Membrane Potential across a High Water Content Anion-Exchange Membrane Separating Two Solutions with a Common Counterion but Two Different Co-ions

A theory for bi-ionic potential, especially for bi-co-ionic potential, which is a potential difference across a membrane due to the difference in co-ions, is proposed. A high water content interpolymer membrane of poly(ethyleneimine) and poly(vinyl alcohol) was prepared as a low-charged anion-exchange membrane. The potential difference Δψ for the system of KCl/membrane/ 1 / 2 MgCl 2 or 1 / 2 CaCl 2 was measured by changing the external electrolyte concentrations c s and analyzed according to the theory. The bi-co-ionic potential Δψ increased with increasing c s at low concentrations, and after the potential Δψ reached a maximum value, the potential Δψ decreased with increasing c s at high concentrations; that is, a bell-shaped dependence of Δψ was observed against c s . This is because the low mobility of co-ions with charge number +2 in the charged membranes increases with increasing c s and finally reaches the mobility in the free electrolyte solutions.