Dierential capacitance studies of the speci®c adsorption of thiosulfate on silver

The di€erential capacitance of a polycrystalline Ag electrode was measured in a NaClO4 electrolyte containing Na2S2O3 at concentrations ranging from 0.1 to 20mM and at electrode potentials ranging from )0.9 to )0.3V vs SCE. The di€erential capacitance measurements were analysed to obtain the surface coverage of speci®cally adsorbed thiosulfate (S2O 2ÿ 3 ) as a function of both electrode potential and bulk concentration. The various forms for the adsorption isotherm at an electrochemical interface which are commonly employed are reviewed and discussed. The adsorption behaviour is best explained by the formation of image dipoles by the speci®cally adsorbed ions, whose energetic interactions are then dominated by repulsion of like dipoles. The adsorption data is quantitatively ®t to an adsorption isotherm for interacting dipoles, yielding an e€ective dipole moment of 0.72D for the system S2O 2ÿ 3 /Ag.