The real meaning of Nernst's steady diffusion layer concept under non-forced hydrodynamic conditions. A simple model based on Levich's seminal view of convection

A model accounting for the role of convection in macroscopically immobile s olutions (viz., not submitted to any macroscopic flow or any density gradie nt) is developed to predict the resulting alterations on electrochemical ex periments performed at long time durations. The model is based on a seminal idea introduced by Levich, and shows that in macroscopically still solutio ns microscopic chaotic motion has the same effect as an apparent diffusion coefficient that depends on the distance from the electrode, x. When the el ectrochemical perturbation affects only the viscous sub-layer adjacent to t he electrode, this apparent diffusion coefficient varies as x(4). The model remarkably predicts the experimental distortions of chronoamperometric cur rents from their ideal Cottrellian behavior. The model is thoroughly tested with success by comparing its predictions to experimental results (current and concentration profiles) obtained for the oxidation of Fe(CN)(6)(4-) in aqueous KCl. (C) 2001 Elsevier Science B.V. All rights reserved.