STUDIES OF DOUBLE LAYER ON CASSITERITE AND RUTILE

The zero point of charge (z.p.c.) and the surface charge densities (q±) of cassiterite and rutile have been determined as a function of pH3 and ionic strength in KNO3 and KCl solutions while the oxide surfaces were in primary equilibrium with the potential-determining ions (H+ and OH-). The oxides acquire a zero surface charge at the z.p.c. (pHs 5.6 for SnO2 and 5.3 for TiO2 in 0.001 M KNO3) through the formation of neutral aquo complexes on the surface. The q+ values of cassiterite in NO3- solution and of rutile in NO3- and Cl- solutions are attributed to the chemisorption of H+ on the neutral surface forming aquo complexes of positive charge. These q+ values depend mainly on aH+ and the oxide electrodes under the above conditions will be reversible to H+. In Cl- solutions, in addition to H+ adsorption on SnO2, Cl- replaces the surface-OH groups and are specifically adsorbed on the surface. These q+ values depend on the chloride ion concentration as well as on αH+. The negative surface charge on oxides originates from acidic dissociation of the surface-OH groups and the q- values depend on aH+ as well as on the potassium ion concentration. © 1969.