Electrochemical Relaxation Techniques D. K. Roe, Oregon Graduate Center, Beaverton, Ore. 97005
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of publications relevant to the title is included in this review. Because of the two-fold increase in articles to be collected and a four-fold decrease in preparation time, compared to the usual situation with a biannual review, I have taken a few short cuts. First, nearly all articles cited were selected from the Electrochemistry Section of Chmnical Abstracts, Vols. 68-75 (1968-71). This limited the most recent publications to about November 1971 and also allowed a minor amount of overlap with the last review under this title (39%). A second expedient was the frequent use of the information in the author’s abstract and/or the Chmnical Abstracts entry for my comments. A direct consequence of indulging in the practice, perhaps the basest of a reviewer’s transgressions, is that a t least one person will be irate for each article included. This confession is not to justify my modus operandi, but to illustrate that well written abstracts and summaries are very important. Electroanalytical Abstracts, which are not conveniently accessible to me, are recommended for an up-to-date survey of the electrochemical literature. I n addition, articles which have appeared in non-electrochemical journals are listed in the newsletters entitled Interface (available from the Department of Chemistry, University of Kansas, Lawrence, Kan. 6604.4). Organization of this review is primarily based on experimental methodology, which is therefore consistent with the title. Over the past four years, however, a significant number of papers have appeared with contents on theory spanning several techniques or the entire subject of electrode kinetics. A general section was therefore warranted, followed by controlled potential, controlled current, ac impedance, coulostatic, hydrodynamic, and other techniques. Each of these sections is divided according to theory, instrumentation, and application. FOUR-YEAR PERIOD
THEORY OF ELECTROCHEMICAL RELAXATIONS
Reinmuth (52%)devoted considerable space in the last review to Delahay’s concept of the a priori inseparability of faradaic and nonfaradaic currents in the treatment of relaxation measurements. By now, the minds of most electrochemists have relaxed from the disturbance created by this assertion, either because details of i t are now somewhat better understood or because the effect is not nearly as significant as originally
implied. After a series of rapid exchanges with dissenters, critics, and cornpetiton, the subject drew to a close insofar as Professor Delahay appears to be concerned. Four final publications (90,9%,168,360) with coworkers Hulub and Susbielles provide a thorough, but mathematically condensed, analysis of the coupling of charging and faradaic processes. Parallel resistance and capacitance, representing an electrode with reversible charge transfer, were shown (90) to deviate from classical linearity with the square root of applied frequency for a selected set of double layer parameters, later acknowledged to be inconsistent (9%). While of no physical significance, the illustration is correct in spite of comments to the contrary (568). Limiting forms (t + 0) of the current-time and potential-time functions for relaxation from potential, current, and charge perturbations reveal (168, 360) that in the presence of strong adsorption of species undergoing charge transfer, the proportionality constants differ from those derived by the classical approach. No quantitative estimate is given for the difference in electrode impedance (or admittance) from the classical treatment, except that it is “small” (560). A slightly piqued tone is noted in those 1968 publications, among the relatively few electrochemical contributions by Delahay during the review period. The experimentalists, however, will be busy for years to come. I n addition to numerous individual publications on relaxation measurements and their interpretation, most of which are noted in the following sections, a rather general survey of kinetics and double layer structure has appeared (570) from the very active Sluyter’s group. Frumkin’s and Delahay’s wellknown contributions to the subject are reviewed and some experimental results are given to illustrate double layer effects. When reacting species are specifically adsorbed on the electrode, the measured rates (using frequencies typically
