Surface Charge of Alumina and Magnesia in Aqueous Media

By streaming potential measurements, the zero point of charge (zpc) of or‐Al 2 O 3 having a fully hydroxylated surface was found to occur at pH 9.2 ± 0.2. By calcining the alumina at temperatures above 1000°C, the zpc was lowered to about pH 6.7. By aging the calcined material in water at room temperature for a period of weeks, the surface was rehydroxylated and the zpc increased. The zpc of MgO occurred at pH 12.5 f 0.5. Because of its high solubility, the zpc of MgO was not affected by heat treatment.