Potential difficulties in obtaining reliable rate constants for thermodynamically unfavorable electron transfer crossreactions between reactants within polymer coatings on rotating disk electrodes and co-reactants in solution

Abstract

The utilization of rotating disk voltammetry to evaluate the rates of electron transfer reactions between redox reagents immobilized in coatings on electrodes and co-reactants in solution has become commonplace [1,2]. The theoretical analyses provided by Andrieux and Saveant [3] have proven to be particularly useful in diagnosing reaction schemes from the dependences of measured plateau currents on the rates of rotation of the coated disk electrodes. Kinetic analyses are simplest when experimental conditions can be selected so that the measured currents are controlled only by the rate of the electron cross-reaction of interest. However, not infrequently, the rate of permeation of the electrode coating by the co-reactant and/or the rate of electron propagation within the coating can affect the measured currents. Andrieux and Saveant have provided instruction on how to treat the experimental data in such cases in order to extract the rate constants for electron cross-reactions [31 and numerous successful studies have been carried out [2,3]. However, considerable care needs to be taken in evaluating the experimental data to avoid serious errors in the rate constants obtained for thermodynamically unfavorable electron transfer cross-reactions. In the present paper, the sources of the potential problems are described and examples are provided to demonstrate the magnitude of the errors that can result.