Isotherms and related data in the electro-adsorption of certain ions on mercury

Abstract

The adsorption of inorganic anions (CN-, CNS-, ClO4-, ClO3- and BrO3-) and cations (Cs+, Rb+ and Li+) on mercury from aqueous solutions of sodium salts and alkali chlorides and iodides (CsI) has been studied by measuring the interfacial tension of the mercury-solution interface as a function of concentration and potential. The specific or contact adsorption of anions and of the Cs+ cation was evaluated as a function of concentration and charge density on the metal. The experimental results for the contact adsorption of anions were found to fit best an isotherm due to Bockris, Devanathan and Müller, which allows specifically for the ion-ion interactions. The standard free energy of adsorption ΔG 0° decreases in the following order: I -‡>CNS->CN-> BrO 3- >ClO4- >ClO 3- >Cl-.§ The experimental results point to the necessity of introducing a correction into the above isotherm, viz., the interaction between ion-image dipole and electrode should be replaced by the work of transferring the ion through the region of low dielectric constant. An empirical equation relating ΔG0° to the ionic radius, ΔG0°≃Arion3 (where A is a constant depending on the shape of the ion), was found to be consistent with the behaviour predicted by Andersen and Bockris on a non-covalent model for specific adsorption.