Effect of membrane constitution on the direction of ionic transport acrossbipolar membranes

Countertransport of ions is defined as the transport of an ion against its own concentration gradient driven by the third driving electrolyte added to the systems. The simulations and the experiments presented show that in th e systems that consist of NaCl, KCl, and CaCl7 solutions divided by a bipol ar membrane, there are two types of the countertransport of K+ and Ca2+ ion s: forward countertransport (the countertransport in the direction of the c oncentration gradient of the driving electrolyte, NaCl) and backward counte rtransport (the countertransport in the direction opposite to the concentra tion gradient). In the system where the negatively charged layer of the mem brane contacts the solution at the high-concentration side of NaCl, the for ward countertransport occurs, and the membrane has permselectivity for univ alent ions. In the system where the positively charged layer contacts the s olution at the high-concentration side, the mode of the countertransport de pends on the charge density ratio of the two charged layers.