Specific adsorption of tetra-alkyl ammonium iodide at the mercury—solution interface and the influence of the double-layer structure on the kinetics of zinc ions

Abstract

The effect of adsorption of tetra-alkyl ammonium iodide at the dme has been examined with Faradaic impedance measurements. The differential capacitance of mercury in the salt solutions of the type R 4NI, where R is CH 3, C 2H 5 and n-C 3H 7 at 25°C as a function of polarization is measured. The parameters of the electrocapillary maximum, E max and σ max, are evaluated. By comparing the differential capacitance curves with those observed in other aqueous salt solutions of the same concentration and assumed to exhibit specific adsorption, it is shown that the cation is specifically adsorbed and that adsorption increases from tetra-methyl to tetra-ethyl to tetra-propyl salts. On the anodic limb of the capacity curve it is assumed that there is simultaneous specific adsorption of cations and anions. The anions thus form bridges between the cations and the mercury surface. The apparent standard rate constant K s for the discharge of Zn(II)/Zn(Hg) in 1 M NaClO 4 was found to decrease in the presence of the quaternary cations and accelerated by iodide only. This phenomena is similar to that observed with the hydrogen evolution reaction at the mercury electrode and is opposite of those observed with anions of the persulphate type.