| Abstract: |
The zero point of charge (z.p.c.) of the electrical double layer of free charges at reversible interfaces is defined, and the experimental methods for determining this parameter are discussed. Adsorption isotherms of H+ and OH− ions on crystalline ferric oxide precipitates as obtained by acid and base potentiometric titrations are reported. From these studies at various ionic strengths (KNO3 indifferent electrolyte), the z.p.c. of α-hematite is established at pH 8.5. A model of the double layer at the Fe2O3-aqueous solution interface is developed. This model is based on the dissociation of surface hydroxyl groups formed by hydration of the virgin surface. In addition, it is shown that the z.p.c. may be determined by considering the adsorption of metal hydroxo-complex ions from solution. According to this analysis it is found that for Fe2O3 the z.p.c. agrees with the pH of minimum solubility of the oxide and with the isoelectric pH as determined by the positively and negatively charged Fe(III) hydroxo-complexes in solution. Although limited experimental data are available, this analysis may be applied to other oxide systems. |
