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Electroanalysis and coulometric analysis Allen J. Bard Anal. Chem., 1970, 42 (5), 22-32• DOI: 10.1021/ac60287a008 • Publication Date (Web): 01 May 2002 Downloaded from http://pubs.acs.org on February 19, 2009

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Electroanalysis and Coulometric Analysis

Allen J. Bard, Department of Chemistry, The University of Texas at Austin, Austin, Texas 7871 2

HIS PAPER surveys the literature (64). The book “Electrochemistry at etry in ” by Milner T and developments during 1968 Solid ” by Adams (2) crit- and Phillips (183) gives a brief intro- through December 1969; a few papers ically and exhaustively reviews the duction to coulometric methods and published before 1968 which have not principles of applying solid electrodes also includes reprints of many of the appeared in previous reviews in this in electroanalytical methods, particu- fundamental papers in the field. series have also been included. larly with applications to the electro- Several reviews have been concerned chemistry of organic substances in both with the application of coulometric aqueous and nonaqueous solutions. methods to drug (163, Z14), clinical and BOOKS AND REVIEW ARTICLES Two additional volumes in the mono- toxicological (dS4), and industrial (42) graph series “Electroanalytical Chem- analysis. Reviews on electroanalysis A number of books containing mate- istry--A Series of Advances,” have also have included discussions of coulometry rial of interest in electrochemical appeared (28). Robin has edited a and other electrolytic methods (1, lf’i’, methods of analysis and basic principles French language book on electrochem- 87’3, 302, 306). Discussions of different of electrochemistry have appeared. ical methods (241); the chapter in it methods of analysis of plutonium (253) Electrochemical methods in general are by Breant (40) reviews electrometric and the noble metals (34) include evalu- treated in Donbrow (7‘1) and the English end-point methods and coulometric ations of different electrochemical meth- translation of Damaskin’s monograph . The monograph “Coulom- ods.

22 R ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 RECENT TRENDS determining trace elements in Pt by the electrical double layer structure Trends in electroanalysis continue separation on a cation exchange resin parameters affect separations of metals away from the older methods, such as column and elution with HC1 into an (23,24). Electrolytic methods continue electrogravimetric methods, and toward cell. The elements Ag, Cu, to be useful in the preparation of very coulometric ones. Interest has con- Nil Pb, and Pd are then deposited on a pure inorganic substances (263) or in tinued in the application of electro- Au cathode and determined by spark the preparation of unusual oxidation chemical methods of different types to source mass spectrometry (11). states of metals, such as solutions of studies of organic systems. Electro- Other Separations. Jones and Mi- In(1) by anodization of In wire in chemical methods can indicate the pres- eure (131, 179) have described a novel HClOa media (278). Gosman (105) ence of oxidizable and reducible groups, electrogravimetric technique suitable studied the homogeneous isotope ex- generate reactive intermediates, and for determination of traces of metals and change rate of Tl(1) and Tl(II1) by even provide information about struc- based on electrodeposition on a Au- simultaneous electrodeposition of T1 ture and energy levels in molecules. plated quartz crystal. The change in from both components in a solution con- Coulometric methods are of special frequency of a crystal oscillator (nom- taining z04Tl. Table I summarizes other interest in electroorganic chemistry, inally 3 MHz) following electrodeposi- work in electroseparations and electro- since the amounts of material and time tion from solutions for 2 to 20 minutes gravimetric methods. scale in these experiments approximates was shown to be a linear function of those in electrosynthesis. This renewed concentration of the deposited metal CONTROLLED POTENTIAL COULOMETRY interest in electroorganic chemistry has (e.g., Cd). The main disadvantage of led to several meetings and symposia the technique appears to be the necessity A summary of controlled potential devoted to the field, including one by of rinsing and drying the crystal before coulometric determinations is given in the Faraday Society during April 1968 determination of the frequency. In- Table 11. The use of coulometric meth- at Newcastle-upon-Tyne and one creased attention has been paid to how ods in the elucidation of reac- sponsored by the Army Research Office and held in Durham, N. C., in October 1968. Adams' recent book (2) also Table 1. Electroseparations and Electrogravimetric Determinations stresses applications to organic systems. Separation The decade began with increased of From Method application of operational amplifiers to AgJ cu Pb, Zn Electrodeposition of Ag or Cu from electrochemical instrumentation; their acidic solution on glassy-carbon use in , coulometers, and electrodes AgJ cu cu, co Ap lication of complexing agents and waveform generators is now routine, Ni Luble layer structure studies in The next decade will probably find in- controlled potential electro- creased application of digital instrumen- deposition tation to electrochemical measurements, Ag Zn Electrodeposition on glassy-carbon from "01 solutions with completely automated coulometric Am Preparation of sources for analysis of several components just a-spectrometry by electrodeposition around the corner. The digital com- from dimethylsulfoxide solutions puter, which has been widely used for Ba, Ca, Sr Controlled potential deposition on mercury electrodes data treatment and computation for the ZlOBi, ZlOpb Deposition by internal electrolysis past several years, will find increased using a lead electrode application in electrochemical problems. Cs, Rb Deposition from aqueous solutions on The powerful technique of digital liquid gallium cathode CUI co Nb, Ti Deposition on Pt- or Cu-coated Pt simulation [see (87) and references Ni, Fe prior to spectrophotometric therein] has and will allow many com- analysis plex mechanisms to be treated theo- cu Co, Ce, Investigations of optimum conditions (26, 69, 229) retically, for many electroanalytical other metals for electrodeposition Cu, Pb Re Electrodeposition on Pt from (196, 197) techniques, even by scientists not con- citrate-tartrate solutions prior to versant with advanced numerical meth- polarographic determination of Re ods. In Zn, Fe Electrodeposition on Pt electrode by Nil Cr internal electrolysis with a Zn anode ELECTROSEPARATIONS AND Mo Fe, V Electrodeposition on a mercury cathode hl0 W Electrodeposition on Pt electrode Electrolytic Sample Dissolution and frqm (NH4)2S0,solution Preparation. Electrolytic methods Mol Re Application of EDTA solutions and have been shown to be useful in the internal electrolysis with Zn anode dissolution of refractory metals, in- to electrodeposition on Pt electrode Pb cu Deposition of PbOr from "01 cluding Tal Kb, W, Zr, V, Hf, Ti, solution at a vibrating Pt anode and their alloys. In this method Pu Electrodeposition on stainless steel the metal sample is made the anode disk prior to a-spectrometry and a Pt or Ta cathode is employed Rare earths Investigations of electrodepositions on a Hg cathode (8). Electrodeposition methods continue Eu Cd Co-deposition of Cd and Eu at Hg to be useful in preparation of samples for electrode followed by controlled spectroscopic techniques. Emission potential stripping of Eu spectrographic techniques were used to Gd, Tb Electrodeposition on Hg cathode from acetate-tartrate solutions determine traces of Ag on glassy-carbon Rh Electrodeposition on rotating Cu disk (186) and traces of Rh on a rotating cathode followed by spectrographic Cu disk (293). Samples of actinides for determination a-spectrometry have also been prepared Zr Be Deposition as the hydroxide on a Pt electrode by pH change produced by electrodeposition (32, 107). An in- electrochemically teresting method has been proposed for

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 23R lometry will be difficult. They conclude Table 11. Controlled Potential Coulometric Determination that in these cases “the extent of a Substance chemical reaction in the diffusion layer determined Method is not worth taking into account if the reaction is slow enough to detect.” Electrolytic reduction to the I11 state followed Am This is fortunate, since the necessity of by coulometric oxidation Am(II1) -+ Am (V) at tin oxide conducting glass electrode including the homogeneous chemical Am Persulfate oxidation to Am(V1) followed by reaction terms in the equations for cal- coulometric reduction to Am(V) at Pt culating the flux at the electrode surface electrpde in HNOa Bi, Pb Deposition of Bi and Pb on Hg electrode considerably complicates theoretical followed by coulometric oxidation of the treatments. For example, Rangarajan amalgam in tartaric acid medium ($35) considered the case Ca, Cu, Zn Indirect coulometric method based on electrolysis of Hg(I1)-DTPA complex at Hg- 0 + nle - I coated Pt electrode cu Determination of ng amounts in undivided k capillary microcell in NHtOH-Hd304 21 -+ D Cu, Na, Li Reduction at Hg electrode in acetonitrile-0.1M I nze R Et4NC104 + - c1 Use of Ag-AgC1 electrode in a pC1-stat for in the light of the work of Karp and determination of free chloride in presence of Meites, and was able to solve the theo- cations.. ..~~. M n Coulometric oxidation Mn(I1) - Mn(II1) in retical equations only with some ap- 0.25v Na4P207,pH 2, at Pt electrode proximations concerning the relative M o Reduction Mo(V1) -+ MOW)in 0.2M rate of the dimerization reaction to that (NH4)2C~0r,pH 2.1, at Hg electrode Pu Dissolution of PuOz by Na2Oz-NaOH fusion, of the electrode reactions. reduction to Pu(III), and coulometric Tenygl (279, 280) proposed a tech- oxidation Pu(II1) -+ Pu(1V) nique, which, although called “pneu- Re, PIIo, W Electrochemical oxidation and coulometric matic ,” is actually more determination in Re alloys Ru Ru(1V) as binuclear complex [RutO]‘+ closely related to a coulometric method reduced to Ru(II1) at Pt electrode in 5M involving gas evolution described by HCl King and Bard. The method is based U Determination in A1-U alloys on electrolysis in two cells connected in U, Fe Reduction at Hg cathode series, one containing only supporting $J, NP Determination in U-Np alloys Determination by oxidation V(1V) - V(V) or electrolyte solution while the other also reverse reduction at Pt electrode in 1.5M contains the electroactive substance of following chemical adjustment of V interest. Evolved gases from both cells oxidative state are led to a differential recording manometer where the difference in ORGANICSUBSTANCES volumes is determined. For example, Sulfur compounds Oxidation in aqueous buffers of phenylthiourea in the blank cell Hzand O2are produced (n = I), thioglycolic acid (n = 2), and by the electrolysis of water. In the cysteine (n = 2) at Pt electrode sample cell less gas is evolved, since some O-phenylene- Oxidation (n = 2) at Pt electrode in HzS04 diamine of the current goes to electrolysis of the electroactive substance, e.g., Although the selectivity of this method is poorer than more conventional coulometric methods, it will probably tion mechanisms, where analytical de- diates are formed in the first electron be useful for the continuous analysis of terminations were not devised, are transfer step, such as the sequence substances undergoing oxidation or re- discussed below. 0 nle I duction at potentials near those for the Electrode Mechanisms. Coulo- + - background processes. metric methods continue to enjoy an k I-P Parry and Oldham (212) showed that important role in the study of elec- the reduction of Pd(I1) in media con- trode reactions, both in the deter- I n2e -+ R + taining ammonia or pyridine occurs in a mination of overall n-values and in that previous treatments gave incorrect two-electron reaction. Johnston and detailed studies of the rates and results because they neglected the Page (130) studied the reduction of the mechanisms of chemical reactions occurrence of the homogeneous reaction Rh-species Rh(NH3)sOH*+ in an am- associated with the electrode reac- within the diffusion layer. The authors tions. Bard and Santhanam (29) have moniacal medium at a Hg electrode. presented a theoretical treatment for By measuring nap*and the amount of given a complete survey of the theory, this reaction and also for simple stepwise H+ consumed during the electrolysis, experimental methods, and applications electron transfer and derived corrected they concluded that the reaction mecha- of coulometry to the study of electrode equations for these mechanisms. They nism was reactions. Meites (176) also reviewed point out, however, that in cases where this field, with special attention to the Rh(NH&OH2+ 2H+ 2e the product I is not itself electroactive, + + - determination of the rates of homo- for example in the ECE scheme Rh(NH3)jH2++ Hz0 geneous reactions. Although theoret- ical treatments in this area have been k The product of this reaction reacted 0 nle -+ I -+ me R less frequent, two studies have appeared + J + - slowly with solvent and more rapidly which examine the role of the reactions the occurrence of reaction in the diffusion with oxygen to yield Rh(II1)-O-O-HZ+, which occur in the diffusion layer and layer is not of importance, even for which could be reduced cleanly in a two- their effect on coulometric n-values and ratios of k to the overall mass transfer electron reaction. Ru(1V) was shown current-time behavior. Karp and rate coefficient (p or 0) of 100. When to exist in HClO4 solutions as a tetra- Meites (136) pointed out that in reaction k/p is greater than 100, detection of the meric species which could be reduced in schemes in which electroactive interme- intervening chemical reaction by cou- two one-electron steps (295). The

24R ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 Ru(II1) species formed from the product formation of (SCN)2 and (SeCN)2, the reduction of geminal dihalocyclo- of the electrolysis could be reduced respectively. The oxidation-reduction propanes in alcoholic or DMF solutions. further to Ru(I1) which is immediately behavior of C1- and HCl in nitro- In all cases the reactions involved two- oxidized by Clod-. In another study methane was investigated and shown to electron reduction with removal of Br- of a Ru species, Itzkovitch and Page involve one-electron transfer to form (in Br-C1 compounds); the stereo- (120) found that Ru(NH3)sCla is reduced chlorine (167). Tsuji and Elving (286) specificity of the reactions was the same with n = 1.02 i 0.02 in an acid sulfate investigated the reduction of different as that found for metallic reductions, medium to a stable Ru(I1) species, acids, such as HNO, and H2Cz04in In another study of the stereochemistry which reacts with Nzto form the complex pyridine and provided coulometric evi- of electrode reactions, Puglisi, Clapper, Ru(NH~)~N~~+.Connery and Cover dence for one-electron reductions at a and Evans (230) investigated the reduc- (58) employed coulometric techniques to Hg-coated Pt electrode. Coulometric tion of benzaldehyde in acidic aqueous study the electrode reactions and species methods have also proved useful in solutions at a Hg electrode. The au- present in NaOH solutions of Os(1V) studies in fused salts. The +2 states thors found that napp= 0.96 and that and Os(V1). In a reinvestigation of of Sm, Eu, and Yb were prepared in a the coupled product of the electrode the reduction of pertechnetate ion in LiC1-KCl eutectic melt by electroreduc- reaction, hydrobenzoin, had a ratio of 1M NaOH, Kissel and Feldberg (143) tion of the trihalides at a W electrode dl to meso form of about the same as that found a three-electron reduction at (128). Fulton and Swofford (96) dem- obtained in the photolysis of benzalde- -1.00 V us. SCE, but a very large onstrated that the oxidation of I- at a hyde (about 1.1 to 1.3). However, in value of nsppand a catalytic reaction for Pt electrode in a NaN03-KN03 eutectic the presence of surface active sub- reduction at -1.27 V. Propst (224) occurred with n = 1.06 f 0.04, with stances, such as tetra-n-butylammonium studied the reduction of Po(1V) and formation of Iz and concomitant de- perchlorate, the dllmeso ratio was sig- suggested a six-electron reduction to composition of Koa- to NO2- and 02. nificantly lower, suggesting perhaps that polonide ion. Propst and Hyder (228) The current interest in electroorganic the dimerization occurs within the also used scanning coulometry with a chemistry and the similarity between electrical double layer. Buchta and conducting tin oxide glass electrode to electrolyses performed for coulometric Evans (41) carried out the reduction of investigate the oxidation of Cf(II1) in and synthetic purposes continues to dibenzoylmethane (DBMH) in a DMSO H2S04 solutions. Re(II1) showed com- produce many studies in controlled medium and found an overall naPpof plicated current-time behavior during potential electrolysis of organic com- 0.55. This value fitted the proposed reduction in 0.5M HC1, with naPpvalues pounds. Several investigations of qui- mechanism involving reduction of 4 between 0.75 and 1.11. The results nones have been reported. O’Brien and molecules of DBMH to yield the coupled were explained in terms of a one-electron Olver (204) showed that the biologically pinacol and 2 moles of the enolate with reduction of a trimeric complex (193). interesting molecule ubiquinone-6 is the transfer of 2 electrons. The JIcCullough and hleites (160) used reduced at a Hg electrode in methanolic current-time curves obtained in this coulometry, among other electrochem- solutions with n = 2.01. Similarly, the reduction were unusual in that the ical methods, to examine the complex durohydroquinone in MeCN at a Pt current after undergoing the usual electrochemical behavior of Nb(V) in electrode was shown to involve an over- monotoric decay increased again and HC1 media. Davis and Montalvo (65) all two-electron reaction (111). Interest formed a peak. This behavior was said showed that the reduction of Mn(II1)- also continues in aromatic nitro com- to result from the decomposition of the hematoporphyrin IX occurs in a one- pounds. The reduction of nitrobenzene pinacol to benzil and acetophenone, electron step at pH 6.2. in ethanolic solutions in 0.1M HCl followed by the reduction of benzil to Coulometry also continues to play an showed the usual nspp= 4.0 and reduc- its anion radical. The peak in current important role in electrochemical studies tion to the hydroxylamine (290). In was caused by the catalysis of the in nonaqueous solvents. The reduction alkaline solutions nonintegral and time- pinacol decomposition reaction by benzil of several Ni(I1) complexes of cyclic dependent nappvalues were obtained, anion radical; the authors coined the amines in acetonitrile (MeCX) was however. The reduction of a number name “electrolytic autocatalysis” for shown to involve transfer of one electron of dinitronaphthalenes were also shown this type of reaction. In a study of the (207). Controlled potential electrolysis to be the nitrohydroxylamine (I%), reduction of perinaphthenone in basic was also employed to study the reduc- except for the 1,8-dinitronaphthalenes media, Cardinali, Carelli, and Trazza tion of ZrClc in RleCX (208). Goolsby which undergo 10-electron transfers and (47) found two one-electron steps; the and Sawyer (103) studied the electro- internal azo- or hydrazo-compound final reduction product was the dihydro chemistry of Au species in hIeCK solu- formation (158). The coulometric re- compound. tions and found that in the presence duction of a number of m-dinitroben- The electrochemistry of amines and of chloride ion, Au(II1) is reduced in a zenes also substituted with alkyl or other organo-nitrogen compounds con- three-electron step to the metal. In halide groups have also been reported tinues to be of interest. Bacon and the absence of chloride ion, Au is oxi- and appear to show nappvalues near 4 Adams (21) investigated the oxidation dized with n = 1.00 j= 0.03 to form for aqueous electrolytes (276). The rc- of substituted anilines, such as p-chloro- stable Au(1) solutions. These authors duction of nitrobenzylhalides in MeCN, aniline, in aqueous media and showed also studied the reduction of superoxide however, shows a one-electron reduction that the reaction occurred with one ion to OH- (102) and the oxidation of to the anion radical which loses halide faraday of consumed per hydroxylamine to N20 (naPp = 2.00) ion and dimerizes (155). A study of the of aniline, and produced coupled prod- (104) in dimethylsulfoxide (DMSO). reduction of N-nitro and nitroso deriva- ucts. Smith and Mann (958) found Decaborane (14) in MeCN was shown to tives of guanidine and urea in aqueous that tertiary aliphatic amines oxidized be reduced in a one-electron reaction buffers has also been reported (154). in MeCN containing some water with (51). hfatsui and coworkers (173) Miller and Riekena (181) studied the naPp= 0.999 f 0.001. In the absence studied the reduction of HgBrz in N,N- reduction of triphenylbromoethylene in of water slightly lower naPpvalues were dimethylformamide (DMF) and sug- DMF solution and demonstrated a first obtained because of the reactions which gested first reduction to Hg and HgBr3- step two-electron reduction to triphenyl- the electrogenerated cation radical can (naPp= 0.67), followed by further reduc- ethylene. Further reduction at more undergo, The electrochemical oxidation tion to Hg (napp= 2). Cauquis and negative potentials yields triphenyl- of p-dimethylaminophenol in acidic Pierre investigated the oxidation of both ethane. aqueous media was shown to occur in a SCN- (49) and SeCN- (50) in MeCN Erickson and coworkers (82) reported two-electron reaction followed by slow and showed both involved napp= 1 with an interesting stereochemical study of hydrolysis of the quinoneimine species

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 25 R to benzoquinone (168). At higher pH’s MeCN at Hg and C electrodes and cessive one-electron steps were involved. the dimethylamine formed as a hy- showed that the primary process is a Santhanam and Bard (247) studied the drolysis product can add in a 1,Gaddi- one-electron reduction leading to the coulometric reduction of triphenylphos- tion to the quinoneimine, producing new alcohol and toluene. McKinney and phine and triphenylphosphine oxide in electroactive species. Ambrose and Nel- Rosenthal (161) showed that the reduc- DMF solutions at Hg electrodes and son (12) studied the oxidation of tion of triphenyl sulfonium ion at an found evidence of a catalytic reaction, carbazole and N-substituted carbazoles Hg electrode gave nap, values between with the steady electrolysis current and found that naPpwas 2.5 to 2.8 for one and two. The reaction products much higher than the background value. carbazole itself, but was essentially 2.0 were Ph2S and PhzHg. This was attributed to the decomposi- for methyl, ethyl, isopropyl, and phenyl In other studies of organic systems, tion of the initial anion radical to bi- substituents. They proposed an initial formate ion was shown to be oxidized phenyl, which then undergoes reduction, one-electron oxidation followed by cou- in DMSO in a one-electron reaction to followed by catalytic reoxidation. pling with loss of protons and a second COZ and formic acid (122). Aromatic Coulometric methods have also been one-electron oxidation of the coupled carboxylic acid esters formed stable employed to investigate compounds of product. The reduction of phenazine in anion radicals in a one-electron reduc- biological interest. Dryhurst and Elv- aqueous acidic solutions was shown to tion in DMF solutions (118). In a ing (74) studied the oxidation of adenine occur with naPpof one for reduction at study of the electroreduction of dieldrin in aqueous solutions at a pyrolytic the first polarographic wave (27). In and aldrin in methanolic solutions, the graphite electrode and found that nap, alkaline solutions only a single drawn- authors concluded that coulometry was was 5.6 to 5.8 for the overall oxidation. out polarographic wave is observed, and not a convenient method of analysis for The oxidations took two days to reach napp is about two. Sadler and Bard these compounds (270). completion, indicating a very slow inter- (243) studied the reduction of several A number of electrochemical studies mediate step in the process or severe aromatic azo compounds in DMF solu- of organometallic compounds have in- filming of the electrode. Dryhurst also tions at a Hg electrode. Reduction at cluded coulometric investigations. Di- studied the reduction (73) and oxidation the first polarographic wave showed napp Gregorio and Morris (68) found that the (72) of 6-thiopurine (TP) in aqueous values of one, with formation of the reduction of diphenylthallium(II1) cat- buffers. The reduction of TP itself at a fairly stable anion radical in all cases. ion at a Hg electrode in aqueous solu- Hg electrode gave evidence of a catalytic Coulometric oxidation (reversal coulom- tions at potentials corresponding to its reaction and high n,,p values, but 6-TP- etry) of the anion radicals was employed first polarographic wave yields nap, = sulfinic acid showed smooth reductions to demonstrate their stability. Reduc- 1.20 i 0.02 and proposed a reduction to purine with an nap, of two. The tion at the second polarographic wave of Ph2T1+ to PhzHg and T1, with side oxidation of TP in 1-If acetic acid at a showed that an additional electron was reactions contributing to the non-integer pyrolytic graphite electrode occurred added to form the dianion. This species nap, value. Reduction at the second with napp= 0.98 on formation of the di- was unstable and probably was pro- polarographic wave yielded naPpvalues sulfide. The oxidation in ammoniacal tonated. The protonated species, how- of 2.00 & 0.02 and produced PhsHg, solutions was more complex and required ever, was oxidizable back to the parent T1, and benzene. A very similar mech- four faradays per mole of TP and pro- azo compound in a single two-electron anism was found for the reduction of duced purine-6-sulfinic acid and purine- step. In a related study, these authors the monophenylthallium(III) dication, 6-sulfonamide. In a continuation of investigated the reduction of 4,4’-azo- which shows nap, = 1.02 f 0.02 and the their studies of nicotinamide salts, pyridine-1,l’-dioxide in DMF (244). formation of PhzHg and Tl(1) at the which are model compounds for the They found that reduction at the first first wave and nap,, = 2.05 * 0.02 at biologically important pyridine coen- wave gave nappvalues of 1.4 to 1.9 which the second (86). Similar mechanisms zymes, Underwood and coworkers (169) was caused by slow decomposition of the are found with other organometallic studied the reduction of 1,l’-ethylene- anion radical followed by further reduc- compounds. For example, triphenyltin bis(3-carbamidopyridinium bromide) tion to azopyridine. At the second fluoride reduces in acid ethanolic solu- and related compounds in aqueous polarographic wave, nap, = 5.88 i 0.04, tions in a one-electron reaction to pro- buffers. Coulometry showed that all with reduction all the way to the duce F- and PhrSnSnPhr (288). Morris of the compounds reduced in a two- protonated azopyridine dianion occur- (191) also found that the aquodiethyl- electron process which produced an ring. Cottrell and Mann (59) studied lead(1V) ion reduces in 0.5M HC10, at a internally coupled product which could the oxidation of aliphatic sulfides, such Hg electrode with naPp= 1.96 f 0.07 be oxidized back to the starting material as dimethyl sulfide, in MeCN and and produces tetraethyllead, lead metal, at more positive potentials. Oxidation showed that in dry MeCN nap, was and small amounts of diethylmercury. of the coupled reduction product with 0.84 f 0.02 with primary formation of Dessy and coworkers (67), continuing oxygen, however, led not to the starting a cation radical which undergoes secon- their extensive study of the electro- material, but to a free radical and then dary deprotonation and coupling reac- chemistry of organometallic compounds, to a species similar to 1,l‘-ethylene- tions. In the presence of 1% HzO, nsPp investigated the behavior of nitroaro- 2,2’-dipyridylium dibromide (Diquat). was 4.0 + 0.1. The oxidation of l-hy- matic mercury compounds, such as Riboflavin was shown to be reduced in droxypyridine-2-thione occurred in a bis(p-nitrophenyl)mercury, at a Hg DMSO solutions at a Hg electrode with one-electron transfer reaction at a Hg electrode in dimethoxyethane solutions. naPp= 0.99 + 0.01 ($77). Reversal electrode in aqueous buffers and pro- The coulometric reduction shows nap, = coulometry showed that the anion duced the organo-mercury compound as 2.1, with stepwise oxidation of the radical was not stable, however, and the a product (153). Donahue and Olver product, oia a radical anion, to the authors proposed that it decomposed in (70) examined the reduction of 8,8‘-di- parent compound possible. The reduc- parallel reactions to produce another quinolyldisulfide (RSSR) in methanolic tion of cyclopentadienyl iron benzene anion radical and a second, nonoxidiza- solutions at a Hg electrode and found hexafluophosphates or tetrafluoborates ble substance. The study of biolog- nap, = 1.99 f 0.05. The reaction was in ethanolic solutions occurred in a two- ically interesting substances by electro- said to occur by reaction of RSSR with electron step producing cyclopentadiene, chemical techniques with the hope of Hg to form (RS)2Hg,followed by a two- benzene, and Fe (18). Rollmann and gaining a greater understanding of electron reduction to form 8-mercapto- Iwamoto (242) investigated the reduc- biological oxidation-reduction processes quinoline, RSH. Yousefzadeh and tion of Co, Ni, Cu, and metal-free has been a fruitful one. However, one Mann (306) studied the reduction of sulfonated phthalocyanines in DMSO must be aware of the differences between alkyl esters of p-toluenesulfonic acid in at a Hg electrode and showed that suc- electrochemical processes, where elec-

26R ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 tron transfer is the major mode of reduc- as sources of metal ions for coulometric Mn(CN)65- (177) and Mo(CN)~~-(178), tion and biological ones where hydrogen titrations. However, in the generation was reported by Mendez and Conde. atom or hydride transfer may also be of La(II1) by oxidation of LaB6 (61-63) , Coulometric methods have also been involved. Although much useful elec- the current efficiency depended upon the employed for generation of species in trochemistry has been performed on particular piece of LaBs employed and solid electrolytes at high temperatures. these systems and valuable electro- was near 109y0. A similar difficulty Several studies involving variation of analytical techniques have been devised, was found when CrB was employed as a the Fe: 0 ratio and determination of the the real relevancy of these studies to source of Cr(V1) (QO), when a current composition of wustite (nonstoichio- elucidation of biological electron transfer efficiency of only 79.5y0 was found. metric iron oxides) have been reported mechanisms as they exist in organisms The need for specific calibrations of elec- (180, 239, 2.40). Another study dealt has not yet been demonstrated. trode material and the current efficien- with a determination of point defects in cies which deviate significantly from NiO, COO, FeO, and Co,Fe3-,04 by lOOy0 probably make these methods coulometric in a calcia-sta- CONTROLLED CURRENT COULOMETRY- useful only in special circumstances. bilized zirconia solid electrolyte at COULOMETRIC TITRATIONS Kostromin and Akhmetov (152) pro- 1200 OC. (259). hnalysis of oxygen in posed another method of generating oxide materials, such as &03, SiOz, Determinations. The application Cr(VI), by anodization of a Cr electrode, TiO2, and FeO, by heating at 2000 “C in of coulometric titrations to high pre- and found current efficiencies near 100% Ar or vacuum and coulometric deter- cision determinations continues to in 0.1 to 2M HC1 solutions at current mination of oxygen, has also been de- be of interest. Marinenko and Taylor densities of 0.3 to 70 mA/cm2. Electro- scribed (148). A number of other re- (170) performed careful titrations of generation of unusual oxidation states ported coulometric titrations are listed primary standard benzoic and oxalic of metals, in cyano complexes, such as in Table 111. acids with electrogenerated base, and found purities of 99.995 and 99.987%, respectively. Based on these values Table 111. Electrogenerated Titrants and Substances Determined by they calculated values of the faraday of Coulometric Titration 96,486.7 i 2.5 and 96,485.4 f 3.4 Electrogenerated coulombs per g.-equivalent, in good titrant Substance determined Reference agreement with values determined by OXIDANTS other methods. The authors again re- Chlorine Reinecke salt, papaverine (indirectly) iterate Tutundzic’s proposal that the Thio [tris(1-aziridiny1)-phosphine sulfide faraday be made the primary chemical Si-H groups in organosilicon compounds Bromine Antimony( I11 ) standard and point out that “the un- Europium(I1) certainty of the faraday is about one Nitrites order of magnitude smaller than that of Methylene blue Tetraalkyl lead compounds chemical standards commonly used.” Amines, amino acids, diamines Knoeck and Diehl (145) showed that Si-H groups in organosilicon compounds high precision titrations with electro- Double bonds generated Fe(I1) were possible, and Cyclohexene found that of K2Cr207 and Iodine Mercury fulminate, by reaction with thiosulfate ion and titration of excess (r\TH4)&elN03)6 yielded values of 99.976 Copper(II), by addition of excess thiosulfate and 99.972, respectively. These authors ion and back-titration also proposed a cell for high precision aerosol, by conversion to SO2 coulometry with external reagent gen- with hot copper Hypobromite Water, by conversion to NHI with NaNHl eration in which two external chambers Nitrogen in steel and metallic titanium by are employed (146). One chamber was conversion to NHI in Kjeldahl apparatus used to generate titrant sufficient for Manganese(II1) Iron(I1) 99.9% of the titration, and then the Chromium(V1) Iron(I1) Iron(II), Tin(II), Antimony(II1) titration was completed using a second, continuous flow, generating electrode REDUCTANTS chamber. Yoshimori and Hikawa (302) Iron(I1) Potassium dichromate Nitrate ion recommended coulometric titration with Plutonium(VI), with back-titration with electrogenerated acid or base to test the electrogenerated cerium(1V) purity of some standard reagents with Selenium(IV), tellurium(IV), by treatment very high precision. with excess KMn04and back-titration Titanium(II1) SeleniumGV). telluriumfIV) Several new coulometric titrants or Tellurium(VIj, tellurium(1v) methods of electrogeneration have been Molybdenum(V1) proposed. Durst and Ross (78) electro- Hexacyano- Ferricyanide ion chemically generated F- by transference manganese (I) Octacyano- Thallium(1) through a solid state Eu*+-doped LaF3 molybdenum(V) memb-ane. The transference was ac- Tin(I1) Selenium(IV), tellurium(1V) complijhed by using the membrane to PRECIPITATING BND COMPLEXING AQENTS separate the test solution from a 1M Silver (I) Chloride ion (62, 232, 233) NaF (pH 5.5) solution containing a Pt Chlorine and bromine in organic compounds (48) cathode. Upon electrolysis, reduction and polymers of H+ occurs and electroneutrality in Sulfide ion Thioacetamide the KaF solution is primarily main- Tetraphenylborate ion and indirect tained by passage of F- ions through determination of potassium ion the membrane into the test solution. Tetraphenylborate ion and indirect (213) The “current efficiency” for the process determination of alkaloids, nitrogenous organic bases, quaternary ammonium salts was 99.2y0. Fletcher and coworkers Continued suggested the oxidation of metal borides

ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 27 R Other Applications. Coulometric Table 111. Electrogenerated Titrants and Substances Determined by and galvanic detectors for analysis Coulometric Titration (Continued) of gas and liquids streams continue Electrogenerated to be improved and developed; 8 titrant Substance determined Reference number of recent devices are sum- marized in Table IV. Coulometric PRECIPITATINGAND COMPLEXINQ AQENTS methods also continue to be applied Aluminum( 111) Fluoride Ferricyanide ion Cobalt(I1) to C and H determinations in organic Ferrocyanide ion Zinc(II), dipropyltin, and dibutyltin substances. Hersch (114) has received perchlorates a patent for an apparatus in which 0, Copper(I1) Anthranilic acid C, and H are all ultimately converted to Lanthanum( 111) Fluoride and oxalate ions EDTA, and indirect determination of CO which is reacted with electrogen- nickel(II), sinc(II), cobalt(I1) erated Iz. Another technique involves Iodide ion Mercury(I), mercury(I1) conversion of C and H to water and determination with an electrolytic hy- ACIDSAND BASES grometer (14). In a similar method, C Base Potassium acid phthalate (269) Carboxylic acids (129, 146, 170) and N were determined by gas chroma- Dichromate ion (146) tography and H by conversion to HzO Boric acid (169) and coulometric electrolysis (56). The Ammonium salts, by passage through cation H content could also be determined by exchange resin column Weak acids, in nonaqueous solvents measuring the chromatographic 0 peak Carbon in alloys, by combustion to COZ produced by electrolysis of the water; Zinc(II), iron(I1) and iron(III), copper(II), by however, the accuracy of the chromato- titration of H+ liberated on electroreduction graphic H determination mas only or hydrolysis (indirect coulometric determination) =t0.2%compared to that of the coulo- Acid Sodium tetraborate metric determination, 10.03%. Cou- Carbonates lometric and galvanic detectors also Lithium hydroxide continue to find application in chroma- Weak bases, in concentrated salt solutions Weak bases, in nonaqueous solvents tographic systems; several detectors Organic nitrogen compounds, by conversion to have been reviewed (113, 262). Alco- “3 hols and aldehydes can be detected by a Karl Fischer Water, in various samples fuel-cell type of detector employing a reagent KOH electrolyte, a Ag oxide cathode and a porous Pt anode at which oxida- FUSEDSALTS tions of the organics occur (60). The Iodine Iodate, in NaN03-KN03 melt by adding I- sensitivity of this detector was said to and excess arsenic(II1) and back-titration be two orders of magnitude higher than hler cury (I) Halide, chromate and “oxide” in molten LiN03-Kn03 a thermal conductivity detector. Little- Tungsten(I1) Ytterbium(I1) in LiC1-KCl eutectic wood and Wiseman (157) determined hydrogenatable material in gas chroma- tographic effluent by continuous reac- Table IV. Galvanic Analyzers and Continuous Coulometric Titrators tion with electrogenerated HZ main- Substance tained at a constant level. Standard determined Method samples of 02 or Hz for gas chromato- ClZin gases Reduction of Clz at Au electrode us. Ag-AgC1 graphic analysis of gases were prepared counter electrode Fz in atmosphere Reaction of F1 with LiCl electrolyte and electrolytically (13). A coulometric reduction of Clz at Pt electrode us. Ag-AgC1 method of detection of amino acids in counter electrode liquid chromatography employing a Cu Mercaptans in gases Continuous titration with electrogenerated (79) has also been de- Ag( I) in ammoniacal electrolyte 02 in gases or liquids Applications and improvements in membrane- (81, 121, scribed (272). covered electrodes or other electrodes for 174, 188, Anodic stripping methods and related reducing Oz 190, 260, techniques continue to be applied to 261 ) metal deposits and metallic samples. 02 in gases, Na, and Solid electrolyte (ZrOz-MgO or ThO?-Y203) (88, 1841 steels galvanic cells with metal-metal oxide 195) Constant current anodic dissolution of electrodes Ni-Fe-Cu electroplates in KCNS solu- 01, peroxides and Galvanic detector with wire metal (e.g., Au, Pt) (147) tion has been employed (150). The hydroperoxides in indicator electrode and hollow tubular anode measurement of the thickness of metal liquids (e.g., Pb, Zn) in KOH electrolyte HZin gases Oxidation in fuel-cell arrangement at porous Pd deposits of Sn, SnFe2,Zn, and a number electrode nyith air-Pt black cathode of other metals has been described (151). H2 in solids and gases Conversion to HCl on catalyst column (e.g., Agasyan and Khamrakulov (4) carried PdCL) and galvanic detection ,of acid out the analysis of solutions of Cu(I1) HZin gases Reduction to hydride at cathode with metal hydride electrolyte and Ag(1) by plating the metals on a AT-compoundsin Conversion to NH3 by passing over Ni catalyst solid electrode followed by coulometric gases or water in HEstream and titration with stripping at a constant current. A electrogenerated H+ similar method for Cu in Zn solutions SOZ in gases Continuous titration with electrogenerated IZ using internal electrolysis with C-Mn02 has also been reported (236). Exten- cathode sions of previously described methods Acidic nr basic Galvanic detectors or continuous coulometric (96, 891 245) for the determination of H in Fe, Ni, substances titrators Cd, and other deposited metals by Water in various Applications and improvements in galvanic or (15, S1, 106, samples coulometric hygrometers b electrolysis of 187, 250, coulometric oxidation have been de- water in P~O~or metal oxiie electrolytes 252) scribed (91, 92). Cathodic reduction of metal salts also has been employed.

28 R ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970 Several Ag compounds, such as AgNO, been given. Peltier and Moinet (218) Model Q-2564 controlled potential cou- and AgCl were determined by placing discussed electric field asymmetry in lometry unit have been discussed (192). them in a Pt crucible, adding NaOH electrolysis cells and demonstrated that Several other potentiostats and coulom- solution to convert them to solid AgOH, with improper cell geometry, Hz can be etry circuits have also been described and reduction at constant current (291). evolved at some points of a Hg cathode (19, 76,80,141,249,296). Phillips and By observing the potential of the cath- even when the control potential is set Milner (219) have designed a modular ode vs. a Hg/HgO and noting the time at a value too positive to expect Hz coulometry apparatus designed for rou- when an inflection in potential occurs, evolution. Christian and Feldman (56) tine and automatic determinations and the coulombs required for the reduction described a microcell with a solution with provision for automatic sample could be determined. Barikov and volume of about 1 ml. and containing changing and data print-out. The unit Songina (SO) report a similar method for sealed-in Pt electrodes which is useful employs a digital voltmeter as a voltage- the determination of metal sulfides by for coulometric titrations. Berlandi and to-frequency converter for current inte- sample dissolution into a carbon paste Mark (56) designed a cell constructed gration and a clock pulse generator to electrode. from pyrolytic graphite and useful with control the sampling rate of digital The application of controlled current solution volumes of 5 to 250 pl for voltmeter and timing. In a typical coulometry to studies of reaction ki- selective controlled potential electro- determination the integrator is cleared, netics has been reviewed by Janata and deposition of radioactive isotopes. Har- a new solution is cycled into place and Mark (123). Coulometric methods were rar described cells for controlled poten- deaerated] the sample is electrolyzed at applied to measure the rates of bromina- tial coulometry with either platinum a pre-electrolysis potential for a time tl, tion of aliphatic ethylenic compounds in (109) or mercury (108) working elec- then coulometry carried out for tz min- acetic acid (76). Paris and Gregoire trodes. An apparatus for simultaneous utes, the results are printed out, and a applied coulometric titrations with elec- spectrophotometric, potentiometric, and new sample is cycled into place. The trogenerated base to the study of com- coulometric measurements has been authors report determination of Mg plexes, and investigated both the designed (171). The optical cell was amounts of Fe, U, Pu, and Cu with a hydrolysis of Be(I1) (209) and the com- fixed to the side-arm of the coulometric coefficient of variation of about 0.2%. plexation of Ti(II1) with picolinic acid cell, and solution was circulated into it Stock (266) has described an apparatus (210) with this technique. by means of bubbling nitrogen gas. A for automatically adding the electro- convenient cell for student electro- active species to a controlled-potential electrolysis cell during the electrolysis. APPARATUS deposition experiments has also been suggested (21). Propst (227) has received a patent for During the last two years, a number Coulometric Titration Apparatus. a previously-reviewed scanning coulom- of commercially available multipurpose A transistorized automatic coulomet- etry circuit in which the voltage scan electroanalytical instruments have be- ric apparatus for determination of rate is inversely proportional to changes come available. Several of these incor- S and C in metallic samples has been in the electrolysis current and has porate provision for both controlled designed (221). The method employs reported some applications of this current and controlled potential opera- potentiometric end-point detection and method (226, 228). tion and include integral coulometers. uses the voltage difference between the Lingane previously recommended a Ewing (84) has reviewed the characteris- indicator electrode potential and the fixed-field (permanent magnet) low tics of several of these. h description end-point potential, through a voltage- inertia dc motor-counter as an inexpen- of a number of commercially available to-frequency converter, to introduce sive (ca. $150) and convenient coulom- potentiostats and a general discussion of current impulses into the solution until eter. The difficulty with its application instrumentation, cells, and methodology the end point is reached. Several pat- was a drop in the count rate calibration in controlled potential coulometry has ents have been awarded for improve- factor at lower input voltages. He has also been given (29). Schroeder and ments in the design of automatic now devised a simple compensation Shain (255) have extended the treatment coulometric titrators (125, 126, 202, circuit which adds a small constant of design criteria for optimum response 231). Simple transistorized constant voltage into the input of the motor (156). and stability of potentiostats. Bewick current supplies, especially suitable for With compensation, the unit is capable (58) has discussed the effect of the in- student coulometric titrations have been of measuring the current-time integral creasingly popular positive feedback suggested (265, 292). Stock’s paper to &0.04% for a 20:l current range, iR-compensators on per- (265), as well as that of Evans (8S), also and to d~O.1~~for a 200:l range. formance and claimed that severe over- suggest some simple titrations appro- Several coulometers based on electro- compensation can readily be produced. priate for student experiments. chemical cells, but of less use in electro- However, another recent paper (57) Potentiostats and Coulometers. analytical determinations, have also discusses methods of determining the Potentiostat design continues to be been reported (139, 140, 297). uncompensated ohmic resistance by aimed at all solid-state component determining the current us. frequency devices with higher output power, ACKNOWLEDGMENT curve and extrapolating to infinite fre- improved stability and shorter re- quency, and reports successful correc- sponse time. Will (299) described a The author is greatly indebted to tion of uncompensated iR at frequencies potentiostat based on a Philbrick P85AH Gaynel Klingemann for her assistance in of 0-10 KHz. Pilla and coworkers control amplifier and a P25AH follower the preparation of this manuscript. (219a) also considered models for iR with a rise time of about 3 psec, 10l2 compensation and conclude that “stable ohm input resistance and with provision LITERATURE CITED overcompensation cannot take place in for iR compensation. The transis- any practical system.” From a prac- torized booster amplifier employed was (1) Abaffy, F., Kern. Ind., 14, 245 (1965). tical standpoint, we have found that capable of delivering +3 amperes at (2) Adams, R. N., “Electrochemistry at Solid Electrodes,” Marcel Dekker, Inc., positive feedback iR compensation is a A10 volts. Stock (264) has designed a New York, N. Y., 1969. necessary and very useful technique for very simple potentiostat for student (3) Agasyan, P. K., Denisova, A. N., electrochemical studies in nonaqueous work which is composed of an integrated Agasyan, L. B., Nikolaeva, E. R., solvents and other highly resistive solu- circuit and two transistors. The output Zuuod. Lab., 34, 129 (1968). (4) Agasyan, P. K., Khamrakulov, T. K., tions, even for measurements at rather for this unit is not specified] but it is Zh. Anal. Khim., 23, 19 (1968). slow times. probably below 100 mA at 10 volts. (5) Agasyan L. B., Nikolaeva, E. R., Several discussions of cell design have Modifications of the popular ORNL Agasyan, fi. K., ibid., 22, 904 (1967).

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Electrophoresis

R. D. Strickland, Research Service, Veterans Adminisfrafion Hospifal, Albuquerque, N. M.

ECAUSE OF SPACE limitations, it has covered hemoglobins is comprehensive, The wide scope of electrophoresis B been necessary to curtail the num- but examples of advances in hemoglobin makes a comprehensive critical review ber of references in this review. Those analysis are merely representative. In impractical; selected sections where im- given have been selected from more than enzymology the literature has been portant progress has been made include 3000 articles, most of which seemed to so extensive that only papers of out- discussions, while others where progress deserve notice. In choosing the articles standing importance have been men- has been less dramatic are treated in to be cited, preference has been given to tioned, and even some of these have been bibliographic style. those concerned with analytical ad- omitted. Considerable weight has been This writer thanks all persons who vances through electrophoresis. Indus- given to the accessibility of journals to responded to the request for reprints trial applications, patents on apparatus, chemists in the United States, but made in the 1968 review, and asks once and iontophoresis have been entirely exclusion of papers on this basis has again for reprints from authors who excluded on this basis, despite the fact sometimes been undesirable; in such would like to have their work considered that each of these categories included instances the Chemical Abstracts refer- for citation in the next review. important contributions. Other topics ence number accompanies the listing. This review extends the coverage of a that have been severely restricted are Reviews are mentioned in the appro- previous one (l243), with most of the hemoglobin research and enzymology. priate sections rather than being references dating between the latter It is believed that the list of newly dis- grouped under a single heading. part of 1967 and the first half of 1969.

32 R ANALYTICAL CHEMISTRY, VOL. 42, NO. 5, APRIL 1970