The effects of counter ion nature on the adsorption of nitrate ion at the mercury/solution interface

Abstract

The adsorption of nitrate ion at mercury has been studied from two systems at constant ionic strength, namely, xM NaNO3 + (0.2 − x) M NaF and x M KNO3 + (0.2 − x) M KF. The surface excess due to adsorbed nitrate ions was determined from differential capacity data using a modified version of the Hurwitz–Parsons analysis which takes into consideration variation in ionic activity coefficients with solution composition. The amount of adsorbed nitrate ion at a given electrode charge density and bulk nitrate ion concentration is shown to depend markedly on both ionic strength and the nature of the counter ion at the outer Helmholtz plane; when the charge in the diffuse layer is positive, an increase in ionic strength results in more anion adsorption and vice versa. A change in the cation from Na+ to K+ also results in increased anion adsorption. The effects observed are discussed in terms of the Stern–Grahame–Levine model for ionic adsorption which is based on an electrostatic description of the charged interface with consideration of discreteness-of-charge effects.