Some examples of practical multicolumn systems

Voogt [4], as long ago as 1959, used a three-column apparatus to separate anionics, soaps, cationics and nonionics. This system is still in use. The first column contained a strongly acidic cation exchanger as the free sulphonic acid. This retained cationics and converted all anions to the corresponding acids. The second column contained a strongly basic anion exchanger acetate, to retain all surface-active sulphates and sulphonates. The third column contained the same resin as the hydroxide, to retain the fatty acids. The effluent contained the nonionics. The sorbed surfactants could all be recovered by elution with hydrochloric acid. 

MacDonald et al. [3] used a system similar to Voogt s, except that the anion exchanger acetate was replaced by the chloride. They recovered cationics and strongly acidic anionics by elution with 1 M hydrochloric acid in 90% methanol, and soaps with 0.5 M potassium hydroxide in 80% methanol. They reported successful analyses of four commercial detergents. 

The same authors also described a simple method for determining the exchange capacity of columns, which consisted essentially of passing small measured amounts of a standard solution of a surfactant through the column, washing after each one, until the surfactant appeared in the effluent. 

In the multicolumn systems it is better to retain soaps and acid-labile compounds on an anion exchanger before the solution passes through a strongly acid resin, which may catalyse esterification of fatty acids with the alcohol in the solvent and hydrolysis of such materials as alkyl sulphates. 

Although ion exchange is a very powerful tool, there are some separations that are more conveniently done by liquid-liquid extraction, and for complex mixtures a combination of the two techniques may be the most effective approach. 

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