Abstract
The amphoteric properties of hydrous γ-Al 2O 3 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 p K int 1 = 7.9 and p K int 2 = 9.1. The extent of adsorption of Ca 2+ and other earth alkaline cations on hydrous γ-Al 2O 3 increases with increasing pH; finite adsorption is observed at pH zpc and at pH values slightly lower than pH zpc. Evaluation of adsorption measurements and alkalimetric titration curves indicates that the adsorption of M 2+ can be described by: AlOH + M 2+ ⇄ AlOM + + H +. Operational association constants for this equilibrium were determined; the extent of adsorption increases in the series Mg 2+ > Ca 2+ > Sr 2+ > Ba 2+. In the pH-range above pH zpc, these ions, upon being adsorbed may reverse the surface charge of the oxide suspension.
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