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 higher oxidation (or lower reduction) state producing two equivalents of its middle redox state. A possible electrochemical signature of that comproportionation reaction is that the faradaic current from the two-electron process (I1,1) might not be twice the current of the corresponding one-electron process (I1,2). In this paper, using redox-active species with well-separated one- and two-electron processes, such as N,N′-di-n-heptylviologen, N-methyl-4-benzoylpyridinium perchlorate, TCNQ, TTF, N,N′-dimethylphenazine and TMPD, it is reported that within a wide range of the experimental parameters, two seemingly different electrochemical methods, namely sampled current voltammetry (SCV), a diffusion-controlled method, and rotating disk electrode (RDE) voltammetry, a convection-dominated method, give equal mass-transfer limited current ratios (I1,2I1,1). These phenomena have been traced to the fact that close to the electrode distance-normalized concentration profiles generated from both SCV and RDE voltammetry are superimposable. Digital simulations have confirmed these conclusions, and have led to the elucidation of the relative roles of the comproportionation reaction rate constant, kf, and the diffusion-layer thickness, δ, in determining the value of the (I1,2I1,1)SCV or RDE ratio: when the diffusion-layer is thicker, the comproportionation reaction time is longer and limiting (I1,2/I1,1) ratios are reached with lower kf values. (The larger δ corresponds to longer sampling times in SCV and slower electrode rotation rates in RDE voltammetry.). Ultimately, the limiting values of the (I1,2I1,1)SCV or RDE ratios are controlled by the relative values of the diffusion coefficients of all three species involved in the comproportionation reaction. According to our results, the (I1,2I1,1)SCV or RDE ratio can afford kinetic information on the comproportionation reaction, and comprises a diagnostic criterion for the relative diffusion coefficients of a redox-active species and its one-electron oxidized (or reduced) form.



Keywords and Phrases

Comproportionation; Diffusion coefficients; Rate constants; Simulations

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Article - Journal

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© 2001 Elsevier, All rights reserved.

Publication Date

01 Mar 2001