Selectivity of Transport of Sodium, Magnesium, and Calcium Ions through a Sulfo-Cationite Membrane in Mixtures of Solutions of Their Chlorides

Abstract

The transport numbers and the selectivity of transport of sodium, magnesium, and calcium ions through a heterogeneous sulfo-cationite membrane in solutions of sodium, magnesium, and calcium chlorides and their mixtures are studied at various current densities. Equations for calculating the selectivity of transport of the ions through the membrane are suggested for two extreme cases. One case corresponds to low currents, when one can assume that the membrane is at equilibrium with the outside solution. The selectivity of transport of the ions is defined in this case by their migration through the membrane. The other case occurs at currents that are close to a limiting value. Under these conditions the transport of the ions through the membrane is defined by their transport through a diffusion layer that forms near the membrane surface. Calculations with the aid of these equations satisfactorily conform to experiment. The deviation from calculation in the case of the magnesium cation is connected probably with its hydration being substantially greater as compared with sodium and calcium. It is established that the selectivity of transport of the ions through the membrane may be controlled to a certain extent by varying the current density.