Ion-selectivity of nickel chromate, molybdate, and tungstate precipitate membranes

The ion-selectivity of chromate, molybdate, and tungstate precipitate membranes of nickel has been examined by measurements of the membrane potential and polarization behavior. The membranes have been estimated to be cation-exchangers of low fixed charge densities from a comparison between experimental and theoretical membrane potentials according to the fixed charge theory in mono-monovalent electrolyte solutions. In electrolyte solutions of di- and tri-valent cations, the membranes became anion-selective. The order of cation selectivity of the membranes in chloride solutions increased as Al3+<Ba2+<Ca2+<=Mg2+<Li+<Na+<K+. The increasing order of co-ion transport through the membranes was SO42-<ClO4-<NO3-<Cl-. From the polarization behavior, it was estimated that Ni2+, CrO42-, MoO42-, and WO42- ions which constituted the membranes were much less permeable than Na+ and Cl- ions. Furthermore, it has been discussed that the surface precipitate film containing CrO42-, MoO42-, or WO42- acts as a protective film against corrosion of underlying metals because of its ion-selective property. (auth.)