**1. Introduction**

#### **General characteristics of ion exchangers**

*Ion exchangers* are high molecular substances, most frequently solid, organic or inorganic, insoluble in water and many other solvents and capable of exchange of their own active ions into those coming from the surrounding electrolyte. From the chemical point of view, they are

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polyacids, polybases or both polyacids and polybases (polyampholyte, amphoteric ion exchangers). Those which exchange cations are called *cation exchangers* and those which exchange anions are called *anion exchangers*. Generally, those exchanging ions are called *ion exchangers*. Some ion exchangers prepared by modification of various types of substances, particularly natural ones, besides capability of exchanging ions exhibit distinct sorption properties [1–5]. The cation exchangers occurring most frequently possess functional groups such as -SO3H, -COOH and -OH, whereas the anion exchangers possess the primary, secon‐ dary and tertiary amine ones and quaternary ammonium ones, quaternary phosphate ones and tertiary sulfone ones. Selective (chelating) ion exchangers and those strongly basic and weakly basic of the polymerization type of the functional trimethylammonium (type 1) and dimethylhydroxyethylammonium (type 2) groups are most widely applied in exchange chromatography. Their affinity mostly depends on the structure, size and change of anion exchanger. Type of functional groups in ion exchangers is decisive about the character of exchange reaction and its applicability.

Besides the general division of ion exchange materials due to the material (organic and inorganic) the skeleton is built, there are many others whose classification is based on the methods of preparation, type of functional groups and skeleton structure. There is still another basis of ion exchangers division conditioned by historical development of this area, i.e. according to their origin – natural, semi-synthetic and synthetic [1–5].
