Estimation of surface ionization constants for amphoteric solids

Modeling of aqueous ion adsorption onto amphoteric solids requires the determination of the equilibrium constants for the surface ionization reactions. From these data, the pH dependent surface charge development can be determined on the basis of any number of models for the electrical double layer. Experimentally, one can obtain the sum of these ionization constants, but an additional relationship is required to determine each uniquely. Numerous schemes have been proposed to accomplish this based on certain assumptions. Unfortunately, data analysis is often confined to a limited pH range, and contradicts the assumptions inherent to each scheme. It is proposed that a reasonable estimate of the ionization constants can be obtained by combination of a graphical method and numerical regression. The methodology was applied to existing data from simple oxide systems and recent data from activated carbons. The results show that the values of the ionization constants are strongly dependent on the pH range used. Consequently, it is suggested that a uniform range be established for the purpose of comparing inter-laboratory results: data within 3 pH units from the point of zero charge toward pH 7 should be taken as the data base for consistent regression results.