Electrotransport of weak-acid anions through anion-exchange membranes

Concentration dependencies of the membrane electric conductivity under alternating current and fluxes of anions through anion-exchange membranes (AEMs) under direct current were measured in 0.002–1.0 M sodium salt solutions of carbonic and phosphoric acids. Several commercial AEMs were studied: AFN, AMX, ACS and ACM produced by Tokuyama Soda (Japan), Anion 204SZRA by Ionics (USA), ADP by Morgan (France) and MA-40, MA-41 by Schekino (Russia). Transport properties of these membranes are interpreted as functions of the internal solution pH. The mechanism of (hydro)carbonates transfer through AEMs is discussed.

The membrane transport properties strongly depend on the intrinsic solution pH, which increases with the external solution pH and dilution of the external solution. When the intrinsic pH is high, the amount of the free OH! ions in strongly basic membranes with quaternary ammonium groups may be great enough to give a noticeable contribution to the current transfer. This explains relatively high conductivity of such membranes in diluted solutions containing multicharged weak-acid anions. In weakly basic membranes with tertiary and secondary ammonium groups, an increase in the intrinsic pH causes a decrease in the ionization degree of the ionogenic groups; hence, a decrease in conductivity. From this follows a low conductivity of these membranes in solutions with high pH containing multi-charged weak-acid anions. Due to the fact that the internal membrane solution is enriched with multi-charged anions with an exclusion of H+ ions, a part of hydrocarbonate ions dissociates when entering the membrane. This explains the decrease in the diluted solution pH and increase in the concentrated, during current flow. Weak-acid anions can act a role of water splitting catalysators by participating in proton-transfer reactions. This mechanism of current transfer becomes essential when the current is close or higher that the limiting one.