CONCENTRATION POLARIZATION IN ION-EXCHANGE MEMBRANE ELECTRODIALYSIS

The distribution of NaCl concentration in the boundary layer formed on the desalting and concentrating surfaces of an ion exchange membrane was estimated theoretically and compared with values obtained by the Schlieren diagonal method. In the desalting chamber, if the convective flow rate (the flow rate component perpendicular to the membrane surface) on the desalting surface of the membrane exceeds a critical value, the NaCl concentration distribution in the boundary layer becomes uniform and takes on the value for the bulk solution. However, if the convective flow rate drops even slightly below the critical value, the NaCl concentration in the boundary layer decreases considerably at the membrane/solution interface. In the concentrating chamber, the NaCl concentration in the boundary layer rises on the surface of a cation exchange membrane and falls on the surface of an anion exchange membrane. This is due to the diffusion constant of Cl- ions being larger than that of Na+ ions.