β Scribed by L. Papouchado; J. Bacon; R.N. Adams
No coin nor oath required. For personal study only.
Cyclic voltammetry (CV) is probably the simplest and most useful electrochemical technique for providing a quick, overall picture of an electrode reaction 1 . However, its simplicity sometimes gives rise to ambiguities, especially when the results are compared with those of other electrochemical methods, e.g., larger scale controlled potential electrolyses. Obvious extensions of conventional CV, including cycling between special potential limits, and potential stepping followed by cycling, have undoubtedly been used by many investigators, but no formal report on applications to practical systems has been made. Schwarz and Shain reported a potential step-linear sweep method for measuring the rates of first-order follow-up reactions, but this was strictly of interest to kinetics 2 , Potential step cyclic voltammetry (PSCV) has been used in this laboratory to elucidate some difficult systems that gave somewhat misleading information by conventional CV. While no claim is made for the originality of the technique, we wish to point out its extreme utility in the hope that it will be more widely applied.
The PSCV method involves two steps. In the first, the potential is stepped out to the region of interest and held constant for 15-30 s. In the second step, the potential is cycled rapidly to observe follow-up products. By this means, the generation time for products is much greater than the usual scan time. We report below the results of PSCV studies on two organic compounds which will demonstrate the value of this method.
A good example is the oxidation of o-methoxyphenol. Figure 1A shows the CV of o-methoxyphenol (MP) with an initial oxidation wave at ca. +0.8 V. On reversing the scan, two couples are seen which were found to correspond to the dimer-like products 3,3'-dimethoxydiphenoquinone and 3,3'-dimethoxy-4,4'-biphenol (DMDQ/DMBP)~and to o-quinone and catechol (o-Q/C). (Due to the high irreversibility of the o-Q/C system, the oxidation of the resulting catechol appears more anodic than that of the DMBP, Le,, the two different redox couples appear "split" in potential appearance. This is not a particularly unusual situation and is not of importance in the present discussion, but is mentioned only to forestall any confusion in evaluating the CV of Figure 1A.)
π SIMILAR VOLUMES