The adsorption of bromide ions on mercury from propylene carbonate solutions of constant ionic strength

Abstract

The adsorption of Br − anions has been studied from propylene carbonate solutions of low ionic strength (0.16 M) at mercury. A maximum on the capacity curve is observed at low Br − ion concentration, a result which is unusual for strongly adsorbed ions. A new method of analysing the interfacial thermodynamic data, which makes use of data obtained in the absence of Br − ion, is presented. Adsorption is shown to be very strong and can be represented by a virial adsorption isotherm over the range that data are available. The adsorption parameters can be rationalized on the basis of the electrostatic model for adsorption with the proviso that the system is complex because of varying inner layer parameters for a given electrode charge density. When the present data are compared with those obtained in other solvents, it is shown that the standard Gibbs energy of adsorption changes with solvent acidity and basicity due to corresponding changes in anion solvation in the bulk, and the Gibbs energy of solvent adsorption on mercury, respectively, the adsorption parameters at the point of zero charge are examined with respect to non-primitive statistical mechanical models for ionic adsorption, discussed recently in the literature.