (W). Kennedy used a sandwich cell with AgbsBr electrolyte as a reversible coulometer (466). A patent has been issued on the construction of silver electrodes of the second kind (482). An unusual potentiometric determination of CS2 (converted to a xanthate) used a Ag/AgCl electrode in alkaline medium (990). A patent has been issued for an AgCl membrane electrode in a light-tight container which gives very low potential drift (684).
Applications of Homogeneous Silver Salt Membrane Electrodes Part A. Silver Sulfide Membrane Electrode Application Condition and comments Standard AgNOs titrant Titrations of C1-, Br-, I- in mixtures, CN- and SCN0.002M AgNO, into -75% HoAc solvent Titrations of 2.5-60 fig of individual halide ions Titration of sulfide Titrant 10-Ll0-1M Na plumbite; CN- interferes. 106 fold excess C1-, Br-, I- S O P and SCN- do not interfere Titrant AgNOa; &Oil- interferes; COS*- and SO*+ do not Titration of sulfide interfere Titration of thiols Hgz+ interferes Titration of nanogram amounts of StTitrant Pb(NOa),, 1M NaOH, 1.5M N2H4 into PbSsatd solutions Titration of sulfide in pulping liquor Used silver titrations and back titrations. Direct potentiometry also used with “known increment” method Potentiom. of SS- in lime-sulfide solutions Plots of (HS) and (Sz-) are constructed from pH and pS*- me& surements Determination of NHI diethyl dithioUses continuous potentiometry and “known increment” method phosphate in flotation m l l solns wlO-7M range Potentiom. detn of Ag+ Uses technique to follow Ag+ adsorption on seiected surfaces Potentiom. detn of Ag+ Table II.
Part B.
Titration of Zr(IV), Fe(III), Th(IV), Hg(I1) Sm(III), La(III), and Ca(I1) with E b T A Titrations of Cu(II), Ni(II), and Zn(I1) with l,IO-phenanthroline, tetraethylenepentamine and EDTA Back titration of Al(1II) with Cu(I1) (4) Titration of nitriloacetic acid
( 5 ) Titration of CuUI) with EDTA and
. stability of complexes (6) Pb(I1) titrations in aq, nonaqueous, and biological media (7) Microtitration of phosphate (8) Semimicrotitration of oxalate
(9) Pbl+ detection for lead poisoning (10) Interferences and titration applications
using Cd¶+
Part C.
Mixed Sulfide Membrane Electrodes
Used Cu(I1)-EDTA intermediate and Cuz+-selective electrode Used Cu(I1)-EDTA intermediate and Cu*+-selective electrode Used excess diaminocyclohexanetetraacetic acid and Cu-select. electrode Uses Cu(I1) titrant and Cu-selective electrode Uses Cu-selec. electrode
(3481
Used Pb-selec. electrode
(789
Uses Pb(ClO4)ztitrant, pH 8.25-8.75 and Pb-selec. electrode Uses Pb(ClO& titrant in 40% p-dioxane-water, pH 3.5-10.5 and Pb-selec. electrode Uses Pb-selec. electrode Uses Cd-selec. elec. in HzOand DMSO
(86.9) (863)
(917) (786)
(788 (110)
Silver Halide Membrane Electrodes Including AgI-AgzS Mixed Case
Application (1) Titration of C1- in plant extracts
(2) Titration of C1- in natural waters (3) Differential potentiom. 1-350 ppb in high purity waters (4) Titration of C1- in soil extracts (5) Potentiom. detn of C1- in milk (6) Microdetermination of C1- and Na(7) Potentiom. detn of Br- in natural waters (8) Pot. detn of C1- and Br- in natural waters (9) Titration of halide mixtures (10) Pot. detn of I- and 1 0 3 ; (11) Pot. detn of I - in organic materials
(12) I- measurement in reacting systems (13) Titration of H (11) (14) Potentiom. of 8 N (15) Titration of CN(16) Potentiom. of CN- in waters, wastes and biological materials (17) Potentiom. of CN- to detn cyanogenic glucosides (18) Potentiom. of CN- in forage
Condition and Comments AgNOa titrant; C1- selec. electrode AgN03 titrant; C1- selec. electrode C1- selec. electrode
Reference(s) (611) (l77)
(2811
AgNOa titrant; C1- selec. electrode C1- selec electrode C1- selec. electrode Br- selec. electrode Use C1- and Br- selec. elecs. I- selec. electrode -gives best resnonse of the halide selective systems Uses AgI membrane electrode Hydrolysis process described Uses I- selec. electrode Uses I- selec. electrode and standard NaI titrant Indirect detn via Ag+ in equil. with AgzCNz Used std. Ni(I1) solutipn 270 pg-270 mg CN-/liter
(3791
(660)
(860)
(740)
(1004) (161
Uses hydrolysis, acid distillation
~~
ANALYTICAL CHEMISTRY, VOL.
44, NO. 5, APRIL 1972
275R
Applications of silver halide membrane electrodes are in Table 11-C. Bottazzini and C m p i (98) have compared Pungortype AgI and AgSS electrodes with corresponding preased pellet membranes. The latter, they claim, are superior for measurements in saturated silver salt solutions. 108- d u t i o n s give unstable redings. There have been issued a number of patents on electrode construction, electrode arrangements, demountable tips, etc. These include vacuum deposition for “all solid state” electrodes (333), use of casting resins (701), mounting in tubes (694), mounting in salt bridge (746), detachable tips (886), use of gaskets (264), and combination construction (390). An obvious readout device (866) and an unusual sample holder (286) were described. Liquid junction configurations include fiber bundles (391),fumed silica (16S),quartz yarn (939), arid a valve (744). Further liquid junction configurations are listed in the glass membrane electrode section. HETEROGENEOUS MEMBRANE ELECTRODES AND APPLICATIONS
Closely related to the crystalline and pressed pellet electrodes, in principles and in applications, are the heterogeneous membrane electrodes. Many of the same materials used in pellet form are also used as ground powders in silicone rubber or other matrices. Use of insoluble ion exchanging materials in insoluble, inert binders is not original with Pungor, but his name is closely associated with these electrodes which are imported from Hungary. Although most of his work prior to 1968 was published with colleagues, the journals were not easily available. Fortunately Covington (177) prepared a review of this work along with other published studies on heterogeneous membranes of which there are numerous kinds (123). A number of other reviews are also available in English: theory and applications (776, 947j, recent developments (774), new results including tests of the selectivity theory (775), properties, “standard” potentials and temperature coefficients (589 in English, 590 in Hungarian). The chloride electrode has been used for C1- determinations in urine (727); iodide electrode has been used to monitor I3lI in milk (116), cyanide ion activity (948), and halides in halogenated pharmaceuticals (200). The selectivity of the iodide electrode in the presence of eight anions forming more soluble silver salts was measured as a test of the theory (766 English), (767 Hungarian). These electrodes cannot be used in dry organic solvents but have been tested and found useful for alcoholwater mixture
