Expulsion of borate ions from the silver/solution interfacial region during underpotential deposition discharge of Bi^III in borate buffer

Electrochemical quartz crystal microbalance (EQCM) studies of the underpotential deposition (UPD) of bismuth(III) at silver were performed in borate buffers in the pH range 8-10. A bismuth borate species is adsorbed at positive rational potentials. At more negative potentials the adsorbed species is reduced to UPD bismuth(o) and there is a concomitant decrease in electrode mass. The surface reduction process corresponds to the expulsion of four BO₂^- anions from the interface according to the reaction Bi(BO₂^-)₄ + 3e = Bi_upd + 4BO₂^-. The EQCM technique has been demonstrated to be well suited to study the mechanism of underpotential deposition of metals and the stoichiometry of adsorbed surface complexes.