Evidence for competitive adsorption of fluoride ions in studies carried out at constant ionic strength: The bromide adsorption on a (110) face of silver

Abstract

The adsorption of the bromide, ion on a (110) silver electrode from mixed solutions of sodium bromide and sodium fluoride was investigated by measuring the double-layer capacity as a function of solution composition. The analysis of the total inner-layer (differential) capacity as a function of the electrode charge brings clear evidence for competitive specific adsorption of the fluoride ion when the bromide concentration is small with respect to the fluoride concentration. The total inner-layer capacity is resolved into its component parts with the aid of diffuse layer theory. The (integral) capacity measured at constant electrode charge is a constant which the fluoride adsorption appears not to interfere. The (differential) capacity measured at constant amount adsorbed as a function of the electrode charge presents a maximum at zero charge, when the fluoride influence is negligible, which may be attributed to the solvent properties. With corrected values of the electrode charge vs. potential plots for a base electrolyte without anionic adsorption, the standard tree energy of bromide adsorption is shown to be linearly dependent on the electrode charge, as for the bromide aqueous solutions/mercury system.