An experimental comparison between the aqueous pH scale and the electron donicity scale

Abstract

We have made an experimental comparison of the aqueous pH scale with the Lewis base donicity scale. This requires an indicator that can function in both kinds of system. We find that the zeta-potential zero charge point for certain oxide materials provides a suitable indicator. The materials are: SiO2, Al2O3, TiO2, and MgO. For each material, we measure the zeta potential in aqueous media as a function of pH. In each case, the zeta potential varies systematically with pH, and changes sign at some value of pH, that we refer to as the zero charge point. We then do similar measurements in a series of organic liquids having different donicities. In this case, the zeta potential varies systematically with the donicity and may change sign. In such cases, we determine the donicity that corresponds to the zero charge point. For a given oxide material, we can then relate a zero charge point on the pH scale to one on the donicity scale. This gives us a consistent experimental alignment of the pH and donicity scales, relative to one another. We compare our results with thermodynamic data, relating to the pH scale, and the hydrogen electrode, and find good agreement.