RADIOMETRIC AND VOLTAMMETRIC STUDY OF BENZOIC-ACID ADSORPTION ON A POLYCRYSTALLINE SILVER ELECTRODE

Adsorption of benzoic acid was studied on polycrystalline silver elect rodes in a 0.1 M perchloric acid solution. The electrodes were obtaine d by electroplating of Ag-black onto a vacuum-deposited silver substra te. The real surface area of such prepared electrodes was calculated f rom a total capacitance of the electric double layer, determined from the plot of the voltammetric double-layer charging current as a functi on of the scan rate. Quantitative adsorption data for benzoic acid, ob tained 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 positi ve. An overlap of the positive- and negative-going T-E plots points to a full reversibility of the surface process, regardless of the potent ial range. In turn, surface/bulk exchange experiments attest to a reve rsibility of adsorption with respect to solution concentration. The ap parent value of the Gibbs energy of benzoic acid adsorption on an Ag e lectrode, 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 o f adsorption. The adsorption of benzoic acid on polycrystalline silver is compared with the adsorption of benzoic acid on other transition m etals. (C) 1998 Elsevier Science Ltd. All rights reserved.