Salts are ion pairs of an anion with a cation. With electrodialysis, it is possible to transfer two salts MA and ma into two new salts Ma and mA, the ion pairs have changed.

As within a conventional electrodialysis unit, the solutions are separated by alternatively arranged anion and cation exchange membranes. The two kinds of compartments formed, are now driven as four compartments: Conc1, Dil1, Conc2, Dil2. The repeated unit into the cell now contains four units. Applying a current, cations within the first diluate (grey compartment set) move into a different concentrate compartment than the anions from this cell. And the cations in this cell, wherein the said anions arrive, are the cations from the second diluate compartment. These anions have changed their partner. Putting together all elementary processes, simply the salt pairs are changed.

An important point in running a salt metathesis process is the mass balance concerning the diluates. Application is very interesting in cases, a diluate stream is available with a high concentration and which has not to be completely desalted. This is the case e.g. for an fermentation, where the product should be removed by a certain extent. After ED-treatment the fermentation solution is recycled into the fermenter. The result is a continuos removal of product at a continuous high concentration level of the diluate. In such application, high current densities may be achieved.

Typical metathesis applications are conversions of Calcium salts of organic acids into their acidic form by metathesis with hydrochloric acid. Often, calcium salts are obtained in fermentation processes or by the tartaric stabilization of wine. If the calcium salts are poorly soluble, they may also be processed into the sodium salts by processing with hot sodium carbonate solution. In each case, a metal free acid of a relatively high concentration is obtained. In cases of soluble calcium salts, a calcium chloride solution is obtained, which may be recycled into the process of obtaining the calcium salt.