**3.1. Ion exchange column**

#### *3.1.1. Batch and column operation*

Ion exchanges are generally employed in the two different modes of operations: batch and column operation. The batchwise operation consists of contacting the whole of an electrolyte solution to be treated with a mass of exchanger and then separating the two phases by means of filtration, decantation, etc. It is quite obvious that for those exchange reactions that do not approach completion, a batchwise operation must be repeated many times before complete transformation is realized. The more unfavorable the equilibrium, the larger the number of stages are necessary for a given degree of exchange or separation.

The operation of an ion-exchange reaction by leaching the exchanger in a columnwise manner achieves most efficiency the same goal that would take many stepwise stages in a batch method. The column operation is essentially an elaborate multiple batchwise technique in which the uppermost portion of the column is constantly contacting fresh electrolyte whereas the lower portions contact the electrolyte not adsorbed by the upper exchanger. This procedure permits the exchanger bed to become fully exhausted at the top and then gradually downward. Ion exchange chromatography is a technique in which resolution of a mixture is achieved by virtue of differences in migration rates of the components in a column packed with an ion exchanger.
