Adsorption of Hydrogen and Hydroxyl Ions on Oxide Mineral

Abstract

The adsorption amount of hydrogen and hydroxyl ions on alumina in water was determined by the acid titration method which was theoretically well explained in this study. It was found that there should be a value of pH, defined here as the equi-adsorption point, at which hydrogen and hydroxyl ions were equal in adsorption amount on alumina. The adsorption isotherms of hydrogen and hydroxyl ions on alumina were expressed in the form of Freundlich isotherm. The equi-adsoption point for alumina was pH 7.4, while the isoelectric point for the same material was pH 9.0 by the electrophoresis measurement. The heat of immersion of alumina was also measured in water at various pH values. The pH dependence of the heat of immersion related closely with the adsorption of hydrogen and hydroxyl ions. The pH value where there is the minimum value in the heat of immersion coincided with the equi-adsorption point. Thus, it was confirmed that the equi-adsorption point should not always be in agreement with the isoelectric point.