Similarly, sodium ions migrating to the right permeate the cationic membranes but are stopped bythe anionic membranes. The overall result is increased salt concentration in alternating compartments while the other compartments are simultaneously depleted of salt. The drawing shown implies that the voltage potential drop caused by the electrical resistance of the apparatus takes place entirely across the ion exchange membrane. This is the case for a very well-stirred cell, in which the solutions in the compartments are completely turbulent. In a well-stirred cell the flux of ions across the membranes and hence the productivity of the electrodialysis system can be increased without limit by increasing the current across the stack. In practice, however, the resistance of the membrane is often small in proportion to the resistance of the water-filled compartments, particularly in the dilute compartment where the concentration of ions carrying the current is low. In this compartment the formation of ion-depleted regions next to the membrane places an additional limit on the current and hence the flux of ions through the membranes. Ion transport through this ion-depleted aqueous boundary layer generallycontrols electrodialysis system performance.
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