Determination of the affinity coefficient of a cation exchange membrane with different ionic strengths

Though it is known that the ion-exchange polymers have a variable affinity for different counter-ions, this parameter is often supposed to be constant in the modelisation of membrane transfers. We have determined the affinity coefficient of three ion-exchange membranes (CM1, CM2 et Nafion(R) 117), d ifferent in natures and usually employed in industrial applications. The eq uilibrating solutions are equimolar mixtures of alkaline chlorides (KCl + N aCl, KCl + LiCl and NaCl + LiCl) and the ionic strength (I) has been varied from 0.1 to 1.5 mol l(-1). The obtained results show that (i) the affinity coefficient decreases at low ionic strength and tends asymptotically to th e unity for high values of I, (ii) this coefficient is function of the natu re and structure of the membrane, (iii) the variation of the inner pressure of the membrane is at the origin of the affinity coefficient variations. ( C) 2000 Elsevier Science Ltd. All rights reserved.