Effect of temperature on the point of zero charge and surface dissociation constants of aqueous suspensions of γ-Al2O3

Abstract

The surface acidity constants, the point of zero charge (p.z.c.) and the concentration of the charged surface groups, AlOH+2 and AlO–, of γ-Al2O3 over a wide pH range have been determined at temperatures between 10 and 50 °C by potentiometric titrations. It was found that a precise regulation of the value of the p.z.c. can be achieved in the pH range 4.45–8.95 by varying the temperature of the impregnating solution between 10 and 50 °C. Increasing the temperature in this range caused an increase in the p.z.c. value. These results demonstrate that the increase (decrease) in the temperature of the impregnating suspension is a very simple method for achieving a marked extension of the pH range in which γ-Al2O3 can adsorb negative (positive) species. Moreover, it was observed that an increase (decrease) in the concentration of the AlOH+2(AlO–) groups, which are sites for adsorption of negative (positive) species, is obtained by increasing the temperature of the impregnating suspension. The p.z.c. values obtained experimentally were in excellent agreement with those calculated from the average of the two dissociation constants, Kint1, and Kint2, corresponding to the deprotonation–protonation equilibria of the surface AlOH groups.