Mapping concentration profiles within the diffusion layer of an electrode: application to redox catalysis.

Abstract

Catalytic reductions of some aromatic halides were performed at a millimetric electrode with several redox mediators. The resulting concentration profiles were monitored amperometrically by placing an ultramicroelectrode inside the diffusion layer produced at the former electrode. The features of redox catalysis and the subsequent structuring of the diffusion layer were investigated experimentally under steady-state conditions imposed by the spontaneous convection of the solution. The concentration profiles established from the probe measurements were in agreement with our theoretical predictions, based on fast kinetics of redox catalysis. Under these conditions, very similar to preparative electrosynthesis, the diffusion layer separates into two domains where pure diffusion takes place and the concentration profiles therein are mainly linear. We demonstrate that the limit between these two zones does not depend on kinetics, but is rather fixed by the product of the ratio of the bulk concentrations of each species and the ratio of their diffusion coefficients.