Studies of the oxide surfaces at the liquid–solid interface. Part II. Fe oxides

The double layer on hematite and magnetite in KNO3, NaClO4, and KCl solutions has been investigated by studying the equilibrium distribution of the potential determining ions (H+ and OH−) at the interface as a function of their electrochemical potentials. Data have been obtained on the pHs of the zero point of charge (5.3 and 6.4 for hematite and magnetite, respectively, in 0.001 M KNO3 solution), the surface charge densities, q±, and the differential capacities C(±) of the double layer.For hematite, the q+ values in NO3− and ClO4− solutions are due to chemisorption of H+ on the surface, with NO3− and ClO4− showing no tendency for specific adsorption. In KCl solutions, high values of q+ were obtained, depending on Cl− concentration that are attributed to the replacement of the surface–OH groups by Cl− (specific adsorption). Magnetite (a semiconductor) shows anomalous behavior with respect to H+ adsorption.The negative charge on the two oxides originates from acidic dissociation of the surface–OH groups which is followed by adsorption of K+ on the surface. At high values of q−, K+ ions are specifically adsorbed on the surface-metal atoms through an O2− bridge.