The Intrinsic Equilibrium Constants of the Surface Hydroxyl Groups of Maghemite and Hematite

The intrinsic equilibrium constants of the surface hydroxyl groups, which describe the ionization of surface hydroxyl groups and the complexation of ions of a KCl electrolyte, were calculated with the surface charge densities determined by the potentiometric titration, and with the surface site densities evaluated by the surface hydroxyl group densities. The values of the intrinsic ionization constants, Kalint and Ka2int of maghemite, are higher than those of hematite, and the differences of these values have a tendency for ΔpKalint<ΔpKa2int The values of the intrinsic complexation constants, *KK+int and *KCl-int, of maghemite are higher than those of hematite. These results could be attributed to the differences of the electron densities of the oxygen atoms of the surface hydroxyl groups, which are linked to those in the bulk those of maghemite are lower than those of hematite. Also, the differences become smaller with increasing number of bonding protons.