Surface acidity constants of alpha-Al2O3 between 25 and 70 degrees C

The effect of temperature on the adsorption of H+ onto corundum was investi gated experimentally by conducting potentiometric titrations at 25, 30, 40, 50, 60 and 70 degrees C. These titrations were used to determine apparent acidity constants, given by the product of an intrinsic adsorption constant and a coulombic term (K-i(app) = K(i)(int)e((-Delta ZF Psi/RT))) at the ab ove temperatures. First,and second intrinsic acidity constants were determined using a consta nt capacitance model (CCM). These constants and the point of zero charge ch ange linearly with inverse temperature. These data were used to determine t hermodynamic constants for the proton adsorption reactions. In the coulombic term, only the capacitance and the surface charge change w ith temperature and both were determined with the titration data at the var ious temperatures. Results show that the change in capacitance can be predi cted with changes in the dielectric constant of water with increasing tempe rature. At a given pH, changes in the: surface charge with temperature can, in turn, be predicted with a linear regression. With the above model, apparent acidity constants of corundum (including the chemical and electrostatic interactions) can be predicted for any temperat ures between 25 and 70 degrees C and possibly higher. These apparent consta nts change over several orders of magnitude in this temperature range (main ly due to a change in the coulombic term) and small temperature changes cou ld have a strong influence on the stability of surface complexes. Copyright (C) 1999 Elsevier Science Ltd.