Elemental Force on Reverse Permeation and Overshooting Phenomenon of Ions

Abstract

In a mixed electrolyte solution, the ionic flux through the membrane is driven not only by a concentration gradient, but also by an electric potential gradient. Therefore, reverse permeation of an ion can occur, which means permeation against the concentration gradient, and also overshooting can occur, which means permeation beyond the equilibrium concentration. We have already revealed these phenomena both theoretically and experimentally. In this paper we defined the electric potential gradient by each ion, (∂E/∂x), and the flux by a concentration gradient, J_IC, and the flux by an electric potential gradient by each ion, J_iEj, in order to reveal the driving force on the reverse permeation or on the overshooting. We analysed the results of permeation through the cellulose membrane on HCl-KCl-H₂O and HCl-CaCl₂-H₂O ternary systems. It was concluded that reverse permeation could occur when the ionic flux by electric potential gradients of coexisting ions was reverse in the direction and large enough, because the electric potential gradient of the ion which does reverse permeation was very small. And it was also concluded that overshooting could also occur when the ionic flux by coexisting ions was the same in the direction and large enough upon the concentration reaching the equilibrium value. We discussed the condition of overshooting precisely.