RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

Potentiometric titrations in indifferent aqueous electrolyte were used to memure the surface charge characteristics of microcrystalline particles of ferric oxides. The net adsorption density (Γ_H⁺ – Γ_OH⁻ microequivalents per gram) is described by equations for the surface excesses of H⁺ and OH⁻, derived by equating the electrochemical potentials of potential-determining ions and indifferent counterions in solution and surface phases, and neglecting counterions in the diffuse layer. The equations contain interaction constants arising from the small but finite distances of separation between planes of adsorbed potential- determining ions and indifferent counterions under conditions of ferric ion solvation at the interface. The BET nitrogen area does not satisfactorily represent the dependence of parameters and interaction constants on particle surface area, for different preparations. The equations were interpreted as showing that adsorbed counterions remain solvated, forming electrostatically bound ion pairs with only a small proportion dissociating into the diffuse layer.