Specific ion adsorption on oxides: Surface charge adjustment and proton stoichiometry

In this paper a quantitative analysis of the coadsorption ratio for oxides,r_OH, that is the number of OH^- ions coadsorbing with each specifically adsorbing metal ion at given pH, is given. This ratio is an important double-layer parameter. It is shown that because of the high accumulation of charge on the surface and the effective compensation of this charge by countercharge in the Stern plane,r_OH can be established with satisfactory precision using Gouy-Stern theory only. Computedr_OH values depend primarily on the distance between the pH and the pzc and to a lesser extent on the indifferent electrolyte concentration. If the Stern layer capacitance is high, it is not a critical parameter. Agreement with data in the literature and new measurements for Cd²⁺ adsorption on rutile and hematite is satisfactory.