A modified radioactive labeling method was used to study bisulfate and sulfate anion adsorption on a Pt(lll) electrode from aqueous HClO4/H2SO4 media. The highest surface concentration of the anion is 3.2 x 10(14) ions/cm(2), which corresponds to a coverage (packing density) of 0.21 +/- 0.01monolayer. Overall, the coverage data confirm our previous results reported in J. Electroanal. Chem. 1993, 348, 451. We found fast adsorption kinetics and a semilogarithmic anion adsorption isotherm. In the "butterfly" electrode potential range [that is, in the range where anomalous anion adsorption was found on the Pt(111) electrode (Clavilier, J. J. Electroanal Chem. 1980, 107, 211)], the charge on the anion and, consistently, electrosorption valency vary with the electrode potential, demonstrating progress in bisulfate dissociation to sulfate, with the increase in potential. On the plateau of the surface concentration-electrode potential plot, we conclude that the adsorbate is a partially discharged (or neutralized) sulfate anion, which displays a net charge of -1.7 e. We also conclude that the species coadsorbed with sulfate are predominantly water molecules rather than hydronium cations. A brief review of the theory of anion adsorption on metal surfaces is presented and a model of sulfate-platinum bonding is proposed.
