In addition to the processes discussed so far, there are two more electro-membrane separation processes in which the driving force is not an externally applied electrical potential but a concentration gradient. The processes are referred to as diffusion dialysis and Donnan dialysis. Diffusion dialysis utilizes anion- or cation-exchange membranes only to separate acids and bases from mixtures with salts. Donnan dialysis can be used to exchange ions between two solutions separated by an ion-exchange membrane. Both processes have so far gained only limited practical relevance .
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