A thermodynamic analysis of common intersection points in potentiometric titration studies of solid surfaces

Abstract

Potentiometric titration curves for solid/solution interfaces often exhibit common intersection points (CIP) at different levels of supporting electrolyte and, for a fixed electrolyte level, at different concentrations of cosolvents. Both situations are discussed. For the first, explicit expressions are derived which summarise the conditions for a CIP to occur in the presence of specific adsorption and which allow formally for solution non-ideality. Attention is focussed primarily on plots of amount adsorbed vs. activity of the potential determining species rather than concentration. However, the latter case is also discussed briefly. The treatment is extended to cases where several ionic species are strongly specifically adsorbed and leads to a useful test for the occurrence of specific adsorption which is applicable in non-ideal systems. The situation where the bulk concentrations of such species are not negligible is also considered. For mixed solvents a simple expression is derived which describes the effect of solvent composition on the concentration of potential determining ions at the zero point of charge (z.p.c.). In this case CIP may arise from competition between medium effects associated with the adsorption of potential determining ions and electric fields in the diffuse double layer. The treatment is applicable to ionic crystals and to surfaces with fixed dissociable groups.