Membranes containing crown ether were prepared. The membranes become positively charged by complexing alkali-metal ions. In these anionexchange membranes, not containing amino groups, pH changes caused by above-limiting currents were very small in contrast to the substantial acidbase generation by conventional A membranes. This is consistent with Simons' model. It is suggested that both suppression of acidification and the dynamic nature of the positive charges may help to avoid fouling.
Polarization in ED at low current densities has been described in detail and analyzed successfully on the basis of the concept of the Nernst unstirred layer [ 1, 21. It is however well known that the curves of current versus voltage do not approach a limiting value assymptoticahy, as expected from theory. The 'limiting' current appears only as a narrow flat region or as an inflexion point [ 31. At higher voltage the current rises again, and this abovelimiting current has been attributed to 'water-splitting', i.e. transfer of H' and m through cation-and anion-exchange membranes respectively, as a consequence of total salt-depletion in the unstirred layers. During the last years it was shown by a number of authors [3-6] that, in the cation exchange (C) membranes studied, even under conditions of severe polarization nearly all of the current was carried-by the salt cation. The significant pH changes observed so far appeared only at anion exchange (A) membranes.
At high, above-limiting, current densities a decrease in the coulomb efficiency of desalting, r), is generally observed. Forgacs et al. [2] have shown