SPECIFIC ION ADSORPTION ON OXIDES - SURFACE-CHARGE ADJUSTMENT AND PROTON STOICHIOMETRY

A quantitative analysis of the coadsorption ratio for oxides, r//O//H, that is the number of OH** minus 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//O//H can be established with satisfactory precision using Gouy-Stern theory only. Computed r//O//H 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 Cd**2** plus adsorption on rutile and hematite is satisfactory.