Adsorption from mixtures containing mono- and bivalent cations on insoluble oxides and a revision of the interpretation of points of zero charge obtained by titration

Acid-base potentiometric titrations of haematite (α-Fe2O3) suspensions in mixtures of the non-specifically adsorbing electrolyte KNO3 and the nitrates of the specifically adsorbing ions Ca2+, Cd2+ and Pb2+ show that the cross-over point of the titration curves, plotted as σ0(pH), can be interpreted as the point of zero charge (pzc) only if there is no specific adsorption or if the amount of specifically adsorbed cations is so small that all of it is quantitatively adsorbed. This finding implies that the usual procedure for obtaining the pzc from cross-over points of titration curves must be reconsidered in the case of specific adsorption. In reality an Esin-Markov coefficient is measured, the relation of which to the surface charge requires further thermodynamic analysis. © 1982.