Effects of impurity and solid-phase dissolution on surface charge titration of aluminium oxide

摘要

Potentiometic and calori-metric acid-base titration of aluminium oxide C (Degussa) was performed. This pyrogenic alumina contains chlorine impurity (about 1 x 10~(-4) mol/g). Its hydrolysis results in indefinite numbers of H~+ and Cl~- ions in aqueous suspension, which disturb the surface charge titration. The chlorine impurity could be removed by heat treatment of alumina powder at 1000℃. The fitting of the titration data for heat-treated alumina in indifferent electrolyte (KNO_3) solutions between pH～5 and pH～9 is good enough assuming only surface charging reactions and any kind of surface complexation model. Outside these material-specific limits the dissolution of the amphoteric solid plays the governing role, especially below pH～4.5. A partial dissolution of the solid phase and the simultaneous equilibria of mononucleous aluminium-species formation have to be inserted into an appropriate model. Calorimetric data can be assigned to chemical reactions in the absence of impurities and dissolution and if the initial state of titration is fixed to the reference state of oxide suspensions. The partial molar enthalpy of the surface protonation process decreases in absolute value from -34 to -28 kJ/mol and that of deproto-nation process increases from 33 to 41 kJ/mol with increasing ionic strength. The standard enthalpy of the surface protolysis reaction on alumina is 34.6 +- 0.6 kJ/mol.