In the presence of very fast comproportionation, sampled current voltammetry and rotating disk electrode voltammetry yield equal two versus one-electron limiting current ratios. Reconciliation through analysis of concentration profiles

A species having three sequential redox states is able to react with its hi gher oxidation (or lower reduction) state producing two equivalents of its middle redox state. A possible electrochemical. signature of that compropor tionation reaction is that the faradaic current from the two-electron proce ss (I-1,I-1) might not be twice the current of the corresponding one-electr on process (I-1,I-2). In this paper, using redox-active species with well-s eparated one- and two-electron processes, such as N,N'-di-n-heptylviologen, N-methyl-4-benzoylpyridinium perchlorate, TCNQ, TTF, N,N'-dimethylphenazin e and TMPD, it is reported that within a wide range of the experimental par ameters, two seemingly different electrochemical methods, namely sampled cu rrent voltammetry (SCV), a diffusion-controlled method, and rotating disk e lectrode (RDE) voltammetry, a convection-dominated method, give equal mass- transfer limited current ratios (I-1,I-2/I-1,I-1). These phenomena have bee n traced to the fact that close to the electrode distance-normalized concen tration profiles generated from both SCV and RDE voltammetry are superimpos able. Digital simulations have confirmed these conclusions, and have led to the elucidation of the relative roles of the comproportionation reaction r ate constant, k(f), and the diffusion-layer thickness, delta, in determinin g the value of the (I-1,I-2/I-1,I-1)(SCV or RDE) ratio: when the diffusion- layer is thicker, the comproportionation reaction time is longer and limiti ng (I-1,I-2/T-1,T-1) ratios are reached with lower k(f) values. (The larger delta corresponds to longer sampling times in SCV and slower electrode rot ation rates in RDE voltammetry.). Ultimately, the limiting values of the (I -1,I-2/I-1,I-1)(SCV or RDE) ratios are controlled by the relative values of the diffusion coefficients of all three species involved in the comproport ionation reaction. According to our results, the (I-1,I-2/I-1,I-1)(SCV or R DE) ratio can afford kinetic information on the comproportionation reaction , and comprises a diagnostic criterion for the relative diffusion coefficie nts of a redox-active species and its one-electron oxidized (or reduced) fo rm. (C) 2001 Elsevier Science B.V. All rights reserved.