Selectivity a practical example

The problem arose because the resin was stirred to equilibrium with the exchanging solution in a batch process, in which the displaced ions remained in contact with the resin. If the treatment is done by passing the solution down a column of the ion exchanger, the displaced chloride ions are carried away in the flowing solvent, so that equilibrium is never attained and the conversion proceeds to completion. Any ion can be completely displaced, even by an ion for which the resin is much less selective, if the conversion is done in a flowing system, because the displaced ions are removed from the contest. Conversion is complete when the concentration of the displaced ions in the column effluent falls to zero. 

Almost all conversion processes from one ionic form to another are much better done in a column than batchwise. The only exceptions are  

Neutralisation of a weakly or strongly acidic resin with an alkali. 

Conversion of a salt of a weakly acidic resin to the free acid form by treatment with a strong acid. 

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