Ion exchange and membrane potentials in cellulose acetate membranes separating solutions of mixed electrolytes

Abstract

The properties of (almost) symmetrical cellulose acetate membranes are studied by measuring the electromotive force (EMF) of concentration cells with the membrane as separator. One or two 1:1 salts with a common anion (Cl −) were used in different concentrations on each side of the membrane and Ag AgCl were used as electrodes. The concentration(s) on one side of the membrane is fixed, whereas one concentration is varied on the other side. A theory is developed based upon ideal Nernst—Planck transport equations in the membrane, Donnan equilibrium at the membrane interfaces and Nernst equilibrium at electrodes. Furthermore, Henderson's continuous mixture hypothesis is used in the case of two different electrolytes. Comparing measurements and theory for pure salts (NaCl and KCl), the fixed charge of the membrane divided by the intrinsic binding constant for the salt can be evaluated. At low variable concentrations, however, considerable deviations from the one-electrolyte theory are observed. The reason is the competition of the H + ion from the autoprotolysis of water with the Na + or K + ion. The measured values of EMF are therefore highly sensitive to pH at low variable concentrations. The variations with pH (pH between 5 and 8) are well described by the theory assuming unaltered fixed charge and a binding constant for H + to the membrane which is around 50 times as great as the binding constant for Na + or K +. The H + ion is distributed almost evenly between aqueous phase and membrane phase, whereas K + and Na + are sterically excluded from the membrane. Comparison between theory and experiment further shows that half of the negative fixed charge is titrated at pH 4 and almost all has been titrated at pH 3. This indicates that the fixed charge in cellulose acetate membranes is caused by carboxylic acid groups (glucuronic acids) as has been reported also in earlier work. A certain asymmetry in the EMF measurements with pure NaCl was observed at high variable concentrations (up to 10 mV), when the membrane sides were changed. The asymmetry is ascribed to the evaporation asymmetry during preparation of the membrane. No asymmetry is observed at low or intermediate variable concentrations.