Effect of anions on the underpotential deposition of thallium(I) on polycrystalline silver

Underpotential deposition (upd) of thallium(I) on a polycrystalline silver surface in solutions containing specifically adsorbing anions (sulphate and chloride) has been studied by voltammetry and the electrochemical quartz crystal microbalance (EQCM). The specific adsorption of thallium cations on the silver surface at potentials positive with respect to the pzc appears to be enhanced by the specific adsorption of anions. This leads to ion pair formation between the specifically adsorbed thallium cation and anion even at potentials positive with respect to the upd region. The decrease in the upd shift is explained by the fact that additional energy is required to break the bond between the formed pair. The electrode mass change measured by EQCM is smaller due to the discharge of thallium cations from the adsorbed ion pairs. The agreement between voltammetric and EQCM data is observed in the potential region close to the bulk thallium deposition where the adsorption of anions is weak and, consequently, there is little formation of ion pairs. The effect of sulphate and chloride anions on the thallium upd process is in accordance with their adsorbing strength. It seems very likely that this scheme can be applied when other upd processes in strongly adsorbing anions containing solutions are considered. (C) 1998 Elsevier Science S.A. All rights reserved.