IMPEDANCE VOLTAMMETRY OF ELECTRO-DIMERIZATION MECHANISMS - APPLICATION TO THE REDUCTION OF THE METHYL VIOLOGEN DI-CATION AT MERCURY-ELECTRODES AND AQUEOUS-SOLUTIONS

The faradaic impedance for an electrode mechanism with a reversible ho mogeneous dimerization reaction following the electron transfer step i s derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced b y Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order che mical reaction. Two limiting cases can be distinguished in which the g eneral expression reduces to the simpler Randles or pseudo-Randles exp ression. Under those conditions, the presence of the dimerization reac tion can be inferred from the potential dependence of the impedance pa rameters. The theory is applied to the reduction of the methyl viologe n di-cation at mercury electrodes in aqueous solution. The rate and th e equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warbur g coefficient, while the potential dependence of the irreversibility c oefficient allows the calculation of the standard rate constant and th e transfer coefficient for the electron transfer step. (C) 1998 Elsevi er Science S.A. All rights reserved.