The coupling of diffusion,migration and chemical equilibrium during the voltammetric reduction of weak polyprotic acids1

A theoretical model is presented for the coupling of diffusion, migration and chemical equilibrium during the voltammetric reduction of weak polyprotic acids. The dissociation reaction of a neutral acid is assumed to influence only the apparent diffusion coefficients of the hydrogen ion and its conjugate base anions. For typical values of equilibrium constants of dissociation reactions and analytical concentration, polyprotic acids behave as either monoprotic acids or diprotic acids, which agrees well with experimental results. A semi-quantitative explanation based on a diffusion layer treatment is proposed for this phenomenon. It is also shown that the ratio of limiting current at low concentration of supporting electrolyte to diffusion-limited current depends on the initial equilibrium concentrations of all species in the solution but not on the kinetics of the dissociation reactions. Furthermore the movement of the uncharged species (the neutral acid) can be influenced by an electric field through the coupling effected by the chemical equilibrium.