Anal. Chem. 1904, 56,7 R-20 R (218) Scholz, E. fresenlus’ 2.Anal. Chem. 1083, 374, 587-571. (217) Schumacher, E.; Umland, F. Anal.-Taschenb. 1081, 2, 197-209; C A I 96, 1 4 6 3 3 ~ (1982). (218) Seminov, I. A,; Komarova, N. I. Zh. Anal. Khlm. 1081, 36, 11041108; CAI 95, 143475f (1981). (219) Shi, C. N.; Lu, C. N.; NI, Q. D.; Chang, M. S.Anal. Chem. 1082, 54, 1119-1 121. (220) Shibalko, G. V.; Stenlna, N. I. Zavod. Lab. 1083, 49, 2-4; CA, 99, l51262g (1983). (221) Shlmodzu Selsakusho, Ltd. Jpn. Tokkyo Koho JP 57 54,741 (82, 54741) (Cl. GOIN27/44), 19 Nov 1982; CA, 99, 3 2 4 7 5 ~(1983). (222) Sklnln, A. I.; Peresypklna, G, D.; Khmelevaya, L. P. Zh. Nauchn. Prlkl. Fofogr. Klnematogr. 1083, 28, 70-73; CA, 98, 100453s (1983). (223) Sleh, D. H.; Dunham, J. M. Anal. Chem. 1082, 54, 1216-1217. (224) Sierra, M. T.; Sanchez-Pedreno, C.; Garcia, M. S.; Martin, J. M. An. Qulm., Ser. B 1082, 78, 108-113; CA, 97, 85598y(1982). (225) Slngh, T. B.; Prasad, B. B. Chem. Anal. (Warsaw) 1081, 26, 541-544. (228) Skavrada, J.; Hladky, 0.; Senkyr, J.; Brezlna, J. Czech. CS 195032 (Cl. (301N27/42), 1 Feb 1982; CA, 97, 84350c (1982). (227) Skavrada, J.; Hladky, 0.; Senkyr, J.; Brezlna, J. Czech CS 200 049 (CI. GOIN27/42), 15 Oct 1982; CA, 98, 1548330 (1983). (228) Soiomatln, V. T.; Malyuta, V. F.; Ermolaeva, T. N. Zavod. Lab, 1082, 48, 18-17; CA, 96, 134939d(1982). (229) Stock J. T. Anal. Chem. 1082. 54, 1R-9R. (230) Stock J. T. Trends Anal. Chem. 1081, 7 , 59-82. Ibid. 1231) ---- - 1082. 7. 117-120. (232) Stuzka,V-Chem. Llsfy 1081, 75, 949-982; CA, 95, 180099g (1981). (233) Stuzka, V.; Zapletaiek, M. Acta Unlv. Palacki. Olomuc., f a c . Rerum Naf. 1082, 73 (Chem 21), 75-88. (234) Sun, H. XlandalHuagong 1082, 77,43-44; CA, 97, 103421v(1982). (235) Suprunovlch, V. I.; Plakslenko, I.L. Zavod. Lab. 1083, 49, 15-17; CA, 98, 172172~ (1983). (238) Svlrldenko, Zh. V.; Orzhekhovskaya, A. I.; Sklenko, E. A.; Shegeda, V. B.; Karchenko, L. A. Zavod. Lab. 1083, 49, 132950~(1983). (237) Takahasl, Y.; Moore, R. T.; Joyce, R. J. Chem. Water Reuse 1081, 2, 127-148. (238) Toledo, A. P. P.; Capelato, M. D.; Carvalho, J. F.; Gonzalez, J. An. Slmp. Bras. Netroquim. Eletroanal., 3rd 1082, 7 , 93-98; CA, 98, 95358h (1983). (239) Truedsson, L. A. J. Chromafogr. lQ82, 234, 47-58. (240) Tsalkov, S. P. Dokl. Bolg. Akad. Nauk 1082, 35, 61-62; CA, 97, 28343r (1982). (241) Tsalkov, S. P.; Bolchlnova, E. S.; Brynzova, E. D. Zh. Anal. Khlm. 1082, 37, 1329-1330; CAI 97, 187920C (1982). (242) Tseng, H. H.; Pau, C. T. K ’ o Hsueh T’ung Pa0 1081, 26, 787; CAI 95, 231290~(1981). (243) Tsentkovskll, V. M.; Evgen’ev, M. I.; Averko-Antonovich, A. A. “Conductometric and Coulometric Methods of Analysis”; Kazan. Khim.Tekhnol. Inst. 1982; CAI 97, 229388h (1982). (244) Umland, F.; Schumacher, E. Bunsekl Kagaku 1082, 30, SI-SIO. (245) Usatenko, Yu. I.; Sukhoruchklna, A. S.; Postnlkov, V. A. Vopr. Khlm. Khlm. Tekhnol. 1080, 60, 44-48; CA, 95, 1 8 0 2 1 9 (1981). ~ I
(248) Usatenko, Yu. I.; Tkacheva, L. M.; Pavlichenko, V. A. Vopr. Khlm. Khlm. Tekhnol. 1080, 60, 51-53; CA. 95, 180220q(1981). (247) Vasserman, A. M.; Bulanova, E. A.; Kunln, L. L. Zh. Anal. Khim. 1082, 37, 1820-1827; CA, 98, 4 6 1 0 9 ~(1983). (248) Vlcente-Perez, S.; Plnllla, J. M.; Sidrach, C. An. Qulm., Ser. B 1082, 78, 298-299; CA, 99, 15637f (1983). (249) Volkova, G. V.; Golubova, E. A,; Dmltrieva, G. A,; Starovoltova, N. M.; Bondareva, N. B. U.S.S.R. SU 1000402 (Cl. COIG55/00), 28 Feb 1983; CA, 99, 325068 (1983). (250) Von Wandruszka, R. J. Chem. Educ. 1082, 59, 781-782. (251) Waite, T. D.; Morel, F. M. M. Anal. Chem. 1083, 55, 1268-1274. (252) Wan, G.; Shen, Q.; Wang, Y.; Shao, S.; Gao, L. YoujlHuaxue 1081, 5 , 350-353; CAI 96, 2 7 9 1 5 ~(1982). (253) Wang, J.; Freiha, 8. A. Anal. Chem. 1082, 54, 1231-1233. (254) Wang, T. Huaxue Tongbao 1083 (3), 28-30, 54; CA, 99, 98496h (1983). (255) Welsz, H.; Fritz, 0. Anal. Chlm. Acta 1082, 139, 207-217. (258) Wojciechowskl, M.; Rubei, S.; Faikowska, W. Anal. Chim. Acta 1082, 74 7 , 387-392. (257) Woolf, A. A. Anal. Chem. 1082, 54, 2134-2136. (258) Xlang, L.; Chen, M.; Gao, L. You// Huaxue 1082 (3), 193-195; CAI 97, 119838~ (1982). (259) Xu, L.; Liu, A. Yao Hsueh Hsueh Pa0 1081, 76, 39-44; CA, 95, 1568361(1981). (280) Ibld. 1081, 76, 132-138; CA, 95, 1 7 5 8 5 5 ~ (1981). (281) Yan, H.; Zhang, X. fenxi Huaxue 1083, 7 7 , 542-544; CA, 99, 115182~(1983). (262) Yokogawa Electric Works, Ltd., Jpn. Kokai Tokkyo Koho JP 82, 86038 (Ci. GOIN27/42), 28 May 1982; CAI 97, 137901s (1982). (283) IbM. JP 82, 88359 (Ci. GOIN27/42), 2 Jun 1982; CAI 97, 137897~ (1982). (284) Yoshlda, S.; Hirose, S. Yakuzaigaku 1081, 4 7 , 140-145; CA, 96, 91728n (1982). (285) Yoshlmorl, T. Rev. Anal. Chem. 1082, 6, 13-48. (288) Yoshlmorl, T.; Hayama, S. Bunsekl Kagaku 1082, 31, 523-528; CA , 97, 2073898 (1982). (287) IbM. 1083, 32, 214-217; CA, 99, 833972 (1983). (268) Yu, G.; Zheng, L. fenxl Huaxue 1083, 17, 12-18; CA, 99, 4 7 2 0 3 ~ (1983). (289) Yunusova, P. T.; Mlrzaev, F. M.; Yunusov, D. Kh. Uzb. Khlm. Zh. 1082 (8), 21-25; CA, 98, 154574a (1983). (270) Zeng, H.; Pan, C. Lanzhou Daxue Xuebao, Zlran Kexueban 1082, 78, 126-130; CA, 97, 2072739 (1982). (271) Zhana, J.; Wey, Y. fenxi Huaxue 1083, 7 7 , 35-37; CA, 99, 90657r (1983). (272) Zhdanov, A. K.; Akhmedov, G.; Akent’eva, N. A. Uzb. Khim . Zh. 1081 (8), 9-13; CA, 96, 192547k (1982). (273) Zheng, J. Huan Ching K’o Hsueh 1081, 2, 176-179; CA, 95, 208869~(1981). (274) Zhou, P.; Yuan, S.; Zhang, J. fenxl Huaxue 1082, 70, 428.-432; CA. 98, 110092n (1983). (275) Zlma, J.; Rousal, M.; Dolezal, J. Microchem. J. 1081, 26, 506-513. (278) Zsigral, I. J.; Bartusz, D. B. Talanta 1083, 30, 54-56.
Dynamic Electrochemistry: Methodology and Applications Dennis C. Johnson* Department of Chemistry, Iowa State University, Ames, Iowa 50011
Michael D. Ryan Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53233
George S . Wilson*
Department of Chemistry, University of Arizona, Tucson, Arizona 85721
With this report the two previous fundamental reviews on electrochemistry have been combined. The intent has been to choose, from an extensive number of publications, work detailing novel developments or indicating important trends. 0003-2700/84/0356-7R$08.50l0
We have attempted to consider new analytical methodology as well as the characterization of electrochemical reactions. References have been selected which, for the most part, have appeared in print from January 1982 through December 1983. 0 1984 American Chemical Society
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A. BOOKS AND REVIEWS Two textbooks which will be of special interest to educators, Plambeck (30A)and Vassos and Ewing (37A),provided very useful and very basic treatments of electroanalyticalchemistry. These texts offer realistic approaches suited for the average entering graduate student without prior experience in the subject. The applicability of electroanalytical techniques to biomedical and biochemical problems is receiving widespread attention, and major works on the subject have appeared. Koryta (25A)discussed general medical and biological applications of electroanalysis; and Dryhurst, Kadish, Scheller, and Renneberg (IOA) described the subject of biological electrochemistry. Ke ser and Gutmann (24A)as well as Milazzo (27A)have e z t e d major treatments of bioelectrochemistry. Digital simulation is a very useful tool for treating Faradaic processes under mixed control and Britz has provided an informative review (6A)of the technique. The relatively new subject of conductive polymers was reviewed under the editorship of Beymour (2A). Several worthwhile titles appeared for non-English monoaphs. These include general treatments of electroanalysis Sanchez Batanero (33A)and polarographic analysis by Geissler (12A).Damaskin and Petrii (9A)presented a second edition of “Introduction to Electrochemical Kinetics”. Detailed considerations of electron-transfer reactions were given by Hamann and Vielstich (13A),by Krishtalik (26A),and by Pavlova and Lomov (19A).The effect of the double layer on electrode kinetics was summarized by Frumkin, Andreev, and Boguslavskii (1IA). Brainina and Neiman (5A)discussed solid-phase reactions in electroanalysis. New additions were added to the series on electrochemistry and electroanalysis. Volume 12 appeared in Bard’s “Electroanalytical Chemistry” (IA)containing chapters on flow electrolysis by Sioda and Keating, voltammetric studies of adsorbed species by Laviron, and coulostatic pulse techniques by Van Leeuwen. Volumes 4,5,and 6 (4A,8A,42A)appeared in the series “Comprehensive Treatise of Electrochemistry”, edited by Bockris, Conway, and Yeager, with assistance by White (4A)and Sarangapani (42A).Volume 4 covers materials science, and volume 5 discusses thermodynamic and transport properties of electrolytic solutions. Volume 6 is of most interest to this review, covering numerous aspecta of electrodics and transport. Included are the followin fundamentals of transport, by Ibl; diffusional transport, %y Marchiano and Arvia; convective transport, by Ibl and Dossenbach; current distribution, by Ibl; porous electrodes, by Chizmadzhev and Chirkov; porous and fluidized-bed electrodes, by Goodridge and Wright; and gas evolving electrodes, by Vogt. The latest addition to “Modern Aspects of Electrochemistry”, edited by Bockris, Conway, and White (3A),includes the following considerations: the effects of ultrasonic vibration, by Zana and Yeager; impedance measurements, by Macdonald and McKubre; photoelectrochemistry, by Khan and Bockris; Clz production, by Novak, Tilak, and Conway; titanium electrochemistry, by Kelly; properties of membrane ionomers, by Mauritz and Hopfinger; bioelectrochemistry, by Findl; and structural considerations in electrocatalysis, by Kinoshita. Volume 8 of “A Specialist Periodical Report: presents a review Electrochemistry”,edited by Pletcher (31A), of porous electrodes by Hampson and McNeil. The successful advances of electroanalysis in the study of biological systems was the subject of a symposium published as volume 201 of “Advances in Chemistry Series”, with Kadish as editor (18A). The present status of various specific aspects of electroanalytical chemistry have received noteworthy reviews. Strippin voltammetry is an old topic which received new reviews f y Heineman (15A)and Wang (40A). Heineman (16A)also reviewed applications of optically transparent thin-layer electrodes for determination of formal reduction potential and electron number. Advances in polarographic analysis were described by Kalvoda (I9A,20A) and by J. Osteryoung (28A). Pungor, Feher, Nagy, and Toth (32A) reviewed applications of electrochemistry in automatic analysis. Progress in the analysis of chemically modified electrodes was reviewed by Karweik, Miller, Porter, and Kuwana (23A);and Zak and Kuwana (43A)described the electrocatalytic significance of such electrodes. An extensive review containing 1438 references was presented by Julliad
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and Chanon (17A)on thermal and electrochemical aspects of photoelectron-transfer catalysis. The subject of flow-through amperometric and voltammetric detectors has been a popular topic of review because of the successful and widespread application of EC detection for liquid chromatography, flow-injection analysis, and the monitoring of process streams. General reviews were given by Brunt (7A),Stulik and Pacakova (35A),and Hanekamp, Bos, and Frei (14A).Trojanek (36A)considered Faradaic as well as non-Faradaic electrochemical detection. Of further value from the perspectives of pedagogy and review is the appearance of the proceedings of the State of the Art Symposium: Electrochemistry (34A)organized by the Division of Chemical Education of the American Chemical Society. On an international note, we welcome information on the development of electroanalytical chemistry in the People’s Republic of China, reviews in English of progress from 1949 to 1979 are cited (21A,38A,22A,39A,41A).
B. MASS TRANSFER Rotating disk and ring-disk electrodes (RDE and RRDE) continue as the primary hydrodynamic tools for voltammetric studies of electrochemical reactions at solid electrodes. Pons, Speiser, and McAleer (33B)discussed orthogonal collocation for simulation of the response of the RDE; and Tribollet and Newman (43B)presented an analytical expression for the Warburg impedance at a RDE at low frequencies. The theory of linear sweep voltammetry is being studied by Andricacos and co-workers;publication with Ross (5B)considered multisweep cyclic voltammetry for a reversible deposition at a RDE and a stationary planar electrode; and a publication with Quintana and Cheh (35B)described the current response for first-order uasi-reversiblereactions. The RDE was used for kinetic stu8es of superoxide dismutmes by Argese et al. (6B) and for electrochemical reactions of sulfide at Ag electrodes by Shimizu, Aoki, and Osteryoung (39B).Albery, Boutelle, Colby, and Hillman (3B)described the use of a ring-current transient at a RRDE for deducing the Faradaic component of a disk-current transient. Fujishima, Karasawa, and Honda (14B)demonstrated use of the RRDE for determination of the rate of the photographic development process. A Hgplated RRDE is useful for study of cathodic reactions as demonstrated again by Daly, Page, and Compton (IOB)and by Brihaye and Duyckaerta (7B).The rotating split-ring-disk electrode offers occasional advantage over the ordinary RRDE, as was described by Okuyama (29B)and by Sakashita, Loechel, and Streblow (37B). Hydrodynamically modulated voltammetry is useful at hydrodynamic electrodes for determination of a small mass transport coupled current flowing simultaneouslywith a large surface-controlled current. Deslouis et al. (12B)and Tribollet and Newman (44B)offered considerations of the frequency response of electrochemicalsystems to a sinusoidal modulation at a RDE. Miller and Rosamilla (26B)expanded significantly the analytical utility of sinusoidal modulation at a RDE by their consideration of its derivative mode. Swathirajan and Bruckenstein (42B)compared the anodic behavior of I- at a Pt RDE under steady-state and hydrodynamically modulated conditions. Novotny and Smoler (28B)used “interrupted convection”for measurements of adsorption equilibrium. In a creative variation of modulated voltammetry, Miller (25B) has described the effect of periodic heat pulses at a RDE in a technique called “thermal modulation voltammetry”. A modest interest has reappeared regarding the enhancement of convective mass-transport at vibrating electrodes. provided a literature review and the Pratt and Johnson (XB) evaluation of a practical design. A solution to the equation of mass transport for a resonant vibrating wire electrode was described by Moorhead, Stephens, and Bhat (27B).Additional consideration of frequency response was given by Liu et al. (24B). A significant surge of activity is apparently related to nonlinear diffusional transport to a single microelectrode and to arrays of closely spaced microelectrodes. There has been some long-standing interest in nonlinear diffusion to planar electrodes, resulting from concern for deviations from the Cottrell equation when applied to “unshielded” electrodes, and for the decreased response of solid electrodes with a significant blockage of active surface sites. The onset of the present flurry seems to be correlated with the appearance of
DYNAMIC
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Anderson (SB)evaluated a film cnrbon fiber electrode for differential pube strip ing vo tammetry; and Dayton. Ewing, and Wqhtman (118)& m a d application of microelectrodes in brain tissues. Amperometric response under mixed kinetidiffusionel control and for a linear potential sweep wa8 considered by Ksenzhek and Lohach (208). We also cite Oldham (308). who derived exact expressions for transient currents a t electrodes having the geometries of a wedge and a groove. Weiaahaar and Tallman (468) attributed the nonlinearity of i-t-l/z plots of data obtained at a KeEf/graphite electrode to the contribution of radial diffusion a t each graphite microparticle in the surface of the composite electrode. Reller, Kirowa-Eisner, and Gileadi (368) applied digital simulation to arra of microelectrodes, and Alhery and Bartlett ( I B ) applied)8such an electrode in the photoelectrochemical decomposition of water. Amatore, Saveant, and Tessier (4B) d y e d the nonlinear diffusional effectsmultii from partial blocking of an electrode surface; and Levart (23B) applied digital simulation to the case of a partially blocked electrode. The availability of an analytical solution for steady-state transport to the RDE has made these electrodes ideal for study of electrocatalytic reactions associated with film-covered electrodes. Freese and Smart (138) described a rotating membrane electrode with the reference and counterelectdes also isolated behind the membrane. Laviron (21B) offered a theoretical study of electron transfer mediated by a redox polymer where current can be limited by mass transport in solution or thrwgh the film. Oyama et aL (328,328)provided experimental and theoretical evaluations of electrocatalytic reactions at a fh-covered RDE. Albery, Boutelle, Colby, and ) transient studies by using potential step Hillman (XIapplied and optical absorbance on a thionine-mated electrode and the transients were discussed in term of a single diffusion model. DAlkaine (9B) also described voltammetric results for a quasi-conducting film. Finall ,Leddy and Bard (ZZB) have used chronoamperometry andlthe RDE to differentiate hetween two models for maas transport at a membrane-covered electrode.
Y-
' / practical carbon-fiber electrodes and composite electrodes produced hy h-pressure m o l d i of mixtures of carbon particles a n d s t i c s (e.g., Kel-0. Yap and Doas? (418) criticized an inaccurate the determinations of diffusion coefficientsb a d on chronoamperometric data a t unshielded planar electrodes even for short times. However, Weiashaar refuted this with data for Fe(CNh," oband Tallman (458) tained on the millisecond time scale. Speiser and Pons (41B) applied the orthogonal collocation method of simulation to a consideration of edge effects in chronoamperometry and voltammetry. For extremely small electrodes in unstirred solutions, spherical diffusion can predominate, especially a t long times when the radius is leas than the diffusion-layer thickness,and a nonzero steady-state current can be observed. Krichmar (198) described the evaluation of a 'nonsphericity coefficient" for the m e of a nonisoconcentration diffusion coefficient. Heinze (16B, 17B)applied digital simulation to the deacription of mass transport a t an unshielded microdisk electrode and concluded the Cottrell e uation should he rewhere b placed hy i = [nFADC6[(rrDt)'lz][ l +%(Dt/R2)1/z] is dependent on (Dt RZ)I*,R b e i i the electrode radius, with values of 1.772-2.254. Hepel and Osteryoung USB) compared results for chronoamperometric transients a t Pt and Au microelectrodes to theoretical predictions for hemispherical and disk electrodes. Excellent agreement was found with theory for a stationary disk electrode. Shoup and Szabo (40B) described an analytical equation, and a digital simulation baaed on a 'hopscotch" orithm, for chronoamperometry a t mid i s k electrodes. alua, Schenk, and Adams (158) applied double potential step amperometry, cyclic voltammetry, and chronopotentiometry with current reversal at small electrodes (R 3-300 wm); and Scharifker and Hills (388)teated microelectrodes for kinetic studies. Cushman, Bennett, and
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C. ELECTRODE KINETICS New methods for studying electrode kinetics include thermal modulation voltammet ( Z B ) where the temperature is modulated by a c h o p p e d L 0; light, and couloamperometry (12C, 3 4 0 The first technique may have the ability to discriminate between solid state and surface recombination in semiconductor electrodes from electrochemical kinetics, while the latter wan used to measure reactions with second-order rate constants of lOe M-' s?. DeLevie (110 has reported on the use of Hadamard transform AC polarography. The development of electrochemical theory has included further work on orthogonal collocation for the simulation of voltammetry (44C) and the DME (33C). Schindler and Weaver (38C) have developed a parallel simulation scheme for nonideal electrochemical transients. Several methods of digital simulation were compared recently by Magno et al. (31C),while Galvez ( 1 5 0 has studied the use of nonlinear perturbation functions a t the DME. The theory for linear sweep voltammetry wan derived for finite diffusion spaced ( 7 0 and rotating disk electrodes (35B). There have been several reports in the past 2 yeam on the effect of electrode material on the reversihility of electron transfers. The effects of heat treatment of glassy carbon electrodes (330)and of paste composition of graphite electrodes (24L) on electron-transfer rates were examined. Amatore et al. ( 5 0 studied the variation of electron transfer rates of organic molecules on several solid electrodes, while Saveant and Teeaier (37C)found additional evidence for the variation of the electrochemical transfer coefficient with potential. Weaver (43C) looked at the comparisons between theoretical and experimental isotope effects for some outersphere electrochemical reactions. Hannia and Aoyagui (170 studied the electrode kinetic parameters of solvated electrons in liquid ammonia. Gabrielli et al. ( 1 4 0 developed a temperature perturbation method for electrochemical kinetics investigations. Because many of the studies of electrode kinetics involve the use of organic solvents, the elimination of uncompensated resistance is always a major problem. Aalstad and Parker (115)have developed a method that uses normalized potential sweep voltammetry to choose the correct ANALYTICAL CHEMISTRY. VOL. 58. NO. 5. APRIL 1884
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value of the uncompensated resistance. The determination of the electrochemical mechanism for a given chemical system has generally been ap roached from a trial-and-error point of view. This was possibe when there were only a few systems for which we had theoretical solutions. Now, the computational techniques have improved enough so that there are a large number of possibilities that can be examined. For the expert in the field, it is still possible to resolve the problem fairly rapidly, but a detailed analysis is beyond the scope of most chemists. Therefore, it is heartening to see the development of computerized methods for the mechanistic classification of electrochemical data. The work of Ruslin (25L) is an example of this approach. Caster et al. (IOC)ave examined the curve fitting of semiderivative voltammetric data, and Oldham (32C) reported on the extraction of kinetic parameters from chronoamperometric and chronocoulometric data. The theory for a number of different pulse methods has been derived recently. Anderson and Bond (6C) developed the theory for digital alternating current polarography. Osteryoung et al. (29C, 30C) have looked at the theory for differential normal pulse voltammetry. The effect of a fiit-order catalytic process (19C) and a prior chemical equilibrium (20C) on the differential pulse polarogram have been derived. In addition, the influence of reactant adsorption in normal pulse polarography was studied (40C). Finally, the theory for cyclic chronoamperometry, chronocoulometry, and chronoabsor tometry with slow electron transfer has been derived (134, along with the theory for the disproportionation (DISP), dimerization, and followin chemical reaction for cyclic chronocoulometry (41C,42t). The development of the electrochemical theory for several important mechanisms has continued during the past 2 years. Several workers have studied the effect of multielectron transfers on voltammetric waves, with the effect of protonation or adsorption of the intermediate (16C, 24-28C). The catalytic mechanism continues to stir a considerable amount of interest. The work of Saveant on electron transfer induced reactions has opened up an interesting area where a product may be electrosynthesized with the passage of a minimal amount of current (3C). This may present additional possibilities for economical large scale electrosynthesis. Rusling and Connors (35C) examined the problem of extracting kinetic data in a pseudo-first-order catalytic reaction where overlapping voltammetric data result. Bhadani et al. (8C) have derived the mathematics of electrochemical polymerization kinetics. Laviron (22C, 23C) has reported on the theory for the 1- e, 1- H+ reaction, with protonations at equilibrium, in addition to work on the CE and EC mechanism for thin layers or surfaces (39C). Some recent developments in cyclic voltammetric and simulation theory have involved fundamental and second harmonic AC cyclic voltammetry with an EC mechanism ( 9 0 , improvements on the determination of dimerization rate constants (21C),and the digital simulation of very rapid chemical reactions (36C). Saveant and others have studied in detail the competition between several mechanistic pathways. This approach is extremely important because most compounds do not react by a single pathway, and the most interesting cases are probably those for which competitive pathways exist. Some examples of these cases are homogeneous vs. heterogeneous electron transfer competition at illuminated semiconductor electrodes (18C) or electrohydrogenation (1C)and the competition between hydrogenation and dimerization (2C). Finally, their work on the effect of water on the dimerization of activated olefins (4C) points out the importance of specific solvation of the radical anion by water in aprotic solvents on subsequent reaction rates.
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D. SURFACE EFFECTS Whereas activity in the study of the electrical double layer seems to have waned, consideration of charging current is critical within the present trend toward (i) fast transient techniques applied to FIAEC and LCEC and (ii) the emphasis on lower detection limits. A simple theory of the double layer was provided by Henderson (100); and Oldham and Zoski (270) described the use of a nonlinear potential ramp to minimize the effect of charging current in semiintegral electroanalysis. The majority of interest regarding electrode surfaces relates to electrocatalytic benefits derived from physical, electro10 R
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chemical, and chemical modifications of the surface. Citations related to chemical derivatization of surfaces are given elsewhere. Kauffmann (160) described the characterization of graphite-coated metallic electrodes prepared by spraying Pt, Al, and Cu with a colloidal suspension of graphite particles. Heat treatment of glassy carbon electrodes was found by Stutts et al. (350) to enhance the reversibility of some reactions. Alternate preanodization and precathodization of lassy-carbon electrodes was determined to be catalytically eneficial by Engstrom (60) and by Ravichandran and Baldwin (310). Ghosh and Bard (90) described voltammetric results for electroactive species incorporated into a clay modified electrode; approximately 50-100 equivalent layers of an electroactive compound in the clay film were electrolyzed. Numerous anodic oxidations involving oxygen-transfer mechanisms are catalyzed by surface oxides at solid electrodes. Until recently, the majority of electrochemicalstudies of oxide electrocatalysis apparently was motivated by considerations of electrochemical energy transformations and emphasized kinetic studies of O2evolution and oxidations of simple organic molecules. Present examples of the latter are by Hira (120), who reported on oxidation of MeOH and EtOH, and by Lamy et al. (200), who studied oxidations of CO, HCOOH, and MeOH at single-crystal Pt electrodes. The application of multistep potential waveforms at Pt electrodes was concluded by Hughes and Johnson (31G, 32G) to be applicable for the reproducible oxide-catalyzed detection of all alcohols, polyalcohols, and carbohydrates in analyses by LCEC; and by Polta and Johnson (290) for all amines and amino acids. Chialvo, Triaca, and Arvia (50) described the changes in surface morphology at a Pt electrode resulting from repeated anodic formation and cathodic dissolution of surface oxide. The potentiodynamic response of ruthenized Pt electrodes was characterized by Lezna, De Taconni, and Awia (220); and O'Grady et al. (260) applied cyclic voltammetry to the study of the four crystal surfaces of RuOz. Katayama-Aramata and Toyoshima (150)utilized XPS in conjunction with electrochemical studies of the anodic oxidation of MeOH at Pd-Sn oxide surfaces and determined that the valence states of Pd were modified by the presence of the Sn oxide. Zak and Kuwana (420) reported that oxidations of catechol, 1,4hydroquinone, and ascorbic acid at a glassy-carbon electrode are catalyzed by the presence of imbedded alumina on the electrode surface. The alumina had been fortuitously introduced as part of an electrode polishing procedure. The application of oxide-catalyzed anodic detection of alcohols at Ni electrodes in alkaline media was among the first to be applied successfully for electroanalysis by Huber and co-workers. Recently Hui and Huber (130) reported application for EC detection of amines and amino acids. Kaulen and Schaefer (170) reported on oxidations of primary and secondary alcohols and diols at Ni oxide electrodes. Maximovitch and Durand (240) applied voltammetry and surface spectroscopy to the characterization of the role of hydroxide layers in the evolution and oxidation of hydrogen on Ni electrodes in KOH. Carugati, Lodi, and Trasatti (30) described pH effects on chlorine evolution at Ti/NiCoz04 electrodes. Metallic ad-atoms on noble-metal electrodes are known to electrocatalyze numerous anodic reactions of organic compounds with potential application for electroanalysis. Bruckenstein and co-workers continue to apply ring-disk techniques to the study of underpotential depositions (UPD) of metallic ad-atoms and recently (370) reported on the kinetics and mechanism for UPD of Ag on Au. Tuseeva and Skundin (380)studied the inhibition of Ag ad-atoms on Au for the Fe3+ Fez+ redox reactions. Vasil'ev (390) reported on the cata ytic effects of ad-atoms and microdeposits on various electrochemical reactions; and Sakamoto and Takamura (320) studied the catalytic oxidation of glucose on Pt electrodes modified by Bi and P b ad-atoms. Changes in the ionic content of the inner-Helmholtz plane can significantly modify electrochemical charge-transfer rates. Fawcett and Markusova (80)presented further evidence of this effect with a study of the effect of adsorbed I- on the kinetics of the reduction of S20f at a Hg electrode. The role and effect of the adsorption of reactants and reaction products on electrochemical reactions continues to receive sparse but important attention. Laviron (210)
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presented a theoretical study of redox reactions with adsorption of reactant under a Langmuirian isotherm. Sasaki et al. (330) considered chronocoulometric response for reactions controlled by both Langmuir and Temkin isotherms;and Lovric (230) developed the theory of EE mechanisms with adsorptions of the intermediate product. Chia, Soriaga, and Hubbard ( 4 0 )studied the oxidation of chemisorbed organic molecules at Pt and reported on the effect of chirality of L- and DL-Dopa. Soriaga, Stickney, and Hubbard (340) reported on the oxidation of aromatic molecules adsorbed on Pt electrodes and presented evidence that surface molecular orientations change with solution concentration. Sustersic and Arvia (360)reported on the dependence of surface coverage for adsorbed hydrocarbons on the electrode potential. Wang and Freiha (410)prescribed the application of adsorptive preconcentration followed by EC detection in FIA. Adsorptive pr6concentration of organometallic species was reco nized to give enhanced square-wave polarographic responsegfor Cd(I1) and Pb(I1) in the presence of picolinic acid by Ramaley, Dalziel, and Tan (300) and was applied for determination of traces of Co(I1) and “1) in the presence of triethanolamine and dimethylglyoximeby Meler and Neeb (250)and used for determination of lo4 to 10- M alkaloids and other surfactants at a static Hg-drop electrode by Kalvoda (140). Finally, Bocarsly and Sinha ( 2 0 ) described voltammo rams for Ni electrodes derivatized by Fe(CN){-/“ and R u ~ C N )which ~ ~ /was ~ achieved by anodic polarization of the electrodes in 0.1 M NaC104. Greater facility is apparent from the literature for applications of surface spectroscopy to the characterization of electrodes. Hinoue et al. (110)described computerized control of internal reflectance spectroscopy with cyclic staircase voltammetry a t optically transparent tin oxide electrodes. Beden, Bewick, and Lamy (ID) applied modulated IR reflectance spectroscopy to a study of formic acid adsorbed at a Pt electrode. Raman spectroelectrochemical investigations were reported for pentaammine(pyridine)osmium(III,II) a t A electrodes by Farquharson et al. ( 7 0 ) and for dithizone atsorbed at a Cu electrode by Pemberton and Buck (280). Koetz et al. (180, 190) used XPS in a study of oxygen evolution at Ru and Ir electrodes in H2S04solutions. LEED was utilized by Wagner and Ross (400) for determination of structural changes at Pt single-crystal surfaces subjected to potentiodynamic cycling. I
E. CHEMICALLY MODIFIED ELECTRODES In the past several years there has been a high level of activity in the area of chemically modified electrodes. More attention has concentrated on incorporation of electroactive sites in polymeric films of varying thickness. The coupling of these sites with the electrodes and with electron transfer reactions of soluble species has been deseribed theoretically by several workers (21B, 2-5E, 30E). The useful films have been described as viscous polyelectrolytes into which substrates and associated counterions may partition and diffuse. The overall reaction may be divided into three major processes: (a) kinetics of electron exchange between fixed and diffusing redox species, (b) substrate diffusion, and (c) charge propagation (“electron or ion hopping”). Immobilization of these polyelectrolyte coatings on a rotating disk electrode has proven useful because application of a Koutecky-Levich equation enables the separation of the rotation-dependent and -independent components of the overall Faradaic current. Important experimental variables in evaluation of the resultin multiparametric equations include electrode rotation s ee$ bulk substrate concentration, ( l O E ) , film thickness, (31Efl and the number of redox sites within the film (31E). Other techniques such as chronocoulometry, chronoamperometry (46E, 49E), chronopotentiometry (26E),luminescence (9E, 39E), Auger spectroscopy (7E), and spectroelectrochemistry (55E) have also been employed for characterization. Studies were also directed toward the characterization of multilayer f i b s or fiis in which more than one redox couple is entrapped. Schneider and Murray (50E)reported on ion exchange partitioning of electroactive cations into organosilane styrene sulfonate copolymers. Bilayer films have also been ormed from viologen polymers (55E). Vinyl pyridine polymers proved popular for binding Ir and Ru complexes (12E, 20E, 25E) and were used for a stripping voltammetric determination of Cr(V1) (16E). Nafion polymer coated
electrodes have also been characterized (41E,42E, 44E) and a method for solubilizing Nafion polymers has been described (41E). Pyrrole (51E),polypyrrole (8E,21E), anthraquinone (19E), ferrocenophane (15E), tetracyanoquinodimethane (17E),polythiophenes (27E,53E),and tetrathiafulvalene (13E) have also been studied. Polymer films have been prepared by dip coating, electropolymerization,plasma deposition, and y-irradiation polymerization (18E). Thin films have been examined as electrochromic materials (28E, 33E, 36E). Film-modified electrodes were also used for catalysis of electron transfer reactions including the reduction of dioxygen (23E, 38E, 40E, 45E) as well as oxidation of chloride (24E), NADH (32E),and ascorbate (52E). A poly-L-valine coated platinum electrode was used to asymmetrically oxidize phenylcyclohexyl sulfide to the sulfoxide with an optical yield of 54% (34E)and for the reduction of prochiral activated olefins (1E). An electrode coated with a functionalized polystyrene was used for the electrochemically controlled release of yaminobutyric and glutamic acid from its surface (37E). Polymer films continued to be applied to the coupling of photochemical and electrochemical processes (6E, 11E, 14E, 22E, 29E, 35E, 47E, 48E). Preparative electrochemistry especially in organic solvents is encumbered by the need to add a supporting electrolyte which must then be subsequently separated from the reaction products. Ogumi and co-workers (43E) have suggested the use of a Pt-olid polymer electrolyte electrode to alleviate this problem. The solid polymer electrolyte (a Nafion film) on which Pt is deposited serves as a separator between a highly conducting aqueous medium and the low dielectric medium containing the electroactive species. The electrolysis of neat acetic acid was demonstrated. F.
COULOSTATIC/GALVANOSTATIC
METHODS There have been a number of reports recently on both the theory and applications of the galvanostatic method. Pikel’nyi et al. (8F) have reported on a dual-pulse galvanostat and its application for the study of complex electrochemical systems. Reller and Kirowa-Eisner (IOF) discussed the analysis of errors in the galvanostatic method and compared its accuracy with coulostatic methods. The relative accuracy of each method depends in a different manner upon the relaxation times of the system. Nagy ( 7 9 examined the effect of uncompensated solution resistance upon the applicability of galvanostatic relaxation techniques and compared it to steady-state techniques for electrode kinetic measurements. The double-pulse galvanostatic method was applied recently to the study of the electrode kinetics of the Bi3+/Bi system (4F), the hexacyanochromate(III/II) system (2F),and aromatic compounds (3F). In the last system, anomalies were found between the double-pulse galvanostatic method and Faradaic rectification (3F). A coulostatic coulometer with a digital counter (5F) was recently designed by Last. Van Leeuwen (11F) looked at dauble-layer relaxation and resistance effects in coulostatics and found the coulostatic method relatively insensitive to ohmic cell components. Nagy and Arden (787 evaluated the errors in the coulostatic technique and compared this method to other dc relaxation techniques for the measurement of electrode kinetics. Reiss and Nieman (9F)reported on the obtaining of quantitative and qualitative information from a single coulostatic decay and showed how the data could be transformed to obtain a curve that resembled a normal voltammogram. A laser-induced coulostatic flash study of n-titanium dioxide in acetonitrile has recently been reported (IF). G. FLOW-THROUGH ELECTROCHEMICAL DETECTORS Citations have already been given for reviews on flowthrough electrochemical (EC) detectors, and their various applications for flow injection analyzers (FIA) and liquid chromatography (LC), by Brunt (7A),Hanekamp et al. (14A), Stulik and Pacakova (35A),and Trojanek (36A). Although satisfadory performance has been reported for a large variety of designs, significant effort persists in optimization of performance through better design. Poppe (50G) stressed the ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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importance of miniaturization for maximizing sample throughout in flow-injection systems. Miner (47G) reported on the need of temperature control for precise measurements. Weber (72G) discussed the dependence of response on flow rate for various electrode geometries used in EC detectors; and Mustoe and Wragg (48G) considered the effect of flow rate in recirculating electrochemical reactors. Hanekamp and de Jong (27G) presented a single equation for characterizing response and concluded the best design for achieving high sensitivity a t solid and polar0 aphic electrodes has the fluid electrode surface. The most stream impinging normal to obvious application of this principle occurs in EC detectors of the so-called “wall-jet”type. Gunasingham and Fleet (26G) reexamined the derivation of the hydrodynamic boundary layer thickness a t the wall jet electrode and explained the decreased sensitivity observed when the nozzle is positioned within the boundary layer. Brunt (14G) and Ivaska and Ryan (33G) have illustrated applications of the wall-jet design. Albery and Brett ( I C ) and Hoogvliet et al. (28G) considered both theory and applications of wall-jet detectors of the ring-disk type. As with rotating ring-disk electrodes, response can be characterized by collection and shielding parameters. More blatant efforts to increase sensitivity by increasing convection within the detector have involved internal stirring by rotating mechanisms (13G, 70G) and a vibrating electgde (51G). We are not yet convinced that the energy dissipated in producing these detectors results in a significant improvement over the simplest of thin-layer detectors with laminar flow. Porous electrodes offer maximum area with minimal internal volume such that electrolytic conversion efficiency can approach the theoretical limit of 100%. Such a detector is absolute with some resulting convenience. Theoretical and experimental consideratipn of porous electrodes appeared by Storck et al. (59G) and by Trainham and Wu (63G). Wang and co-workers described applications of porous electrodes in EC detectors of the wall-jet type (65G) and with internal stirring (70G). Greatest sensitivity f& EC detection is obtained for constant applied potential. To increase the analytical resolution of the detector system for unresolved chromatographic peaks, multiple electrodes operated at different potentials can be advantageous. Although the number of electrodes to be operated simultaneously appears unlimited in theory, predominant effort has been with two electrodes which are hydrodynamically coupled or independent. The most significant effort on applications of dual-electrode detection to LC was by Roston and Kissinger (54G); the review by Andrews et al. (6G) is noteworthy. Applications of dual detection are cited by Allison and Shoup (2G) for thiols and disulfides, by Goto et al. (25G) for metabolites of biogenic amines, and by Lunte and Kissinger (44G) for pterins. In spite of some disadvantages due to handling of the bulk metal, Hg remains popular for use in cathodic EC detection. Debowski et al. (22G) and Lloyd (43G) evaluated polarographic detection in HPLC for determinations of several nitro compounds. Scholz and Henrion (55G), Stulik, Pacakova, and Podalak (BOG),and Trojanek and Krestan (64G) described modifications of the design of polarographic detectors. In comparison to bulk Hg, the hydrogen overpotential at Hg-film electrodes prepared on solid metal substrates is not as large. Nevertheless, the use of Hg-film electrodes offers some analytical utility. Alexander and Akapongkul(4G) demonstrated use of a Hg-Cu electrode for cathodic detection of Cd(II), and Bratin, Kissinger, and Bruntlett (12G) applied a Hg-Au flow-thro h detector for easily reduced nitro compounds and obtainedyetection limits at the picomole level. Glassy carbon received additional attention as an electrode material for flow-through detectors. Bratin and Kissinger (11G) studied the influence of dissolved O2 on use of the material for reductive detection and reported a larger overpotential for O2reduction than observed at a Hg-Au electrode. Hoogvliet et al. (29G) described the use of a silicon coating on glassy carbon to produce a leak-free carbon-glass seal. Interference by dissolved O2 cannot be overlooked in cathodic detection at large negative potentials regardless of the electrode material. Reim (53G) described a system for postcolumn deoxy enation to decrease the problem. The appropriate use of ckal detectors can also give some relief from the interference (6G, 54G). It is now safe to say that whereas
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the possibility of interference from O2 must always be considered in reductive detection, it no longer stands as a barrier to successful detection. The applicability of flow-through EC detection in FIA systems has been adequately demonstrated for simple (usually one-component) samples. The adoption by Eggers, Halsall, and Heineman (24G) of the amperometric detection of NADH in an enzyme-coupled immunoassay flow-injection system is especially novel. The full voltammetric potentiality needed to resolve components of more complex samples can be achieved in FIA only under very rapid scanning of applied potential wave forms. The application of rapid linear sweep and cyclic sweep voltammetry in FIA was considered by Janata and Ruzicka (34G) and by Theogersen, Janata, and Ruzicka (61G). Applications of transient voltammetric techniques also received extensive attention for EC detection: normal pulse and staircase voltammetry at glassy carbon and carbon fiber electrodes by Caudill et al. (18G); differential pulse at a polarographic detector by Alexander and Marpaung (3G); differential pulse (68G) and reverse pulse (66G) voltammetry by Wang and Dewald; and AC voltammetry at a tubular electrode by Compton and Sealy (21G). Anderson et al. (5G) described the microprocessor-based control of transient techniques for LCEC which is equally applicable for FIA with EC detection (FIAEC). Last but not least is the coulostatic voltammetric detector described by Last (40G) capable of scan rates of 3 VIS. The concept of voltammetric stripping analysis with media exchange is perfectly suited to the concept of FIAEC where stripping can be achieved by any wave form desired after the sample has passed through the detector. Several applications appeared: square-wave stripping of Cd at a polarographic detector by Buchanan and Soleta (15G); potentiometric stripping of Cu, Cd, and Pb by Hu, Dessy, and Graneli (30G); differential pulse stripping of L-cysteine fiis on Hg electrodes by Martins and Johansson (45G); Pb at a wall-jet glassy carbon electrode by Wang, Dewald, and Greene (67G); use of dual detectors by Wang and Dewald (69G);and stripping of Cu, Pb, and Cd from a graphite-cloth electrode by Yaniv and Ariel ( 73G). There are a few noteworthy examples of surface-modified and biologically active electrodes in flow-throughEC detection. Castner and Hawkridge (17G) applied a methyl viologen modified Au foil electrode for a study of the heterogeneous kinetics of metalloproteins. Sittampalam and Wilson (56G) coated a Pt electrode with a thin film of cellulose acetate to minimize protein fouling during applications to analysis of biological fluids. The negatively charged membrane is also effective in excluding small anionic molecules such as ascorbate which are often electrochemicalinterferences. Protection of the electrode was also utilized in an enzyme immunoassay by Eggers, Halsall, and Heineman (24C). An ascorbate enzyme electrode was used by Matsumoto et al. (4613) for FIA detection; and Thompson and Krull(62G) have described the desi n and operation of a cell which used supported bilayer lipicf membranes. Space and time do not allow a complete review of the applications of EC detection in liquid chromatography (LCEC). A review by Krull, Bratin, Shoup, Kissinger, and Blank (37G) is recommended. Also noteworthy is a review by Krull and Lankmayr (38G) of derivatization techniques in HPLC, aspects of which are pertinent to LCEC. A complete bibliograph of the LCEC literature has been prepared by Shoup (57G7. Selected citations are listed in Table I which were chosen because they represent an extensive area of application or because of some unique aspect of the detection.
H. ANALYTICAL VOLTAMMETRY The theory for the current response of new voltammetric wave forms has been given in the section on Electrode Kinetics. This section will deal mostly with analytical procedures. Andrieu et al. (3L)developed a computerized method to increase the speed of voltammetric analyses. In this procedure, they recognized that only a small fraction of the entire voltammogram was analytically useful. Thus, they used a fast scan to find the peaks, followed by a slow scan which only traversed the peak potential regions. Overall, this provided a rapid analysis time, combined with high sensitivity. Wang and Dewald ( 6 0 described a highly sensitive stopped-flow voltammetric method with potential scanning while Cox and
DYNAMIC ELECTROCHEMISTRY
Table I. Selected Applications of EC Detection in Liquid Chromatography electrode analyte comments C C C C C C Pt Pt Pt, Au, C
cu Ni C
Hg Hg/ c Hg Hg Hg Hg Hg film Hg film
urinary catecholamines phenols hydrazine reducing sugars pterins biogenic amine metabolites carbohydrates amines and amino acids Cuz+,NizC,Cozc,Cr(II1, VI) Cdz+,Coz+,Cuz+,Pbz+,Hg2+,NiZ+ amino acids sugars polyaromatic amines aromatic amines Sz-,CN-
Tc0,Pt(I1) complexes penicillamine, glutathione, cysteine lynestrenol, digoxin, glycosides nitro compounds nitro compounds thiols and disulfides
Jedral(13H) reported on a square wave amperometric method with two polarizable electrodes, which gave a response independent of stirring rate. In a novel application of cyclic voltammetry, Loget et al. (36H) studied the electrochemical behavior of some silver-based dental alloys. The use of voltammetric methods for drug analysis contained reports on the oxidation of chlorpromazine hydrochloride (51H),several neuroleptics (34H),cimetidine and its Plotsky metabolites in urine (62H),and cephalosporins (2"). (44H) described the differential voltammetric measurement of catecholamines and ascorbic acid at surface-modifiedcarbon filament microelectrodes. The reduction of explosives such as propylene glycol dinitrate, nitroglycerin, dinitrotoluene, and trinitrotoluene ( 2 0 on silver electrodes was studied by Fine and Miles. Differential normal pulse voltammetry was used for the anodic oxidation of iron(I1) (1IH). The determination of trace silicon at a glassy carbon electrode (61H) and the identification of aromatic structures in coal extracts (323 by differential pulse voltammetry were also reported. A membrane electrode for the determination of carbon dioxide and oxygen ( I H ) was developed by Albery and Barron. The use of anodic stripping voltammetry has continued to expand. Gerlach and Kowalski (23H) have described a generalized standard addition method for intermetallic interferences. There were several reports in the literature about the use of graphite-sprayed electrodes for anodic stripping voltammetry (32H, 33"). The use of hydrodynamic methods to increase the efficiency, sensitivity, and ease of use for stripping voltammetry was described by several workers. Brihaye et al. (7-9H) used the ring-disk electrode for the analysis of cadmium, lead, copper, antimony, and bismuth. A wall-jet ring-disk electrode ( I G ) ,a miniature rotating disk electrode (56H),and a graphite-epoxy microelectrode for in vitro and in vivo measurements were used for anodic stripping voltammetry (57H). There were several reports on the use of anodic stripping voltammetry in flowing systems such as the on-line monitoring of effluent with a flow-through reticulated vitreous carbon electrode (59H),a dual coulometricvoltammetric cell (69G),and a flow-throu h cell with differential pulse voltammetry (37H). The comPbination of anodic stripping voltammetry with a flow injection system was also reported (67G). A method of using the adsorption of PADAP complexes on ClSbonded glass beads prior to anodic stripping analysis was described (54"). A cell for the anodic stripping voltammetry of lead in microliter volumes was devised by Miwa et al. (38H). The use of anodic and cathodic stripping voltammetry for the analysis of heavy metals in biological media and food was reviewed by Vire et al. (55"). Anodic stripping voltammetry was used for the determination of trace elements in pharmaceutical tablets (58H),in gunpowder residues (IO"), and methylmercury in the presence of inor anic mercury (27H). The influence of long-chain amine anfammonium salts on
anodic amperometry single and dual detectors pretreated electrodes Cun(phen), as mediator dual detectors dual detectors oxide-catalyzed anodic detection oxide-catalyzed anodic detection anodic detection of dithiocarbamates detection of pyrrolidine dithiocarbamates anodic detection oxide-catalyzed anodic detection anodic detection anodic detection indirect anodic detection cathodic DME, HMDE indirect anodic detection tensammetry cathodic detection dual detection indirect anodic detection
ref 35G 54G, 58G 52G 71G 44 G 25G 31G, 32G 49G 9G 10G 36G 16G 19G 20G 8G 41G 7G 74G 23G 22G, 43G 39 G 2G
the anodic stripping voltammetry of thallium, lead, tin, cadmium, and indium was studied (14H). Anodic stripping analysis was also used for the immunoassay of serum proteins (16H)and the tneasurement of lactate, glucose, and hypoxanthine following biocatalytic preconcentration (35H). Anodic stripping voltammetry was used for the measurement of gold in bone tissue (25H,26H) and for the determination of atmospheric hydrogen sulfide (30"). Cysteine was determined by anodic stripping voltammetry with a copper amalgam electrode (52H,53H). Cadmium binding by soil-derivedfulvic acid (4H) and metal-organic interactions in seawater (43H, 46H) were examined by the use of anodic stripping voltammetry. While less popular than anodic stripping voltammetry, cathodic stripping voltammetry has continued to grow, especially in regards to the analysis of selenium, arsenic, and sulfur. There were several reports on the analysis of selenium(1V) (5H, 17H,29H), including its measurement in biological samples (5H) in a mixture with a tellurium (17H). In the latter case, both selenium and tellurium could be measured simultaneously (17H). Cathodic stripping voltammetry was also reported for the analysis of arsenic in the presence of copper (47H),and for the analysis of sulfur in sera (49H) and semiconductor material (18H). In addition to sulfides, organosulfur compounds are quite amenable to analysis by cathodic stripping voltammetry. For example, primary, secondary, and tertiary thioamides (15H),2-thiouracil at a rotating silver disk electrode (24H),codeine, papaverine, and cocaine (31H), and trace amounts of penicillins (22H) were determined by this method. In the case of thioamides, the DC polarographic, cyclic voltammetric and differential pulse polarographic response of these compounds were studied (15H). In addition to these compounds, glutathiones (48"), organosulfur compounds such as mercaptides, disulfides, and thiuram sulfides (2H),2-mercaptopyridine and its N-oxide, 2-mercapto-3- yridinol, and mercaptosuccinic acid (50H),and methylated agnine (42H) were studied by cathodic stripping voltammetry. Lastly, Palecek and Hung measured nanogram quantities of osmium-labeled nucleic acids by stripping voltammetry (41H). The use of voltammetric methods for in vivo monitoring has expanded greatly over the past 2 years. It is now possible to perform both steady-state amperometricmeasurements and voltammetric scans to identify the chemical species present. For example, differential pulse voltammetry has been used to discriminate in vivo between ascorbic acid, catechol, and indoleamine (6H), dopamine and ascorbate (19H), and 5hydroxyindolecompounds (12H). ONeill et al. have measured arcadian changes in homovanillic acid and ascorbate levels in the rat striatum (39H) with linear sweep voltammetry at a carbon paste electrode (40H). Plotsky et al. have reported on the measurement of catecholamine release with in situ voltammetric microelectrodes (45H), while Forni (21H)has ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
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developed a new multifiber electrode for continuous in vivo measurements of neuromediators. I. CHARACTERIZATION OF REDOX SYSTEMS The past 2 years have continued to show how rigorous attem ts to relate experimental results to theoretical data can provi e useful chemical information on redox processes. An elegant example of this approach is the work of Evans et al. (121,37-391) on conformationaleffects associated with electron transfer reactions. This work provided fundamental chemical information on the pathway of the reduction of vicinal dibromides (381) and conformational changes associated with the reduction of bianthrones (391). Coleman et al. (51) showed the dramatic effect that stereochemical orientation of functional groups can have on the peak potential by observing a (300 mV difference in oxidation peak potentials between the endo and exo forms of two norbornyl dithioethers. Other examples are the study of the structural consequences of the reduction of a cobalt complex (211), and the spin-state dependence of heterogeneous electron-transferrates (291). These works demonstrated that the accurate measurement of electrochemical parameters is not just an academic exercise but can also provide important chemical information. This is especially useful when electrochemistry is combined with some synthetic imagination in designing appropriate redox compounds. The synthesis and electrochemical characterization of models of enzyme active sites have become fertile areas for electrochemical study. The study of synthetic clusters provides fundamental understanding of factors controlling the potential of these clusters, which can also mimic the catalytic processes. Modeling of molybdenum active sites by Holm (411), Spence (31), and Coucouvanis (61),and the work of Mascharak et al. (341) and Cleland et al. (41) on iron-sulfur active sites are such examples. These same molybdenumiron-sulfur and iron-sulfur clusters have been used to mimic the catalytic processes, such as the reduction of hydrazine (281) or dinitrogen (461). The reduction of carbon dioxide has also been catalyzed by a cubane iron-sulfur cluster (471). The electrochemistry of vitamin B12has also been an active area of electrochemical research that has yielded important information on the biological activity of this important compound. Saveant et al. (13-151), Rubinson et al. (431), and Kim and Birke (301) have quantitatively measured the kinetic and thermodynamic factors that control the electrochemistry of vitamin Blz. The reaction of related iron-porphyrin complexes that involve the formation and cleavage of the iron-carbon bond was studied by Saveant et al. (21,321). Goff et al. have used a series of iron-porphyrin complexes to understand the difficult question of the site of oxidation in iron porphyrins (421) and have used an oxidized difluoro complex of iron(II1) porphyrin to catalytically oxidize cyclohexene (271). In the area of bioorganic chemistry, Dryhurst et al. (I71-20r) have shown how a detailed examination of the products and intermediates in the electrochemical and enzymatic oxidation of uric acid can shed important information on both processes. The number of reports of redox catalysis has increased significantly in the past 2 years. The work by Saveant on electron transfer-induced reactions has been discussed earlier. Herbranson and Hawley (231) have also studied electron-induced chain reactions of azides, while other workers have looked at the indirect reduction of triorganohalogermanes (71) and diphenyl sulfide (221). Evans and Xie (110,in their study of the reduction of bianthrones, have examined the homogeneous redox catalysis of bianthrone reduction by way of quinone. Dasgupta and Ryan (80 used strong reductants such as propylene viologen and europium and vanadium compounds to study the kinetics of the indirect reduction of spinach ferredoxin. The catalytic wave produced by the reduction of oxygen in the presence of superoxide dismutase was studied by rotating disk electrodes (6B,401). The measurement of the redox potentials and the electron uptake, both by the enzyme itself and in the presence of substrate, of the nitrogenase complex was accomplished by Lough, Burns, and Watt (331). Hershberger et al. (24-261)examined the electron transfer catalysis of metal carbonyls which involved a chain mechanism. The factors affecting the oxidation of coal slurries by electrooxidized iron (91, II) or cerium (11)leading to carbon monoxide or carbon dioxide were studied.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
In addition to electrochemical studies of interesting redox compounds, there have been a number of studies of interesting solvent systems. Laurent et al. (31n have studied organic electrosynthesis in ordered systems. McIntire et al. (351) have looked at nitrobenzene in anionic, cationic, and nonionic micelles, while Eddowes and Graetzel (101)have investigated the oxidation of tetrathiofulvalene in micellar solution. Uribe et al. (481) have studied the reduction of carbon monoxide in liquid ammonia, while Bard et al. (161,441)have used liquid sulfur dioxide to generate compounds with very high oxidation potential. Nakabayashi et al. (361) used a combination of benzene and a crown ether as a medium for electrochemical and photoelectrochemical reactions. Stimming and Schmickler (451) were able to use aqueous acid solutions to make electrochemical measurements a t 120 K.
J. BIOELECTROCHEMISTRY Faradaic electrochemistry has found increasing application to the study of protein electron transfer properties. Of particular interest are the continuing studies (9J, 1 0 4 pioneered several years ago by Hill’s group at Oxford to facilitate direct and rapid electron transfer between an electrode and a protein redox center. The importance of rapid but reversible adsorption of proteins at the electrode-solution interface has been emphasized (104. A number of studies of cytochrome c electron transfer at modified gold ( I I J ,29J, 304 and carbon paste ( 7 4 electrodes have been reported as have measure33J), adrenodoxin (14J),cytochrome ments of ferredoxins (W, c3 (34 2W), and cytochrome cg3 (45). Mitochondrial electron transfer and that of several proteins have been coupled directly to an electrode using soluble cytochrome c as a selective mediator ( 6 4 134. Spectroelectrochemistry was used to measure the temperature dependence of cytochrome potentials (314 while a spectrofluorometric cell was used to observe cytochrome structural changes (274. Channel flow hydrodynamic voltammetry was applied to the measurement of cytochrome heterogeneous electron transfer rates (54. Heterogeneous electron transfer was evaluated by spectroelectrochemistry (84. The redox potentials of lactate oxidase have been determined (284. Potential dependent enzyme activity was studied in a thin-layer cell (174. Direct attachment of enzymes to electrode surfaces also proved popular (154 16J,374. Hameka and Rechnitz (124 have reported on refinements of the theory of biocatalytic membrane response. Immobilized enzyme electrodes were used for the determination of L-lactate (20J, 384, ethanol (394, triglyceride6 (354, and acetylcholine (364. Electrochemical assays for brain ascorbate (254and catecholase (234 have been developed. Biological tissue (194 and bacteria (184 have been incorporated into biosensors. Composite biocatalytic layers have been used to promote consecutive or competitive enzymatic reactions which, when coupled to an amperometric sensor, provide a basis for analyses (21J, 24J, 264. Electrochemical measurements are now being applied to immunoassays. Electrochemical “labels” have been used to indicate the extent of an antibody/antigen (hapten) reactions. Thus electrochemical detections of nitroestriol (344, In3+ (16H),Pb2+(14,and NADH (24G) have formed the basis for nonradiometric assays. Amperometric detection of glucose released by immune lysis of glucose loaded liposomes has been reported (324.
K. SPECTROELECTROCHEMISTRY Spectral techniques used in concert with electrochemical measurements have continued to attract considerable interest. Infrared techniques have been used to study hydrogen (4K) and carbon monoxide (2K) adsorption. Electrode modulated infrared spectroscopy (EMIRS) was used to characterize adsorbed species (3K), and diffuse reflectance was used for characterization of chemically modified carbonaceous electrodes ( 1 I K ) . Specular reflection spectroscopy was used to 17K,22K). The adobserve radical ion intermediates (16K, sorption of cytochrome c3 on a gold electrode was observed by spectroreflectance (1OK). Fujihira (9K) developed the theory for “illumination step chronoamperometry” in which an electroactive product is generated by passing radiation through a transparent electrode. Blount and Hawkridge (7K) continued their development of derivative cyclic voltabsorp-
DYNAMIC ELECTROCHEMISTRY
tometry in studies of cytochrome c direct electron transfer. The importance of using highly purified preparations was emphasized. Spectroelectrochemistry was used to characterize the electrochemical kinetics of chlorpromazine (12K, 14K). Anion binding to (tetraphenylporphinato)iron(II) was examined in a thin-layer cell (13K). A major limitation of traditional transmission spectroelectrochemistry is the very short “effective” optical length which results when the light beam is oriented normal to the plane of the electrode. McCreery and co-workers (18K)have shown that if the light beam is passed essentially parallel to the surface at an angle of about 1.2O, an effective cell thickness of about 5 pm results but the optical path length can be 1cm. Greatly improved sensitivity is the result. Microsecond spectroelectrochemistry is made feasible by external reflection from cylindrical microelectrodes (19K). A number of long path length thin-layer spectroelectrochemical cells have been designed (8K,20K, 21K, 23K). It has been pointed out by Zak and co-workers (23K) that such a long path length, small volume cell will be useful in quantitating adsorption on electrode surfaces. A cell has been designed for direct insertion in a spectrophotometer (1K).Special cells for vacuum line studies (5K), anaerobic measurements (6K),and electrochemistry in organic solvents (15K) have been reported. L. INSTRUMENTATION/ELECTRODES Relatively few publications appeared for new analog instrumentation. Parus and Perone (22L) described the application of a battery-powered coulostat with appropriate shielding and grounding for minimizin noise in pulsed-laser photoelectrochemical studies. Aalstacf and Parker (1L)discussed the use of positive feedback to eliminate the detrimental effects of uncompensated cell resistance in hetero eneous kinetic studies. Guterman and Ben-Yaakov (11L)%escribed a nonlinear network to be used as a “dummy cell” for differential-pulse polarographic analyzers to simulate response of electrochemical cells. Cai et al. (5L) described an instrument for measuring impedances which uses a smaller ac perturbation than in conventional bridges with the benefit of less deviation from equilibrium for systems which cannot support a large current density. An analog circuit for semidifferentiation (d‘/dtv) of olarographic data was described by Oldham and Zoski (20Lf Construction of bipotentiostats (i.e., four-electrode potentiostats) was described by Farkas et al. (9L) and Figaszewski et al. (IOL). A significant decrease is noted in the proliferation of howto-do-it publications related to digital circuits and computer applications. This attests to the commercial availability of suitable components, the ease of interfacing these with electrochemical apparatus, and our rapidly increasing aptitude for preparing software. Last (16L) described a coulostatic coulometer with a digital counter; the applicability of this device for EC detection has already been noted (40G). Siegerman (26L)provided easily followed instructions for the use of the IEEE-488 interface for computerization of electrochemical experiments. Micro-, mini-, and macrocomputer instrumentation for transient voltammetric experiments was described by Anderson and Bond (2L),Andrieu et al. (3L), Britz (4L),Li et al. (17L),Price et al. (23L),and Wasa et al. (30-32L). Paul et al. (21L) related instrumentation for twin-electrode voltammetry. Tkalcec, Grabaric, and Filipovic (27L) discussed the application of a computerized system for determination of stability constants by semiintegral analysis of i-E data. Finally, Rusling (25L) provided a worthwhile description of the computerized mechanistic classifications of i-E curves. Studies of electrode materials have tended to emphasize the desirable qualities of carbon in its many forms. Rice, Galus, and Adams (24L) presented interesting correlations between the composition and surface treatments of graphikpaste electrodes and the charge-transfer rates for model redox systems. They reported that the addition of pasting liquids decreases charge-transfer rates in comparison to those at dry graphite electrodes; oxidative pretreatment increased char e-transfer rates. Kauffmann, Laudet, and Patriarche (14LY reported on differential pulse anodic stripping voltammetr at a Hg-film carbon-paste electrode. Jnioui, Metrot, and Jtorck (13L)reported the electrochemicalproduction of graphite salts by intercalation of H2S04 in a packed-bed graphite electrode. Kauffmann et al. (15L) reported that
satisfactory results were obtained for oxidation of clozapine at a graphite spray-modified A1 electrode; the related compounds clothiapine and loxapine were more difficult to oxidize and oxidation products were strongly adsorbed. Lundstrom (18L)generated some new enthusiasm for pyrolytic carbon with a description of the preparation, characterization, and applications of a film electrode for oxidative detection of several organic compounds. Wang (28L)reviewed applications of reticulated vitreous carbon (RVC); Calasanzio et al. (6L) described the use of RVC as a thin-layer electrode; and Hobart et al. (12L) applied RVC for construction of optically transparent electrodes. Microelectrodesconstructed from carbon fibers are most interesting and aspects of mass transport to microelectrodes have already been noted (1IB). Edmonds and Ji (8L) have described the effect of electrochemical pretreatment on detection sensitivity. Finally, attention is given to several beneficial configurations of electrodes and cells. A pulsed-flow mercury electrode was described by Dias, Buess-Herman, and Gierst (7L) which is reported to overcome some disadvantages of the DME and HME. Nagaoka and Okazaki (19L)described a polarographic cell for low-temperature voltammetry; solidification of H was observed in the capillary at
