Bi-ionic chloride-nitrate equilibrium in a commercial ion-exchange membrane

Chloride-nitrate equilibrium was studied using the Neosepta ACS anion-exchange membrane. Ion-exchange isotherm and selectivity coefficient were performed on the basis of the Donnan equilibrium and Helfferich’s equilibrium relationship. Fixed ionic group concentration and water activity coefficients in the membrane were constant for any concentration of the nitrate and chloride ions in the solution. It was shown that the selectivity coefficient is more than unity and that the membrane has a good selectivity for nitrate ions.

We have shown that the activity coefficient of the counter-ions (chloride and nitrate) involved in our experiments was constant in the membrane. The activity coefficient of the sorbed water was also constant for any ionic concentration of the counter-ions in the sample. We have also given proof that the selectivity coefficient was more than unity in these conditions. Consequently, the ACS ion-exchange membrane behaves as a homogeneous membrane and presents good selectivity at equilibrium for nitrate ions.