K. U., Toropov A. P., Gordienko, A. A., Dokl. A d . Nauk Utb. SSR, 27,32 (1970). (932)Valasek I. E., Majranovskii, V. G., Sachova, d. K., Samochvalov, G. I., (931)Wsmanov,
Khim. Farm. Zh., 2,51 (1968). (933)Valcher, S.,Ghe, A. M., Del-Monte, M. G., Ann. Chim. (Rome), 60, 708 (1970). (934)vbBeek, L. K. H., Helfferich, J., Jonker, H., Thijssens, T. P. G. W., Recl. Trav. Chim. Paus-Bas.. 86.. 749 (1968). (935)Van Beeumen, J., DeLey, J., Anal. Biochem., 44,254 (1971). (936) Van Der Meer, D., Red. Trau. Chim. Pays-Bas, 88, 136 (1969). (937)Zbid. 89, 51 (1970). (938)Van-kerchove, C.,J . Pharm. Belg., 25, 215 (1970). (939)Zbid., 26,304(1971). (940)Van Tkhan, N., Avrutskaya, I. A., Fioshin, M. Y., Elektrokhim., 4, 1508 (1968). (941)Vegnere, V. Y., Stradins, J., Ozol, Y. Y . Aren, A. K., Zzu. Akad. Nauk. Latv. hSR, Ser. Khim., 1969,394. (942)Vegnere, V., Stradins J., OZO~S, Arens, A., Latv. P S R dinat. Akud. Vestis, K i m . Ser., 1970, 164. (943)Vera, J., Vidal-Abarca, J. B., Serna, A., Sancho, J., An. Quim., 66, 311, 317 (1970). (944)Vesheva L. V., Argova, T. B., Reishakrit, L. S., Zavod. Lab., 37, 276 (1971). (945)Vesheva, L. V., Argova, T. B., Reishakhrit, L. S., Forsova, T. V., Zh. Obshch. Khim., 40, 1927 (1970). (946)Vesheva, L. V., Ovchinnikova, R. A., Reishakhrit, L. S., ibid., 41, 975 (1971). (947’)Vig, S. K., Indian J . Chem., 7, 501 (1969). (948)Vig, S. K., Bannerjee, N. R., Electrochim. Acta, 16, 157 (1971). (949)Vodzinskii, Y.V., Vasil’eva, A. A., Korshunov, I. A,, Zh. Obshch. Khim., 39, 1196 (1969).
(950)Volke, J., Oehchlager, H., Lim, G. T., J . Electroanal. Chem., 25, 307 (1970). (951)Volke, J. Ryvolova-Kejharova, A.,
Manousek,
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Organic Electrode Processes,” Academic Press, New York, N. Y.,1969. (986)Zuman, P., Fortschr. Chem. Forach., 12, l(1969). (987)Zuman, P., “Topics in Organic
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Polarographic Theory, Instrumentation, and Methodology Richard S. Nicholson, National Science Foundation, Washington, D.C. 20550
T
m v m w FOLLOWS the pattern of the previous one (238), selectively covering literature from December 1969 to December 1971. The scope is defined by my interpretation of the assigned title, but in any case “polarographic” is taken to include the other major techniques. Papers involving only applications generally are not cited. Books and Reviews. Several books related to the topics of this review have been published in English or translated into English. The textbook on electrochemistry by Koryta, Dvorak, and Bohackova, which was first published in Czechoslovakia in 1966, can now be read in English (188). HIS
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Two books by Zuman have been published (338,333). One of these (333) is a set of reprints covering the past twenty years, and the other (332) is based on a 1967 series of lectures. Both deal with organic polarography. The electrochemistry of organic compounds also is the subject of a book by Mann and Barnes (212). And speaking of adsorption, the authoritative book by Damaskin, Petril, and Batrakov has been translated from Russian (92). Similarly, the original 1969 “Progress in the Electrochemistry of Organic Compounds” edited by Frumkin and Ershler is now available in English (120). Headridge (139) has written a book
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which he says is designed to introduce electrochemistry to inorganic chemists. His book does not possess the scope or rigor of some purely electrochemical texts, but it is short and readable, and might be a good place for nonelectrochemical colleagues to get a start. Polarography is the subject of a French book by Pointeau and Bonastre (868), and a chapter by Muller (229). Muller’s contribution is part of two volumes devoted to electrochemistry in the newest version of the famous “techniques” series edited by Weissberger. These two particular volumes (314, 516) are of general interest and additional chapters will be mentioned below.
Some others of the series-types also deserve recognition. Volumes 4 and 5 of the one Bard edits are in print (88,991. Each of these contains chapters that are cited later, but it is appropriate at this point to mention the chapter by Guidelli (133). This review treats chemical reactions in polarography in a unified and logical fashion. Both surface and volume reactions are considered. I n addition, the various mathematical approaches that have been used down through the years are described and compared. The style is not very electroanalytical-there is even a modicum of Onsager themodynamics-but the serious student of polarography will find the chapter interesting. Another series is the one edited by Delahay (93). This entire volume should be of interest, but the excellent chapter by Butler on nonaqueous solvents is most germane (7’1). A new series has Hills as editor (I&), and reviews the 1968-69 electrochemical literature. I n an established series, number 6 of “modern aspects” edited by Iiockris and Conway has been published (41). Finally, volume 9 of the series edited by Eyring was published in two parts (119, 1 1 3 ) . This is said to be an advanced treatise devoted to electrochemistry-not electroanalytical chemistry. This distinction naturally brings to mind the tome authored by Bockris and Reddy. This two volume work (48, 43), by Bockris’ own account (do), should help lower the (overpotential) gap that exists between the very few electrochemists and everyone else. Equally personalized, but on a different topic and with a different flavor, is Kolthoff’s description of sowe of the electroanalytical greats of the past (183). Instruments, Cells, Etc. I n the old days, polarographic instrumentation was beautifully simple: a battery, slide wire, and galvanometer. But times change, and today’s polarographer is a p t to feel obsolescent if he doesn’t use a minicomputer. And perhaps rightly so, since minicomputers will do everything from digitizing a potentiometric titration a t 20 KHz to obtaining natural abundance carbon-13 NMR spectra in nearly routine fashion. The latter application will prove to be a milestone in the history of chemistry. Perhaps comparable developments are impossible in electrochemistry; nevertheless, some very fascinating applications are beginning to appear. The most obvious of these is Laplace transformation of experimental data along the lines suggested by Wijnen twelve years ago. This approach, conceptually expanded a great deal, is being crusaded now, and it makes a new ball game of doing experiments and making measurements. For example, potentiostats needn’t possess infinitely short risetime, just a measurable one which causes a
measureable perturbation of the chemical system. Representative discussions of these important ideas are contained in papers by Birke (36), Levart and Schuhmann (197), and Pilla (262, 263). Pilla, who is a pioneer in this field, has written an authoritative review that should 8ppear in 1972 (866). Although more along polarographic lines, the work of Fujiwara et al. (123, 189) also provides a glimpse of the future v i s - h i s minicomputers. They have completdy computerized their polarograph, but rather than doing A/D followed directly by D/A, as some might, they analyze data in terms of autocorrelation functions and Fourier transformation. Their results give only a hint of the power of this approach. A more explicit indication can be gleaned from the really fascinating review by Smith (890). Although this article is concerned primarily with ac polarography, it projects some very significant applications of minicomputers. Some of these applications are mentioned later in the present review. Shirai (186) and Niki and Shirai (239) also have discussed computerization of polarography. I n the second paper, they evaluted the well-known effect of depletion on the diffusion current of individual drops. Ogle et al. (242) have claimed that digitization is useful for analyzing unresolved polarographic waves. Finally, Goldsworthy and Clem (128) have designed what they call a digital potentiostat [Smith (g90) also discusses this topic]. This involves negative feedback of current pulses, with one advantage being that A/D conversion is not required. These same authors have described related a p proaches to polarography and other electrochemical measurements (77‘). Digital potentiostats notwithstanding, the analog variety and op amps are still nice to have around. Most of the effort continues to center on compensation of ohmic (IR) losses. Discussions of what you can and cannot do via positive feedback are exemplified in papers by Pilla (264) and Lamy and Malaterre (190). Smith, in the review recommended above, acts admirably as translator for these various discussions. Devay et al. (101, 102, 196) have described a form of I R compensation involving positive feedback of an error signal derived from a superimposed small amplitude ac. Their approach appears to possess no advantages over more conventional methods and seems ill-advised. Finally, it sometimes happens that solid state op amps don’t have sufficient output voltage to compensate properly; when that happens the circuits of Bottei and Boczkowski (68) represent one solution. I n spite of all the electronic marvels, cell design remains an important consideration in reducing I R losses and
making fast measurements. Many of these factors, with a view toward rapid response, have been discussed in detail by Mumby and Perone (230). They evaluated both cell and potentiostat designs, construction of Luggin capillaries, and proper placement of the Luggin for optimum response and IR control. Bode plots are used extensively and clearly show the profound effects that interelectrode capacitance can produce. Belew et al. (33) also considered proper cell design for compensation of I R losses, but with polarography rather than the wonderland of nanoseconds in mind. They pondered the problem of how to keep a Luggin probe near the moving surface of a conventional DME. The trick is to usean unconventional DME, in this case a Smoler electrode. The Luggin probe can then be placed above the growing drop and remain within 0.1 radius of the drop during its entire life. They report measurements essentially free of IR drop in solutions of specific resistance as high as 22,000 ohm-cm. Several papers deal with circuits that can be used to measure (not eliminate) uncompensated resistance. For example, iMcIntyre and Peck (209) developed a simple interrupter technique that permits measurement of uncompensated resistance when the working electrode is under control. A similar approach was used by Newman (837) in connection with an investigation of the potential distribution a t a rotating disk electrode. Schwartz (280) has pointed out that the circuit of McIntyre and Peck is limited because it can not be used for experiments requiring both cathodic and anodic current. This fact prompted Schwarts to design a bipolar interrupter that can be used in polarographic configurations. This circuit is said to be an improvement over yet another one due to Bruckenstein and Miller (66). This latter circuit was designed primarily to permit closed loop, rapid conversion from potentiostatic to galvanostatic control (and vice versa). Other papers in the area of instrumentation include the description of polarography and construction of a polarograph for high school students (36). This instrument is reminiscent of those old days alluded to above. Also said to be simple and inexpensive is a droptime control and current sampling device designed by Means and Mark (218). They state that they required low cost and simplicity because of the large number of polarographs in their laboratory-a fairly unique problem in the United States. The accurate measurement of unregulated droptime is the subject of a paper by Papeschi et al. (248). They combined a precise timer (crystal controlled osciilator) with a decade counter and circuit for detecting drop detachment.
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For those who like to eliminate the droptime oscillations altogether, but don’t think brute force damping is the way, the approach of Borello et al. (67) provides an interesting possibility. They employed one of the op amp peak finder circuits to locate and remember only the maximum current during drop life. The result is an undamped polarogram, but i t does not exhibit the usual drop oscillations, which some organic chemists find so distracting. Their results are not ideal, but then there are more sophisticated circuits which probably would yield improved performance. Finally, Eynard et al. ( 1 1 1 ) developed a polarographic machine that is completely automatic, including peristaltic pumps for adding reagents. It was not a vintage period for new cells, but there are a few that should be mentioned. Hanzlik (139) has discussed general aspects of polarographic cell design and described in detail two new cells for three-electrode operation. Scarano et al. (878) developed a cell for continuous analysis of flowing solutions. A rapidly dropping mercury electrode is placed in a flow-through cell possessing low holdup. Minimizing loss of mercury is the aim of a design by Tur’yan (W).While certainly a laudable objective, the result is too cumbersome to be useful. Another novelty is the cell by Bruckenstein and Gadde (64) that attaches directly to the source of a mass spectrometer. The working electrode is a variable porosity frit that permits gases formed during electrolysis to enter the source. The electrode is interesting, but its scope is yet to be demonstrated. When it comes to cells for nonaqueous electrochemistry, the perennial probl e m are residual gases (oxygen) and water. Even very small concentrations of either of these can have an influence on electrochemical behavior. To eliminate water completely is impossible, so the best objective is to minimize it, and in some cases to have its background level as reproducible as possible. Everyone has his pet method, but in my opinion the most efficacious one is a vacuum line. Bard has been an advocate of this approach for several years and recently described the system he uses (81). Mills et al. (881) have developed an apparently excellent vacuum system suitable for polarography. The design includes Teflon (Du Pont) stopcocks to avoid contamination by vacuum grease (no mention is made of O-ring deterioration). Their experimental data indicate that the cell is very effective in reducing residual oxygen and water (ca. lO-5M in THF). The cell is capable of three-electrode operation and is suitable for coulometry. Rericha and Sir (871) also designed a cell for polarography under vacuum, but it does not appear to be as useful as the one of Mills 480R
and coworkers. At the other (pressure) extreme, Polievk’tov (269) et al. have constructed apparatus that permits polarography a t pressures up to 3000 atmospheres. This system also is designed for three-electrode control; some preliminary results on the protonation of pyridine by water at high pressure are included. The marriage of electrochemistry and ESR spectrometry is still viable, and therefore the cell designed by Goldberg and Bard (187) is a noteworthy development. I n contrast with commercial ESR cells for intra muros electrogeneration, their cell permits true three-electrode operation and recording of conventional voltammetric curves. Preliminary data suggest that the cell will be very useful, especially in studying short-lived paramagnetic species. Less versatile, but also less expensive is the approach Ambrose et aE. (9) have suggested. They note that heat shrinkable Teflon does not contain paramagnetic impurities, and therefore is very convenient material for construction of ESR cells. Reference Electrodes. I n general, I uncovered very little activity in this area. As For reviews, the publication by Covington (83)is useful although not polarographically oriented. The article by Butler (71) already has been mentioned, but it’s worth citing twice. Feltham and Spiro (116) have reviewed what is and is not known about platinized platinum electrodes. There is a quest for the ultimate electrode to use in propylene carbonate, largely because this solvent is so popular with the battery builders. Caiola et al. (72) have investigated the lithiumlithium chloride couple and found it useful in the presence of tetrabutylammonium chloride. They also pointed out that the conductivity of propylene carbonate solutions of tetrabutylammonium chloride passes through a sharp maximum a t about 0.6M. Kirowa-Eisner and Gileadi (174) studied the silver-silver perchlorate couple and concluded that it also is a useful reference electrode. They determined an exchange current density in 0.1M lithium perchlorate of 20 pA cm-*, which means that an electrode area of greater than 10 cmz is required for twoelectrode polarography. Finally, Fried and Barak (119) have compared all six of the propylene carbonate electrodes suggested to date. They conclude that the best system is a calomel-tetraethylammonium chloride couple. They further claim that the electrode is useful for either two- or three-electrode polarography; the exchange current density, however, is about the same as the silversilver perchlorate couple. I n the miscellaneous category, Malkova (810) discussed the major factors that determine the stability and repro-
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ducibility of calomel electrodes. Grebenik (130) considered possible reference electrodes for sodium chloride-calcium chloride melts at 600 “C and concluded that a tungsten electrode is suitable for polarography. For dimethylformamide, Manning and Purdy (813) recommend cadmium amalgam-cadmium chloride, while Berardelli et al. (34) have determined the standard potential for silversilver chloride in methylformamide. Although not a reference electrode, the mlt bridge described by Clem et al. (78) is very interesting. The bridge is based on gelling 1M potassium chloride with 7% fumed silica. The authors claim the bridge is superior in every respect to unfired Vycor tubes.
Things Related to Half-Wave Potentials. Certainly a major objective of modern electrochemistry is the ability to correlate and predict electrochemical behavior on the basis of contemporary concepts of chemical structure and reaetivity. Presumably something like this is part of the reasoning behind the very many papers published each year describing linear correlations between Ell2 and anything. There are so many of these publications that none will be cited specifically, except to mention Hoijtink’s review (160) of HMO correlations. Interestingly enough, the majority of correlations work regardless of whether the polarographic wave is reversible or irreversible, although most authors, given their choice, seem to prefer the reversible kind. This predilection is strange to me, because, surely a t this stage in the development of electrochemistry, the correlations with irreversible waves are far more interesting, since in this case El/*presumably is proportional to some kind of activation energy. Frequently this activation energy corresponds to a following chemical reaction, or the l i e . But in some cases-especially organic molecules in aprotic solvents-the activation energy corresponds to a fairly well defined electron transfer reaction where the structure of reactant and final product is readily determined spectroscopically, if not already well known. I n this case, some correlations could provide important insight into the structure of the activated complex during electron transfer, which is a step toward meaningful understanding of electrochemical reactivity on a molecular level. Polarography and E1/2probably are not the way to get the necessary values of k,,and a minimum level of sophistication for any MO calculations is probably SCF/ CNDO. Nevertheless, it is interesting to speculate about whether some of the well-known quantum mechanical reactivity parameters (e.g., localization energies) based on models of the transition state for homogeneous reactions might not also correlate electrochemical
reactivity. Of course, the influence of the field associated with the double layer is a very nasty complication, but one that, almost certainly has to be considered. It is possible, however, to account for induced electric polarization by employing perturbation methods, and this has been done using HMO wave functions. It is very surprising that with few exceptions no carefully designed research of this type has been published. By far the most signal exception is the work of Peover and coworkers (1968, 1960). The most recent research of this type is contained in a paper by Huntington and Davis (164). During the course of studying some aminoquinones, they measured half-wave potentials and (apparent) k, values for six bis(alky1amino)-benzoquinones and found a correlation between k, and proton coupling constants derived from ESR spectra. They tentatively interpreted this correlation as suggesting that the molecular structure of the transition state for electron transfer is similar to the (known) structure of the semiquinone anion. Unfortunately, the stoichiometry involves two electrons and two protons, and therefore the actual process for which the (apparent) standard rate constants were measured is ambiguous. And finally Smith’s review has to be mentioned again (290). He has discussed Peover’s research, as well as some of this own, and also has included a table of standard rate constants and charge transfer coefficients for a series of compounds not previously reported. If you like to speculate (vide supra) but don’t like to work getting the necessary data, this might be a place to start. Of couree, there has been considerable progress in studying homogeneous electron transfer reactions and making correlations with Marcus’ theories. Much of this work involves coordination chemistry, with recent examples due to Bartelt (28) and Vlcek (310). And in the same vein, an article by Bond (47) is very thought-provoking. Closely related and equally fascinating are the spectrometric measurements reported again by Crow (89). As Crow suggests, the analytical applications also are interesting. Incidentally, this article by Crow is one installment of a six-part series dealing with applications of polarography in coordination chemistry (86-90). Along slightly more pedestrian lines, there naturally have been many papers dealing with the use of polarography to measure equilibrium constants. This topic is reviewed by Saraiya and Sundaram (276)in an article particularly notable for its extensive bibliography. The long accepted methods of analyzing polarographic data to determine coordination numbers and equilibrium
constants have been scrutinized by Klatt and Rouseff (179). They used simulated data and a computer to evaluate the effect of experimental uncertainty in half-wave potentials. Some of their conclusions may be disarming to those long indoctrinated in the ways of DeFord and Hume. I n fact, their results prompted a further “theory guides, experiment decides” study by Klatt and Lewis (180). They reinvestigated the lead tartrate system, and concluded that half-wave potential shifts must be determined with a precision of 0.5 mV or else data from some other technique must be sought. Other possible hazards in using polarography include the effects of adsorbable ligands. Bond and Hefter (62) have shown that anomalously large stability constants can result from complexation in the double layer when strongly adsorbable ligands are involved. Of course, one also worries about competitive complexation with components of the “indifferent” electrolyte. This fact is obvious to most chemists, and explains why perchlorate or nitrate salts are usually preferred for adjusting ionic strength. But even there one has to be careful, as Bond has convincingly demonstrated ( 4 6 ) . Another frequent complication is reversibility of the electrode process. Bond also has considered this situation (46), and more recently Momoki and Ogawa ( W 4 ) have recommended approaches to the same problem. I n a similar vein, but not having to do directly with studying coordination chemistry, Ruzic et al. (276) have described a procedure for extracting reversible half-wave potentials from quasi-reversible polarographic waves. This same subject also is the topic of a paper by de &vie and Pospisil (96). Finally, Breslow has extended his use of electrochemical measurements to estimate otherwise inaccessible equilibrium constants. This time Breslow and Chu (61) report pK.’s for several substituted triphenylmethanes and cycloheptatriene. They predict future applications of this approach. Things Related to Current. Two years ago I noted a paper by Oldham who claimed to have a new approach for solving diffusion problems. Oldham and Spanier have now produced a more detailed expod (846), and it seems to me that the new method is really just new nomenclature for a convolution integral. Problems that are not tractable through Laplace transformation will not be any more or less tractable with their method. Oldham and Spanier do derive an interesting relationship that appears to provide a new approach to data analysis. Both the principle and the method, however, are essentially the same as the one due to Pilla and others referred to above; in fact the approaches are mathemati-
cally equivalent. Oldham and Spanier’s equation can be written directly (but with ditrerent symbols) from Duhamel’s theorem. From a pragmatic viewpoint, all of these methods, regardless of the symbols you prefer (partial semiderivatives, Laplace transform, etc.) ultimately reduce to a DO LOOP. Which is not to criticize the basic idea, because it is both important and sound as indicated earlier. Guidelli (132) has considered in a general way the effects of stepwise reduction and disproportionation. Kastening (171) also considered disproportionation, but in a more explicit manner (he gets answers). Kastening evaluated his theory by measuring rate constants for disproportionation of the radical anions of several substituted nitrobenzenes. Speaking of disproportionation brings to mind the ECE mechanism. This time the torch has passed to Sobel and Smith (289) who have solved the expanding plane model for the entire polarographic wa,ve. Their model further assumes Nernstian electron transfer for both couples. One interesting aspect of this work is an evaluation of the commonly employed steady state approximation. I n this case (and probably in many others), the approximation works much better than a look a t the mathematics might suggest. Before leaving the ECE mechanism (and then only briefly), the paper by Jezorek and Mark (169) is worth mentioning, because they claim to have uncovered an ECE sequence involving three electrons. Hopefully (for the mathematicians’ sake), this is a rare case. Other polarographic theory includes a paper by Verdier et al. (308) who have derived a new form of Matsuda’s general equation for reduction of metal complexes. Adsorption is considered by Guidelli (131) who based his treatment on a diffusion layer model and Langmuirian adsorption of reactant and product. Laviron and Degrand (198) also have considered the effect of inhibition caused by adsorbed reactants or products. Several nuances of potentiostatic electrolysis a t stationary electrodes with different geometries have been treated by Barnartt and coworkers ( 2 6 , 2 7 , 1 6 8 , 1 6 4 ) and by Bachmann and coworkers (16,16). Logarithmic analysis of polarographic waves continues to be investigated. Ruzic has presented two more installments (273, 274) on his method of analyzing overlapping waves. Ginzburg (126) has discussed the shape of logplots for quasi-reversible waves, where for a limited range of standard rate constants a nonlinear plot can be expected. Sheinin and Ginzburg (283) also have investigated the most suitable method for graphically analyzing overlapping waves.
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Other research dealing with polarographic currents includes the investigation of maxima and determination of diffusion coefficients. I n the former area, a laser interferometry study by O’Brien and Dieken (9.40) is illuminating. The effect of magnetic fields always is fascinating if not dramatic; the extent of the effect on polarographic maxima is reviewed by Fujiwara and Umeaawa (121) and discussed in a paper by Fujiwara et al. (122). The measurement of diffusion coefficients, particularly by nonelectrochemical methods, obviously is important. Recognition of this fact prompted Adams several years ago to develop a method based on capillary diffusion followed with the aid of tritium labeling. Tritium labeling is not all that handy, however, and this fact has prompted Bacon and Adams (17) to adapt fluorescence analysis to their capillary diffusion cell. Also concerned with diffusion coefficients, Baticle et al. (29) have compared three different methods, while Jones and Fritsche (166) have wisely cautioned about having experimental conditions conform as closely as possible to the model on which theoretical equations are based. The use of Faradaic rectification to measure diffusion coefficients is discussed by Korchinskii (187), and the measurement of interdiffusion coefficients of silver and gold based on a modified Cottrell equation is described by Oldham and Raleigh (244). AC Polarography. Some of the most interesting activity is in the area of second harmonics. The enhanced analytical sensitivity is well known; the less widely known sensitivity toward kinetic effects is now well established theoretically. These theoretical predictions have been confirmed experimentally by McCord and Smith (206). Glover and Smith (126) have devised analog instrumentation that gives excellent results, including automatic recording of second harmonic phase angles. To record these accurately is no mean feat, as the virtual absence of such data in the literature suggests. In this connection, the measurement of second harmonic phase angles by Kojima and Fujiwara (182) using a method totally different from that of Glover and Smith is quite interesting. These authors used the computerized polarograph mentioned earlier (189) to perform ac polarographic experiments. Data are accumulated in digital form and then Fourier transformation is performed numerically by the on-line computer. The resulting Fourier spectra elegantly illustrate such things as harmonics, phase angles, effect of charging current, etc. Second harmonic phase angles are readily recorded as a function of dc potential and, as expected, display a marked dependence on elec4821
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trode kinetics. While their system is not yet as developed or polished as the analog system of Glover and Smith, their results make some of the advantages of Fourier analysis abundantly clear. Other second harmonic activity includes theory for a preceding chemical reaction due to McCord and Smith (206), and experimental evaluation of theory for a succeeding reaction by the same authors (204). Some aspects of earlier work by McCord and Smith are the subject of a paper by Senkevich et al. (281). Devay and coworkers have investigated second harmonic ac polarography both experimentally and theoretically in a series of papers (98-100). Fundamental harmonics also have received their share of publicity. Negative faradaic admittance leading to spontaneous oscillations is the subject of articles by Smith and Sobel (291), and de Levie and coworkers (94, 96). Moreira and de Levie (226) have extended previous theoretical treatments dealing with the coupling of interfacial and diffusional impedances to include effects of homogeneous chemical reactions and passivation. The theory, apparatus, and applications of amplitude-modulated ac polarography have been discussed by Zheleatsov (328-330). Brocke (62) has used the effect of demoddation caused by the nonlinear faradaic admittance to enhance sensitivity about tenfold over conventional ac polarography. Similar enhancement has been achieved by Sluyters and Breukel et al. (687) who used a superimposed triangular wave to facilitate electronic subtraction of capacity current. The use of a triangular wave is said to reduce equipment costs considerably below that required for phase selective or square wave polarography. The necessary circuitry for phase selective measurements is described by Matsuda et al. ( I 17 ) . I n the analytical area, Bond and Canterford (61) have considered the problems of using ac polarography to determine a minor component reduced a t more negative potentials than the major component. Bond (48) also has reviewed some of the principles of ac (and dc) polarography when using a DME with short, controlled droptimes. Bond and Canterford (60) have investigated the simultaneous determination of two depolarizers by ac polarography. They concluded that even though the derivative form of the curves may be visually appealing for analysis of mixtures, if the peaks overlap at ail, it is best not to rely on peak heights. These same authors also investigated the myth that deoxygenation is never required when using ac polarography (4.99)
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The entire topic of sine wave methods has been reviewed authoritatively by Sluyters-Rehbach and Sluyters (288).
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Last, but by no means least, is the review article by Smith (290)that already has been mentioned several times. This review is recommended reading even if you don’t think ac polarography is for you. Pulse Polarography. Pulse polarography has been around for a while, but it has not received a lot of attention. With modern electronics, especially computers, and some commercial instruments at attractive prices, this situation is certain to change. The analytical advantages of pulse polarography are well known, but there may be other advantages that are less obvious. This belief is beautifully illustrated in a recent paper by O’Deen and Osteryoung (241), who used pulse polarography to study halide ions in molten nitrates. I n this system, conventional polarography gives anything but conventioml polarograms. The pulse polarograms, on the other hand, are nearly textbook quality and readily amenable to anslysis. Presumably the dramatic difference is because of the very small amount of electrolysis per drop that occurs with pulse polarography in the norma! (or integral) mode, since the potential is a t the foot of the wave most of the time. No doubt similar advantages will be found in other cases. For those not as familiar with pulse polarography as they should be, the topic has been reviewed by Burge (70). The effect of reversibility in pulse polarography ic, discussed by Oldham and Parry (243) who considered two different pulse methods. The use of normal pulse polarography on a statiomry electrode has been discussed theoretically by Belavin and Mikhailov (31). They suggest that one advantage of a stationary electrode is ths t polarograms for oxidation and reduction can be recorded simultaneously. They not only claim the obvious mechanistic advantages, but also some analytical ones which are illustrated in a subsequent paper by Budnikov et al. (68). A description of the apparatus used by Mikhailov and coworkers also has been published (82). Pulse polarography is nicely suited for minicomputerization; an example is the paper by Keller and Osteryoung (1‘72). Chronopotentiometry. If asked, most pros would say that chronopotentiometry is of very limited value. Of course, one always can find some novel (and questionablej applications. such as the work by Verdier and Bennes (306, 307) on the determination of stability constants from chronopotentiometric constants. But Lingane, who ought to know, is still largely pessimistic in his most recent review (199). He does, however, make some thoughtful remarks about possible applications. The major problem is still the deter-
mination of transition times in a way that is simple and reliable, yet does not include contributions from charging current. Olmstead and I once recommended the method of Laity and McIntyre based on our theoretical calculations which, incidentally, have since been evaluated experimentally (946). Unfortunately, this method is cumbersome compared with better known graphical ones. The fairly simple graphical method recently described by Deroo et al. (97) would, therefore, seem to be encouraging, since it is said to be as effective as the method of Laity and McIntyre. Unfortunately, the LaityMcIntyre method theoretically is valid for only one particular electrode process. This fact is illustrated by Ershler et al. (110). They have applied the mathematical method developed by Olmstead and me to the case of adsorption and find that the Laity-McIntyre method is not satisfactory. Nevertheless, work continues, and Herman and Blount (142) have considered the various models used to describe adsorption, whereas Podgaetskii and Filinovskii (257) have treated adsorption for a n isotherm with two plateaus. Naturally the effects of kinetic complications also have received their share of attention. Vukovic and Pravdic (319) used finite difference methods to extend the theory of cyclic chronopotentiometry to second order reactions, including disproportionation. Uranium(V) served to evaluate the numerical calculations. Similarly, Blount and Herman (38) have published new calculations for the ECE mechanism. Several sequences involving higher order kinetics have been treated mathematically by Dracka (104,105). Baraboshkin and Vinogradov-Zhabrov (18) have determined the shape of chronopoteniometric waves for step-wise reactions, while Baranski and Galus (19) have treated internal electrolysis and corrosion (simultaneous chemical oxidation of the electrode) in a separate paper (20). Finally, in what seems to be a marriage doomed for divorce, Bek et al. (SO) and Chernenko (73) have derived equations for chronopotentiometry performed a t a rotating disk electrode. A few papers dealing specifically with instrumentation have appeared. Herman et al. (143) have equipment that starts the experiment without the use of a mechanical switch, and then reverses the current after a preselected time. Rabuzin et al. (266) also have developed electronic circuitry designed for automatic cyclic chronopotentiometry. English’s instrument will reverse current and is said to be especially useful in nonaqueous solvents (109). With high resistance, or large current pulses, IR drop can be a problem. Thus, the circuit of Flinn et al. (117) that isolates the reference electrode during current inter-
ruption or pulsing may be of interest. Another circuit that permits rapid switching from potentiostatic to galvanostatic control is described by Knots et al. (181). A circuit by Bruckenstein and Miller that operates in a similar fashion was mentioned earlier (66). Finally, chronopotentiometry is one of the topics in a review by Murray (931). Cyclic Voltammetry. The burgeoning applications of cyclic voltammetry during the past two years admirably attest to the popularity this technique presently enjoys. Development of more theory continues to occupy the attention of many investigators, as does the everpresent ECE mechanism. Thus, Nadjo and Saveant (933) have extended their theoretical work with applications to reduction of uranium, and Feldberg has extended his series on “nuances” to include cyclic voltammetry (115). Andrieux and coworkers (12) discussed single sweep and cyclic curves in terms of a dimerization following Nernstian electron transfer. They described several possible methods for measuring dimerization rate constants. Shuman (884) has calculated the effect of a very rapid chemical reaction of arbitrary order following Nernstian electron transfer, and with coworkers he has evaluated some of the results experimentally (285). White and Lawson (317)have performed calculations for metal deposition and dissolution from a solid electrode. They assumed several different models for activity of the deposit, several electrode geometries, and quasi-reversible electron transfer. They also have studied the problem experimentally (318). Uncompensated resistance interests several investigators. Imbeaux and Saveant have devoted two more papers to the topic (156, 156), while Wells (316) has approached the problem from a more pragmatic view. Farsang and coworkers have considered the influence of depolarizer concentration on cyclic curves from both theoretical (114) and experimental (899) viewpoints. Effects of adsorption also have been the subject of investigation. Hulbert and Shain (153) derived theory for the case of Nernstian electron transfer foilowed by rate controlled adsorptiondesorption. They subsequently used these calculations to determine apparent rate constants for adsorption and desorption with the methylene blue system. Lavjron (192) has treated the case of product adsorption further complicated by dimerization, while Nesterov and Korovin (236) considered several aspects of adsorption. Tyssee (301) has noted the occasional occurrence of inverted peaks, and conjectured that these may be diagnostic for the intermediacy of adsorbed free radicals. I n the miscellaneous category is the effect of a magnetic field on linear scan polaro-
grams as described by Fujiwara and coworkers (194). They find that in some c&8e8peak currents are suppressed by a magnetic field and they interpret this phenomenon in terms of polarographic maxima of the second kind. There have been some developments in instrumentation, and as expected this technique is no safer than any other from the minicomputer. Digital hardware that functionally duplicates an online computer system for optimization of linear scan experiments has been designed and tested by Jones and Perone (166). Perone and coworkers (849) also have developed a sophisticated on-line computer and display system that permits the operator to interact in real time with the computer and thereby optimize its operations. Their method in particular permits convenient base-line correction for linear scan and cyclic voltammetry. Another use of an online computer is described by Gutknecht and Perone (1S6). They have developed an empirical five-parameter equation which can be adjusted to fit any experimental polarogram. These empirical equations are then used to deconvolute overlapping curves. They find that peaks separated by as little as 40 mV can be deconvoluted and used successfully for analytical purposes. In what is a more innovative approach to the same problem, Sybrandt and Perone (297) have applied some principles of pattern recognition (learning machines) to analysis of mixtures with overlapping waves. With their experimental data, the learning machine is about as good (Le., 40 mV) as the deconvolution technique. On the other hand, if there is no experimental deviation in peak potentials, the resolution approaches 2 mV, which is something to think about. I n a more down-to earth vein, circuits for generating triangular waves for some reason continue to be published. The latest ones are due to Ramalay (269), Myers and Shain (232), and Bull and Bull (69). Finally, the whole topic of cyclic voltammetry (including ac polarography) has been reviewed by Brown and Large (63). Stripping Analysis. Stripping analysis also means inverse polarography, amalgam voltammetry, etc. Regardless of the name, what invariably is involved is electrolytic preconcentration followed by electrochemical analysis. The sensitivity can be truly impressive and in some cases unmatched by any other trace analysis technique. Hence, in these trying times of concern for the composition and quality of our environment, it is not surprising that the literature shows increased activity in this area. Of course, a majority of these papers involve applications and will not be cited here.
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The review by Stromberg and Zakharova (996) is worth mentioning if only because of its extensive literature coverage, and the review by Monien (996)because it is more accessible to readers of the present review. Although restricted to film stripping, the review by Brainina (69) is both comprehensive and readily available. I n the area of theory, chronopotentiometric methods have received a surprising amount of attention. The paper by Baranski and Galus involving corrosion during chronopotentiometric stripping was mentioned earlier (90). Guminski and Galus (134) have evaluated theoretically the effects of finite electrode volume. They treated both linear potential scan and chronopotentiometric stripping. For example, they derived an equation that relates the ratio between oxidation transition time and preelectrolysis time to the diffusion coefficient and electrode radius. They reported good agreement between theory and experiment. Zakharov and coworkers have devoted a series of publications primarily to chronopotentiometric stripping analysis ( 3 2 4 , 3 2 6 , 3 2 7 ) . I n a separate paper, Zakharova et al. (326) have considered in detail the diffusion of several metals in mercury and measured temperature coefficients. Stromberg and coworkers also have discussed effects of temperature both theoretically and experimentally (170, 294). Stromberg and Kon’kova (996) have derived equations describing the several possible effects of using ammonia-containing electrolytes, while Vinogradova et al. (309) have evaluated the same effects experimentally. The influence of metal complexation has been treated mathematically by Brainina and Neiman (60). Similar factors have been studied by Zieglerova et al. (331) who also considered medium exchange and described a new flow-through cell that is said to be convenient. Booth et al. (66) have gone a step further and built a completely automated system for stripping analysis. Their rig is programmed mechanically by a cam-operated cycle timer. Linear scan is the most common technique for stripping, but other methods possess some very real advantages. I n this regard, Bond (44) has compared several electrochemical methods of analyzing for tin and finds that ac stripping is most sensitive. Lendermann et al. (194) also have compared several method#, including pulse polarographic stripping, with particular emphasis on the effects major constituents have on trace analysis. High senstivity for lead and cadmium has been achieved by Miwa et al. (222), who used an amplitudemodulated ac technique to do the stripping. Kemula (173) has used a different approach to improve sensitivity. He employs two identical electrodes, but 484R
usea different preelectrolysia t h e a and then records the difference in stripping current during simultaneous linear scan. With this ditrerential approach, charging current and impurities not deposited during preelectrolysis have much l e s influence. Several authors have considered diverse electrodes, but variants of carbon are most popular. Eisner and Mark (107) found wax-impregmted electrodes to be superior to pyrolytic graphite for analysis of silver. Kopanica and Vydra (186) have evaluated the rotated glassy carbon disk, while Monien and Jacob (9.97) tried rotating carbon paste. As one might guess, electrode composition is important in this case; Chulkina et al. (76) have investigated this problem by systematically varying the pertinent composition parameters such as particle diameter, etc. Florence (118) has used a glassy carbon electrode in a very clever fashion. He notes the many limitations of HMDE’s and the construction of mercury film electrodes, and consequently suggests in situ formation of a mercury film on glassy carbon as a logical part of the preelectrolysis step. To do this he simply adds mercuric nitrate to the solution and then preelectrolyzes at a potential where both mercury and the trace metal ions are reduced. The films he generates this way are extremely thin (he claims 10’ cm) and give rise to the best resolution so far reported. The method appears to be extremely useful and promising. h eeb et al. (236) have investigated the (well known?) effects of back diffusion that occurs with HMDE’s of the extruded drop variety. Van der Leest (306) has described a method that is said to yield very reproducible mercury films on platinum black electrodes. Finally, Eliseeva and Sinyakova (108) have developed a thin layer cell that permits stripping from as little as 2.5 pl of solution. Disks, Rings, and Things. The popularity of various rotated electrode configurations continues unabated, with theory seemingly outstripping significant experiments in the all too familiar style of electroanalytical chemistry. The topic has been reviewed in a publication by Albery and Hitchman (6). Daguenet and Aouanouk (91) have derived equations for reversible and irreversible waves under conditions of laminar flow, while Heckner and Moeller (140) have considered only irreversible processes. Prater and Bard (260) have started a series on rotating ringdisks with the first paper devoted to generalities and steady state and transient currents in the absence of kinetic complications. Their “theory” is courtesy of a very big digital computer and some finite difference equations. The Levich
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theory haa been extended to the case of a rotating spherical electrode by Chin (76), while Radoi prefers to rotate cylinders (967,968). Although Radoi c l a i i some advantages when the cylindrical electrode is eccentric, the discussion of eccentricity by Bardin and Dikusar (94)may be of more general interest. They have derived a very simple expression that permits correction of the Levich-Riddiford-Gregory theory for any eccentricity in an experimental disk system. They also have tested their theory experimentally and found that it works satisfactorily. Similarly, Smyrl and Newman ($96) have devised modifications of the Levich equation to account for edge effects resulting from radial diffusion. I n turn, Newman’s theory of current distribution has been reconsidered by Albery and Hitchman (7) who derived a simplified version. I n a more pragmatic approach, Bruckenstein and Miller (66) performed experiments to test the correctness of Newman’s theory. The experiments consisted of deposition of copper from solutions containing insufficient supporting electrolyte. Subsequent measurement of the surface variation of deposit thickness depicted the extent of nonuniform current distribution. Not surprisingly, they found that nonuniform current distribution on the disk has little influence on the collection efficiency of a ring-disk. Current and rotated disks also are the subject of a paper by Chernenko et al. ( 7 4 ) , but their interest, for some reason, is linearly scanning the current. Also in the weird twist category is the paper by Miller et al. (219) wherein they advocate what they call isosurface concentration voltammetry. What this means is that current is scanned while rotation rate is varied in such a way that the ratio of current to the square root of rotation rate remains constanthence, in some cases, surface concentration of depolarizer is invariant during the experiment. To go one step further, Lopatin et al. (901) have derived the proper equations for the effect on limiting current of temporal variations in bulk concentration. Equations for calculating diffusion impedance a t a rotating disk are given by Levart and Schuhmann in two papers (196, 198), while Albery et al. have derived phase shift equations for ac perturbation (6). They also have tested their theory experimentally, only to find that it works. The study of transient currents at ring-disk electrodes is the subject of other experimental and theoretical work by Albery and coworkers (1,2,4). Electrode processes that occur in a step-wise fashion on rotating electrodes are treated theoretically by Kiss and Farkas (175-177). Coupled chemical
reactions are the subject of papers by several workers. Aouanouk and Daguenet (14) solved the case of a preceding chemical reaction, while McIntyre is interested in surface catalyzed Homogeneous catareactions (N7). lytic and ECE reactions have been treated by Prater and Bard (961,969). Their theory includes both first and second order reactions. Second-order disproportionation is the subject of Holub's paper (148) which prompted a paper on the same subject by Bonnaterre and Cauquis (69). These authors also have solved the case of a dimerization following quasi-reversible electron transfer (64) and evaluated this theory in a separate paper (66). There were no major breakthroughs in the area of instrumentation, but one perplexing problem has been solved in splendid fashion. The problem: If a vacuum line is the proper way to do nonaqueous electrochemistry, and your thing is rotating electrodes, then how do you do that on a vacuum line? The solution: You simply put the motor and everything else inside a great bottle and attach the bottle to the vacuum line. According to Maloy et aZ. ( H I ) , outgassing takes a little longer than usual because of the motor windings, but after that one can operate the system at lo-' Torr. That act is hard to follow, but some other instruments probably should be cited. Getting data in a convenient fashion (ie., square root of rotation speed) is the subject of papers by Creason and Nelson (84) and Miller and Bruckenstein (980). The latter authors considered several new techniques, including the constant concentration method mentioned above. I n some cases, of course, it may be better to settle for 8112 than ~ 1 1 2 . Although this idea is old hat, Albery et aZ. (3) are the first to apply it to rings and disks. They have designed a simple analog computer circuit that performs the necessary Laplace transform integration. Solvents. Apparently no new solvents of great significance to electrochemistry have been discovered during the past two years, although studying electrode processes in nonaqueous media continues to be a popular and often very useful endeavor. Some of the important advantages, principles, and tricks of the trade are described in papers by Bard ( $ I ) , Kolthoff (184), and Saveant ( H 7 ) . Although not devoted to electrochemistry, the volume by Riddick and Bunger (272) on organic solvents has been revised and may be of interest. Various electrolytes for nonaqueous solvents have been compared by House et al. ( I @ ) , while Mohammad and Kosower (223) have cautioned about the profound influence a supporting electrolyte can sometimes have on solvent
polarity and consequently on electrode kinetics, particularly when the transition state is polar. They have devised a n empirical scale to measure solvent polarity based on the position of a charge transfer band. Although nonaqueous solvents are usually associated with organic compounds and synthesis, the field of inorganics is an important and growing one as the review of Laube and Schmulbach (191) attests. This article is based on a comprehensive literature survey through circa 1969. Dimethyl sulfoxide is a very useful solvent and the standard method of purification described by Reddy (270) should be of interest. Electrochemistry in dichloromethane is the subject of a paper by Coutagne (81). Propylene carbonate continues to receive its share of attention. Jasinski (169) has reviewed the subject with emphasis on battery applications. McComsey and Spritzer (203) have evaluated propylene carbonate for polarography, while Courtot-Coupez and L'Her (80) have determined potential limits and the influence of water. Dimethyl sulfone, which melts a t 127 "C, has been investigated by Bry and Tremillon (67).The art of electrochemistry in fused salts is reviewed both by Hladik (146) and Usmanov et aZ. (302). I n the final analysis, our constant companion (water) is the most important solvent, and therefore the latest thinking about its purification and quality, as described by Hughes et al. (162),is worth mentioning. Other Electrode Systems. Solid electrode voltammetry has been reviewed by Piekarski and Adams (261), while Alder et al. (8) have evaluated a lot of solids with which one might do the voltammetry. They studied some thirty different materials, ranging from metals to semiconductors, as possible electrodes for anodic voltammetry. Their conclusion is that with the possible exception of chromium and tungsten carbide, vitreous carbon is the best electrode material. Indepenpendently, Jennings et aZ. (161) have determined the reproducibility that can be achieved using a vitreous carbon electrode. They found a standard deviation of less than 2% for peak currents for reduction of a quinone in both aqueous and alcoholic solvents. Several electrode systems involve convection, but in less customary fashion than rotating electrodes or the DME. For example, systems where the diffusion layer is periodically destroyed by convection have been popular, especially with some Italian workers. Two representative approaches are described by Schiavon et al. (279) and Valcher (303, 304). Slightly more conventional is the dropping silver electrode described by Hoff (147) who claims it is very useful in molten salts.
With an HMDE one usually prefers not to have convection, but sometimes it happens anyhow. This problem has been studied by Guminski and Galus (196) who used electrodes of several geometries and arrived a t conclusions similar to those of Martin and Shain several years ago. On the other hand, if one really wants convection, vibrating the electrode is a logical approach. Cover and Folliard (89)claim some advantages for vibrating DME's, especially in terms of elucidating electrolysis mechanisms. Vibrating electrodes also have been treated theoretically by Lopatin et al. (200) for irreversible electrode reactions. Their theory naturally includes the influence of vibration frequency on limiting current. One of the most novel convection systems is described by Stock (293). I n an effort to mollify the problems of electrode fouling, he has attempted to perfect the technique of continually abrading the electrode surface. He uses a submerged drum that rubs a platinum disk electrode. I, for one, always have been thankful for the number of interesting problems that can be studied with a HMDE. Tubular electrodes, which frequently involve convection, continue to be investigated. An expression for limiting current when there is a preceding chemical reaction has been derived by Aouanouk and Daguenet (19). Mason and Olson (247) have described a procedure for making tubular carbon electrodes. They evaluated the electrode with several chemical systems, and discussed effects of electrode history and fouling. Blaedel and Boyer (37) have designed a flow-through cell involving a tubular platinum electrode. They advocate a differential amperometric measurement which is shown to reduce background currents significantly. They are able to monitor concentrations as low as 10-M. Moving the electrolyte rather than the electrode seems to be an increasingly popular way to acheive convection. Matsuda and Yamada (216) have developed a circulatory electrolysis cell assembly that provides very welldefined hydrodynamic conditions. They have reduced a general theory of Matsuda to the case of a ring-disk immersed in a laminar flow, and then evaluated the correctness of these equations with the aid of the circulatory cell. I n general, the results are excellent. I n a subsequent publication, the same authors have claimed some advantages for a wedge-shaped electrode sitting in a laminar flow of electrolyte (323). They therefore derived the appropriate equations and also evaluated them experimentally. The theory for spherical electrodes, including the DME, in a flowing electrolyte has been developed by Matsuda (216). Some of Matsuda's
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theory WM evaluated by Dikusar and Bardin, who described their results in two publications (26,103). Although the Matsuda theory is derived on the assumption of a laminar parallel stream, Dikusar and Bardin found that the theory also is applicable to turbulent streams and tubular electrodea. Finally, measurements in flowing streams are the subject of work by Pungor et al. They have investigated silicone rubberbased graphite electrodes; their results can be read either in Hungarian (263, 864) or English (834,866). Thin layer electrode systems are very useful in certain applications, and therefore the review by the two authorities in this area, Hubbard and Anson, is very timely (161). A few other publications dealing with thin-layer electrochemistry come within the scope of the present review. First, a very easy to construct thin layer cell has been developed by Sheaffer and Peters (886). Their construction is based on a Teflon needle containing a wire electrode; capacity of the cell is a few microliters. A more complicated cell, but one which effectively reduces ohmic potential losses has been invented by Tom and Hubbard (698). The new design also has the important advantage of providing more uniform current density distribution. Konopka and McDuffie (185) have described how twin-electrode thin layer measurements can be used effectively to determine diffusion coefficients. Optical measurements a t (through, near, etc.) electrodes continue to be popular topics for study. Major activity still centers primarily on methodology, with a relatively few papers involving only applications to new chemical systems. Von Benken and Kuwana (311) have described the construction of optically transparent platinum and gold electrodes by standard vapor deposition techniques. Optical properties of the electrodes as well as some preliminary electrochemical results are presented. Heineman and Kuwana (141) have subsequently described how these same electrodes can be electrolytically plated with mercury. By using very thin films of mercury (ca. 10” cm) hydrogen evolution is moved cathodically about 400 mV, without seriously degrading the optical properties. Other electrodes include the gold minigrid which has been evaluated for simultaneous spectral monitoring by Petek et al. (250). An equally novel but predictable development is a rotating ring-disk where the ring is optically transparent. Fabrication and evaluation of this system are described by McClure (208). Other marriages of note include the thin-layer system described by Kissinger and Reilley (178). Their work involves specular reflectance from metai film electrodes 486R
incorporated into thin-layer cells. Experimental evaluation consists of some of the standard oxidation reactions. Winograd and Kuwana (381) have described internal reflection measurements in terms of standard optical constants. They also use this paper to demonstrate some of the advantages of signal averaging. Rates of electron exchange reactions determined from reflection measurements also are disc u d by these two authors (380,326). The appropriate finite difference equations for determihing first order and second order rate constants with optically transparent electrodes have been solved by Grant and Kuwana (129) and Blount et al. (39), respectively. Each of these papers includes experimental applications. Optical measurements have been used by several workers to study adsorbed layers on electrodes. Spectra of several rare-earth complexes on tin oxide electrodes are given and partially interpreted by Moskalev and Kirin (628). Other papers dealing with this same general topic include ones by Plieth (866),McIntyre and Kolb (208), Mark and Randall (614, and Hansen (137). Miscellaneous. A few synthetic chemists are busy making some new and very intriguing macrocyclic compounds (so-called “crown” compounds). Some of these possess quite unique properties which make them relevent to important biological processes (319). Current synthetic efforts are being directed toward making compounds designed to possess steric selectivity; these may well prove to be enzyme-like in their behavior. Few, if any, applications or studies of these compounds are as yet evident in the polarographic literature, but that probably will change. Another very important field where electrochemistry may have some impact is high pressure liquid chromatography. There is no doubt about the future of this technique. One of the problems, however, is with detectors, which are a far cry from being developed to the extent enjoyed by GLC. Electrochemical methods might prove to be the key, especially if some of the extensive technology cited earlier in this review is brought to bear. In this regard the work of Joynes and Maggs (167, 168) may be of interest. Electrochemistry in the solid state is a relatively unexplored field as the review by Hladik (146) illustrates. Nevertheless, this is a potentially interesting and important area. An application that partly illustrates this point is described in a paper by Coburn (79), who performed some experiments involving solid state electrolysis in the source of a simple quadrapole mass spectrometer. Spectrometric methods of all kinds
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5 , APRIL 1972
will continue to be important for such things as examining electrode surfaces and solvent phases. I n this connection the papers by Watkins and Tvarusko (813) and Janz (168) are of interest. The former paper deals with the observation of diffusion layers via laser interferometry, and includes an interesting detector system of the vidicon type. These detectors will become more common and probably influence the way in which a variety of measurements are performed. The paper by Janz deals with a n old technique, Raman spectrometry. With its unique selection rules and commercial laser spectrometers being purchased by most labs, more extensive use of this technique by electrochemists seems likely. A much newer area is electron spectroscopy (ESCA, Auger, etc.). Although it’s not clear just how useful these methods will be to organic chemists, they are extremely powerful tools for studying electrode surfaces. Some early examples have been reported by Dunn and Harris (106) and Jenkins and Weedon (160). Of course, there are other ways to examine surfaces, one of these being with high energy particles. Amsel and coworkers (10, 11) recently have investigated this potentially fertile area. Among the many clever analytical methods is one due to Kaplin et al. (169). A number of years ago Delahay observed that the apparent standard rate constant for reduction of cadmium on a HMDE decreases if the electrode is exposed to the solution for an extended time. Similar effects, usually attributed to trace organics, have been observed and sometimes reported by many investigators. Kaplin and coworkers have now taken advantage of this effect, in terms of peak current suppression, to estimate total organic trace impurities in deionized water used to clean surfaces of semiconductor components. The last reference I wish to mention is a paper by Inoue et al. (157). They have described some preliminary polarographic studiesof a polymer undergoing helix-coil conformational transitions. This was accomplished by “adding as a structural probe” a small amount of reducible metal that int,eracted with the polymer. This kind of thinking is very reminiscent of the ideas that led to the concept of spin-labeling, and makes one wonder whether electrochemistry can make comparable contributions to chemistry. Perhaps the necessary discoveries are taking place right now, and will be reported in the 1974 version of this review. LITERATURE CITED
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