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

Catalytic reductions of some aromatic halides were performed at a millimetr ic electrode with several redox mediators. The resulting concentration prof iles were monitored amperometrically by placing an ultramicroelectrode insi de the diffusion layer produced at the former electrode. The features of re dox catalysis and the subsequent structuring of the diffusion layer were in vestigated experimentally under steady-state conditions imposed by the spon taneous convection of the solution. The concentration profiles established from the probe measurements were in agreement with our theoretical predicti ons, based on fast kinetics of redox catalysis. Under these conditions, ver y similar to preparative electrosynthesis, the diffusion layer separates in to two domains where pure diffusion takes place and the concentration profi les 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 o f the ratio of the bulk concentrations of each species and the ratio of the ir diffusion coefficients.