Isoelectric points of metal oxides at high ionic strengths

High ionic strengths induce a shift in the isoelectric point (IEP) of rutile to pH values substantially higher than its pristine IEP. When a critical concentration of electrolyte is exceeded (0.6 mol dm-3 for rutile and 0.35 mol dm-3 for hematite), there is no IEP at all and the electrokinetic potential is positive even at very high pH values. Similar shifts in the IEP have been observed for anatase, alumina, indium oxide, and niobium oxide. The critical NaI concentration is low for the oxides of metals whose valence is odd, and it is high (or it does not exist) for the oxides of elements whose valence is even. There is no simple correlation between the critical NaI concentration and the pristine IEP. The above results regarding the shifts in the IEP of oxides at high NaI concentrations must not be generalized for other 1-1 salts; e.g., cesium nitrate induces a substantial shift in the IEP of silica and no shift in the IEP of alumina while the opposite is true for NaI.