The Point of Zero Charge and the Isoelectric Point of γ-Fe2O3 and α-Fe2O3

The difference of the point of zero charge (PZC) and isoelectric point (IEP) values for γ- and α-Fe2O3 were measured and are quantitatively explained by the difference in crystal structure. Great care was taken to assure the purity of the samples and that measurements were taken under identical conditions. The PZC and the IEP for the “unhydrated” samples, were the same for each crystal structure, with the values for γ-Fe2O3 being more acidic, by 1.1 or 1.2 pH units, than for α-Fe2O3. This difference can be quantitatively explained by Parks’ equation, in terms of the difference in the coordinations of the Fe3+ ions in the two crystal structures.