Ion Exchange Equilibrium Constant and Permselectivity between Two Ions

5 Ion Exchange Equilibrium Constant and Permselectivity between Two Ions 

When an ion exchange membrane is equilibrated with a mixed salt solution, the ratio of ions in the membrane phase attains an equilibrium, as for an ion exchange resin.20 The preference of the ion exchange membrane for one of the two counterions is expressed by a separation factor, a , in which the concentration of ions is expressed by equivalent, 

In the electrodialysis of a mixed salt solution, preferential permeation of specific ions is interesting in academic studies and important in practical application. The preferential permeation of ions is defined as the permselectivity between two ions or the transport number of A ions relative to B ions. Details are given in Chapter 5. 

6 Diffusion Coefficient of Electrolyte through Ion Exchange Membranes 

When an ion exchange membrane is used in electrodialysis, one side of the membrane contacts a dilute solution and the other a concentrated solution. There is a diffusion flux of electrolyte and non-electrolyte of low molecular weight through the membrane. Because the diffused amount directly affects current efficiency and the purity of products, the diffusion coefficient of electrolytes through membranes is important in practical applications. The following simplified method may be used to determine the coefficient after one side of the membrane has contacted a concentrated solution and the other side a dilute solution (in some cases, pure water) for a given period, the amount of electrolytes diffused through the membrane into the dilute solution is determined, and the diffusion coefficient is calculated by the Fick equation. 

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