A Cyclic Voltammetry Experiment for the lnstrumental~nal~sis Laboratory Richard P. Baldwin,' K. Ravichandran, and Ronda K. Johnson University of Louisville, Louisville. KY 40292 Electrochemically based experiments, which have traditionally occupied a prominent bosition in the undergraduate instrumental analysis laboratory, can generally be placed in either of two categories, those involving ioteniiometric methods of analysis and those involving voltammetric methods. Of the latter classification, by far most approaches have employed mercury electrodes, either dropping or stationary, and have utilized electrochemical techniques whose major applications lie almost exclusively in the area of quantitative analytical determinations. Thus, experiments based on classical DME polarograpby (1-3),normal and differential pulse voltammetry ( 3 , 4 ) ,and anodic stripping (5, 6) have most often been recommended. In rect2ntyears, however, there has been a renewed interest in numerous applications of electrochemical techniques outside the area of traditional polarographic analysis. In particular, the investigation of electrochemical oxidations at solid electrodes has become a verv active research area. These tmes of studies have utilized ppincipally the technique of &lic voltammetw (CV) which. in addition t o a somewhat limited range of quantitative applications, possesses extensive capabilities for the kinetic and mechanistic analvsis of electrode processes. In fact, unlike most other voltammetric methods, C\' has experienced wide usage - bs. non-analvtical chemists in the study of numerous organic, inorganic;and biological redox processes. Surprisingly, however, experimental procedures introducing the principles and applications of CV in a format suitable for use in the instrumental analysis laboratory have not been rapidly forthcoming. This is despite the fact that CV is perhaps the most straightforward of the voltammetric techniques, involves relatively inexpensive and easyto-operate instrumentation, and can easily avoid the hazard and inconvenience inherent in the use of mercury as an electrode material. In this report, we will describe some redox systems which we have employed with success in our laboratory program to illustrate both the nature of the CV technique and its application in the characterization of organic electrode processes. Theory
One of the principal advantages of CV is its inherent simplicity. The CV experiment consists simply of the application of a re~etitivelinear scan over a fixed notential ranee a t a stationary electrode and the continuou~monitorin&f the current which resulb. The primam exwrimental variable. the rate a t which the potential is scanned,brovides a direct means for acquiring kinetic information concerning the rates of the electrode pr&esses under study. For a reversible, diffusion-controlled. electron transfer reaction at a stationary electrode, the CV should contain distinctly peaked anodic and cathodic waves for the electrolyzed species since no convective mass transfer is employed and the time interval between forward and reverse potential sweeps is relatively short. For such an ideal electrochemical system, the peak current i, in amperes is given by
Author to whom correspondence should be addressed 820
Journal of Chemical Education
where n is the number of electrons oer molecule oxidized or reduced, A is the electrode area, u isthe rate of the potential scan. D is the diffusion coefficient of the electroactive s~ecies. and C' is its bulk concentration (7). Thus, it can reahily he seen that, for a perfectly reversible process, the current is directly proportional to both u1l2 and C*. Further, both the anodic and cathodic peak potentials are independent of scan rate, separated by afixedpotential difference AE, of 59ln mV. Additionally, in the absence of anv significant chemical complications, the peak currents obseriedfor the anodic and cathodic scans should both, of course, be identical. In practice, the real electrochemical systems most often encountered seldom possess the instantaneous electron transfer rate assumed for perfectly reversible systems and often are complicated further by the occurrence of preceding or succeeding chemical reactions which directly affect available concentrations of the electroactive species at the electrode surface. Actually, it is these "non-ideal" processes which are usually of the greatest chemical interest and for which the special advantages of CV are the most suitable. In general, for irreversible systems, decreases in the rate of charge transfer are characterized by voltammograms containing anodic and cathodic peaks which are increasingly drawn out in shape and separated from one another. As the~scanrate is increased, this phenomenon is often observed even for redox systems which amear oerfectlv reversible at slower u's. The heteroeeneous eiictrori transfer rate constant k , for such "quasi-re&rsible9' systems can be determined exoerimentallv bv a varietv of CV approaches, the most convedient of whkhWisbased-on the relation ~
where a is the transfer coefficient for the electron transfer, y is the ratio of the diffusion coefficients of the reactant and product species, a = nFuIRT, and $ is a tabulated function whose values are provided in Table 1. By adjusting the potential scan rate such that ( n u , ) , the product of the peak separation and the number of electrons involved, matches a
Table 1. Varlatlon of Tabulated Parameter a wHh Peak Potentlal Separation (from ref. (7)) $ 20 7
6 5 4 3 2 1 0.75 0.50 0.35 0.25 0.10 A P S Y ~ B I OL
= 0.5.
nAEp(mV) 61 63 64 65 66 68 72 84 92 105 121 141 212
tabular value of $, k, is obtained. Under these conditions, k, can be computed directly. Chemical reactions associated with the primary electron transfer nrocess can occur in numerous forms. each of which must be considered using a theoretical approach tailored to the specific reaction mechanism in effect (7-9). In general, however, since the potential in CV can he scanned over a wide ranee and a t variable rates. the formation of electroactive reaction products or intermediates can usually be conveniently monitored via the CV approach.
-
.
.
Another significant advantage of CV is in the extremely modest equipment investment required. The two indisprnsable items& the cyclic voltammej control apparatus (khich serves to scan the electrode potential as required and to transform the CV current into a useahle voltage signal) and an X-Y recorder. In this work, a modestly priced, commercially available CV unit (Model CV-lB, Bioanalytical Systems, Inc., West Lafayette, IN) was utilized. As the purchase price of this device was $500, it is well within the budget range of most instrumental analysis courses. The recorder used in this work was the Houston Instruments Model 2000 X-Y recorder. The use of this or other conventional X-Y recorders will allow scan rates up t o roughly 300 mV/s. This will he a limiting factor in some CV applications but does not substantially hinder its use in the undergraduate lab. A digital multimeter (Keithley Model 169) was used to monitor the cell potential convenientlv. A three-"electrode cell configuration was employed throuehout this work. The workine electrode was a carbonpaste'&rode using Nujol oil as t
