Determination of the affinity coefficient of a cation exchange membrane

Though it is known that the ion-exchange materials have a variable affinity for the different counter-ions, this parameter is currently neglected in t he modelisation of membrane transfer. We have determined this characteristi c for three electrolyte pairs (KCl + NaCl, KCI + LiCl and NaCl + LiCl) and for three cation exchange membranes (CM1, CM2 and Nafion 117). The ionic st rength of the equilibrating solution was maintained constant but the molar fraction varied. We remark that all the affinity coefficients are quite dif ferent from 1, so the phenomenon can not be neglected in the resolution of the transfer equations. This coefficient increases linearly with the extern al molar fraction of the most voluminous hydrated counter-ion. These coeffi cients are in favour of the less hydrated counter ion and decrease linearly with its molar fraction. (C) 1999 Elsevier Science Ltd. All rights reserve d.