Application of the Pitzer model for the estimation of activity coefficients of electrolytes in ion selective membranes

The Pitzer model can be applied to predict the mean activity coefficients o f salts in aqueous electrolyte solutions as a function of the molality. In this work it is shown that the mean activity coefficients for NaOH and NaCl solutions as predicted by the Pitzer model are in good agreement with expe rimental data from literature. Hereafter, the Pitzer model was extended to the description of the activity coefficients in ion selective membranes. Ex perimental sorption data of NaOH and NaCl obtained for sulfonic DuPont memb ranes with EWdry = 1.10 and 1.50, respectively, were used to determine the Pitzer parameters describing the interaction between the fixed charged grou ps of the membrane and the other ions dissolved in the membrane. In the app lication of the Pitzer model to the membrane, it was assumed that the fixed charged groups of the membrane can be considered as 'ordinary' ions evenly distributed in the aqueous solution inside the membrane. The results indic ate that the Pitzer model can be used to describe the experimental mean act ivity coefficients in sulfonic DuPont membranes, provided the molality of t he membrane, as used in the Pitzer relations, is multiplied by a constant r epresenting a shielding term. This shielding term, f(shielding), is a scali ng factor depending on the equivalent weight. (C) 1999 Elsevier Science B.V . All rights reserved.