Determination of the diffusion coefficients of ions in cation-exchange membranes, supposed to be homogeneous, from the electrical membrane conductivity and the equilibrium quantity of absorbed electrolyte

Two ion-exchange membrane characteristics, the quantity of absorbed electro lyte and the electrical membrane conductivity have been measured and correl ated for three cation-exchange membranes (CM2, CMx and MK-40), two electrol ytes (KCl and LiCl), over a large concentration range of the solution (0.1 M less than or equal to C-0 less than or equal to 3.0 M). The membrane cond uctivity has been measured according to a French standard. However, because of the lack of standards fixing all the operating conditions for good dete rmination of the second parameter, we established an experimental protocol, validated by a statistical study and made a comparison with the theoretica l Glueckauf's equation. This method will be proposed to be standardized for this category of measurements. Relationships have been established between the two parameters allowing us to determine both the counter-ion and the c o-ion diffusion coefficients in a cation-exchange membrane, considered as h omogeneous. The values of these coefficients for the counter-ions are in go od agreement with those obtained from other methods. The variations of the computed diffusion coefficients of counter-ions and co-ions for the differe nt studied systems are discussed in terms of internal interactions with the charged polymer matrix and the other diffusing species. The discussion als o takes into account the free water content of the polymer. (C) 2001 Elsevi er Science Ltd. All rights reserved.