Measurement of the surface potential of individual crystal planes of hematite.

A device for measuring surface potentials of individual crystal planes was constructed. The surface potentials of the (0 1 2), (1 0 -2), (1 1 3), and (1 1 -3) crystal planes of hematite were measured as a function of pH at different sodium nitrate concentrations. Results of measurement enabled differentiation between the planes, showing agreement with the surface potentials obtained with a single-crystal hematite electrode. At low ionic strength there was no significant difference in potential between the crystal planes, whereas at relatively high ionic strength the difference was noticeable. In the absence of counterion association, but also in the case of their symmetric association taking place, point of zero potential (pH(pzp)) coincides with other zero points, i.e., with the isolectric point (pH(iep)) and the point of zero charge (pH(pzc)). If the counterion affinities toward association are not equal, the pH(pzp) is shifted in the same directions as the pH(pzc). The shift in the point of zero potential to the basic region was more pronounced for the (1 1 -3) plane than for the (1 0 -2) one, indicating a higher affinity of anions for association with oppositely charged surface groups compared to cations. It was demonstrated that measurements of surface potentials of individual crystal planes could help to better understand the equilibrium at solid/liquid interfaces.