Diffusion coefficients of ions through ion-exchange membranes for Donnan dialysis using ions of the same valence

The transfer of ions through an ion-exchange membrane during Donnan dialysis was studied theoretically and experimentally. By applying the Nernst-Planck equation to the flux of ions in the ion-exchange membrane, an equation was derived for Donnan dialysis in which feed ions and driving ions were of equal valence. The theoretically derived equation was fitted to the experimental values by adjusting diffusion coefficients. Thus, the diffusion coefficients of ions in the ion-exchange membrane which satisfied all the experimental data were determined. It was clearly shown that the ratio of the diffusion coefficient in the membrane to that in solution remained constant at 70 for a system of monovalent feed and monovalent driving ions, and remained constant at 175 for a system of bivalent feed and bivalent driving ions. The flux of ions in Donnan dialysis was found to be influenced by the concentrations of both feed ions and driving ions. Monovalent ions showed a larger flux than bivalent ions, suggesting that they were better as driving ions. The flux of ions was scarcely affected by the kinds of co-ions. It was necessary to pretreat the membrane with driving ions. (C) 1997 Elsevier Science Ltd.