A theoretical treatment of cation exchangers - I. The prediction of equilibrium constants from osmotic data

Abstract

The theories put forward to explain ion-exchange phenomena in natural and synthetic materials are examined. It is suggested that a reasonable interpretation of ion exchange in swelling materials can be made by treating the exchanger phase as a concentrated aqueous electrolyte. It is assumed ( a ) that the free energy of the exchanger is a function of its volume and its composition only and ( b ) that the exchanger phase obeys the laws of concentrated electrolyte solutions so far as they are known. The water uptake at different vapour pressures of polystyrene sulphonates of different degrees of cross-linking has been investigated for various monovalent cations. The data indicate that the anions do not contribute towards the statistical entropy terms of the solutions, but otherwise the osmotic coefficients of the monovalent cations have similar trends as in the case of strong 1:1 electrolytes. An approximate calculation may be made of the distribution constants of the exchangers when in contact with solutions containing two cations. With a few exceptions the values obtained are in reasonable agreement with both the absolute values and the trends observed experimentally for different ions, different cross-linkage and different solution compositions.