Investigations of SO 42− adsorption at the Au(111) electrode by chronocoulometry and radiochemistry

Abstract

Chronocoulometry was employed to measure charge density at the Au(111) electrode surface. Thermodynamic analysis of the charge density data for ionic adsorption from a solution with an excess of a supporting electrolyte is described and used to study SO 4 2− adsorption at the Au(111) electrode from a 0.1 M HClO 4 solution. The Gibbs excesses determined from the charge density data are compared with the Gibbs excesses determined from radiochemical measurements using 35S-labelled sulphate solutions. Very good agreement between the Gibbs excesses determined by the two techniques is observed. Essin-Markov coefficients for SO 4 2− adsorption from two series of solutions were determined: (i) at constant pH and variable K 2SO 4 concentration; (ii) at constant K 2SO 4 concentration and variable pH. The Essin-Markov coefficients were used to identify the nature of the adsorbed species (HSO 4 − or SO 4 2−). Our results indicate that SO 4 2− ion is the adsorbed species even if HSO 4 − predominates in the bulk of the solution. Chronocoulometry and radiochemistry were used to study SO 4 2− adsorption at the Au(111) electrode. Very good agreement between the Gibbs excesses determined from chronocoulometric and radiochemical measurements was observed. This agreement is an important verification of the reliability of these two in-situ techniques for measuring surface coverages. Although the two techniques give the same physical quantity, which is the Gibbs excess of adsorbed ions, each of them probes different properties of the interface and therefore has different limitations. The quantity measured by chronocoulometry is the electrode charge density. The surface concentration is calculated from σ M employing one integration step and one differentiation step and assuming the validity of the electrocapillary equation. Therefore the chronocoulometric technique can be applied only within the double-layer region of the gold electrode. However, since both the Gibbs excess and the charge density are measured simultaneously, this technique provides not only the surface coverages but also an additional electrical variable needed for the interpretation of the coverage data. In contrast, radiochemistry provides information about the Gibbs excess from direct counts of the radioactivity when the electrode with adsorbed radiolabelled material is pressed against the scintillator window. Therefore this technique can be used to study adsorption on both reactive and non-reactive materials. However, the technique is restricted by the efficiency in squeezing out the solution containing radioactive species from the thin layer between the electrode surface and the scintillator window. When the count rates from solution trapped in that gap are high with respect to surface counts, the adsorption measurements are difficult and ultimately become impossible to conduct. In fact, the method cannot be employed for adsorption measurements when the bulk concentration of labelled solute is higher than 10 −2 M [1]. In addition, the radiochemical technique provides information about the Gibbs excess only.