INVESTIGATIONS OF SO4(2-) ADSORPTION AT THE AU(111) ELECTRODE BY CHRONOCOULOMETRY AND RADIOCHEMISTRY

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 SO42- adsorption at the Au(111) electrode from a 0.1 M HClO4 solution. The Gibbs excesses determined from the charge density data are compared with the Gibbs excesses determined from radiochemical measurements using S-35-labelled sulphate solutions. Very good agreement between the Gibbs excesses determined by the two techniques is observed. Essin-Markov coefficients for SO42- adsorption from two series of solutions were determined: (i) at constant pH and variable K2SO4 concentration; (ii) at constant K2SO4 concentration and variable pH. The Essin-Markov coefficients were used to identify the nature of the adsorbed species (HSO4- or SO42-). Our results indicate that SO42- ion is the adsorbed species even if HSO4- predominates in the bulk of the solution.