Ion-exchangers of this heading are cross-linked polymers, generally in granular form, containing active ionic groups (usually sulphonic, carboxylic, phenolic or amino groups). These active ionic groups enable the polymers, when brought into contact with a solution of an electrolyte, to exchange one of their own types of ions for one of those (of the same sign, positive or negative) contained in the solution. These are used in water-softening, milk-softening, chromatography, for recovery of uranium from acid solutions and of streptomycin from broths and for various other industrial purposes.
The most common ion-exchangers are chemically modified styrene-divinylbenzene copolymers, acrylic polymers or phenolic resins.
This heading does not cover ion-exchange columns filled with ion-exchangers of this heading (heading 39.26).
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