Determination of specific adsorption of some simple anions at a polycrystalline silver-aqueous interface using differential capacitance and kinetic probe techniques

The specific adsorption of chloride, bromide, iodide, azide, and thiocyanate has been studied at an electropolished polycrystalline silver-aqueous interface using differential capacitance measurements. For chloride, bromide, and azide, quantitative estimates of the surface concentration of specifically adsorbed anions were obtained from capacitance-potential data in mixed fluoride electrolytes having a constant ionic strength of 0.5. The dependence of the measured capacitance upon the ionic strength of sodium fluoride was also investigated in order to check the behavior of the polycrystalline surface in comparison with the predictions of conventional double-layer models. Estimates of the specifically adsorbed charge densities of chloride, bromide, and thiocyanate anions were also obtained from a "kinetic probe" technique which entailed monitoring the response of the outer-sphere reduction rate of CO(NH₃)₅F²⁺ and Co(NH₃)₆³⁺ to the addition of the appropriate adsorbing anion. At the average potential of zero charge for the polycrystalline silver surface, the standard free energies of adsorption - Δ₃^o for chloride, bromide, and azide were found to be within ca. 5 kJ mol^-1 of the corresponding quantities obtained at mercury electrodes. However. significantly greater increases in - Δ₃ ^o in the sequence Cl^- < N₃^- < Br^- are seen at silver compared to mercury. Electrochemical roughening in chloride media, giving silver surfaces displaying intense surface Raman scattering, yields only minor changes in the surface concentration of specifically adsorbed chloride anions.