Membrane potentials in charged membranes separating solutions of weak electrolytes

Abstract

The transport of weak electrolyte ions through a charged membrane has been studied. The membrane potential across cation- and anion-exchange membranes was measured for acetic acid and glycine methyl ester hydrochloride aqueous solutions. Membrane potentials were correlated to a transport theory using the Donnan equilibrium and the Nernst–Planck equation of ion flux by introducing the dissociation constant of a weak electrolyte in aqueous solution. The anion-to-cation mobility ratios in the membrane were determined via a non-linear regression method. In the case of acetic acid solutions in an anion-exchange membrane, the ratios were about 0.0001–0.001 times larger than those in water. On the other hand, they were about 10000 times larger than in water if a cation-exchange membrane was employed. Therefore, the ion mobilities of weak electrolytes are significantly affected by groups of fixed charge in the membrane. In the case of glycine methyl ester hydrochloride, the mobility ratio was the same as in water, which means that it behaves as a strong electrolyte. It is suggested that the transport phenomena of weak electrolyte ions in a charged membrane can be explained by the above-described transport theory.