Radiometric and voltammetric study of benzoic acid adsorption on a polycrystalline silver electrode

Adsorption of benzoic acid was studied on polycrystalline silver electrodes in a 0.1 M perchloric acid solution. The electrodes were obtained by electroplating of Ag-black onto a vacuum-deposited silver substrate. The real surface area of such prepared electrodes was calculated from a total capacitance of the electric double layer, determined from the plot of the voltammetric double-layer charging current as a function of the scan rate. Quantitative adsorption data for benzoic acid, obtained using the thin-layer radiometric method, indicate that benzoic acid is surface active in the entire range of potentials available to electrochemical studies on silver in aqueous solutions. The Gamma vs E plots show that adsorption increases as potential becomes more positive. An overlap of the positive- and negative-going T-E plots points to a full reversibility of the surface process, regardless of the potential range. In turn, surface/bulk exchange experiments attest to a reversibility of adsorption with respect to solution concentration. The apparent value of the Gibbs energy of benzoic acid adsorption on an Ag electrode, as estimated from the Langmuir isotherm, is equal to -31 kJ mol(-1). A kinetic analysis of the radiometric data suggests that the surface process rather than the diffusion is a rate-determining step of adsorption. The adsorption of benzoic acid on polycrystalline silver is compared with the adsorption of benzoic acid on other transition metals. (C) 1998 Elsevier Science Ltd. All rights reserved.