RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The amphoteric properties of hydrous γ-Al₂O₃ and the distribution of earth alkaline cations in the oxide-electrolyte interface was evaluated from alkalimetric and acidimetric titration curves of aqueous dispersions. In NaCl electrolytes (25°C), pH of zero point of charge is pH_zpc = 8.5; acid-base properties can be characterized in terms of a heterogeneous two-protic acid with intrinsic acidity constants pK_int¹ = 7.9 and pK_int² = 9.1.

The extent of adsorption of Ca²⁺ and other earth alkaline cations on hydrous γ-Al₂O₃ increases with increasing pH; finite adsorption is observed at pHzpc and at pH values slightly lower than pH_zpc. Evaluation of adsorption measurements and alkalimetric titration curves indicates that the adsorption of M²⁺ can be described by: AlOH + M²⁺ ⇄ AlOM⁺ + H⁺. Operational association constants for this equilibrium were determined; the extent of adsorption increases in the series Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺. In the pH-range above pH_zpc, these ions, upon being adsorbed may reverse the surface charge of the oxide suspension.