Abstract: |
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 pHzpc = 8.5; acid-base properties can be characterized in terms of a heterogeneous two-protic acid with intrinsic acidity constants pKint1 = 7.9 and pKint2 = 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 pHzpc. 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 pHzpc, these ions, upon being adsorbed may reverse the surface charge of the oxide suspension. |