ION-EXCHANGE REACTION

any of a class of chemical reactions between two substances (each consists of positively and negatively charged species called ions) that involves an exchange of one or more ionic components. Such reactions are widely used for purposes of purification, concentration, and ion separation in industry and in the laboratory. During an ion exchange process, ions attached to or incorporated within the molecular structure of insoluble substances, called ion exchangers, exchange with ions present in a surrounding solution. The process has also been called base exchange and exchange adsorption. Ion exchangers are essentially insoluble acids or bases, the salts of which are also insoluble. Ion exchangers may undergo exchange either with positively charged ions (cations) or negatively charged ions (anions). In the former case the process is referred to as cation exchange and in the latter as anion exchange. Numerous substances, of both natural and synthetic origin, possess ion exchanging properties. These include proteins, synthetic resins, cellulose, bone, living cells, silicate minerals, and soils. For commercial and laboratory uses the inorganic ion exchangers include the natural zeolites, glauconite and greensand, and synthetic zeolites such as the sodium alumino-silicates. Inorganic ion exchangers have been prepared from insoluble salts, heteropoly acids, and hydrous oxides. These include compounds such as zirconium and titanium phosphates, zirconium and tin oxides, ammonium molybdophosphate, and many other insoluble compounds possessing ion exchanging properties. Inorganic ion exchangers are of special interest because they are more stable than organic ion exchangers under certain conditions, such as high temperature and ionizing radiation. The most important organic ion exchange reactions employ synthetic resins. The synthetic resins are plastics that may be chemically modified to impart or enhance their ion-exchanging properties. Ion exchangers can take a variety of geometric shapes, forms, and internal structures. Among the most important forms are sheets, or membranes, which are widely used for electrochemical purposes. The structure of the synthetic resins can be modified to enhance or impart desired properties. These include the chelating resins, into which are incorporated chemical groupings possessing a preferential affinity for a given ion or group of ions. Another type of ion-exchange material is paper impregnated with ion-exchanging substances for use in paper chromatography. any of a class of chemical reactions between two substances (each consisting of positively and negatively charged species called ions) that involves an exchange of one or more ionic components. Ions are atoms, or groups of atoms, that bear a positive or negative electric charge. In pairs or other multiples they make up the substance of many crystalline materials, including table salt. When such an ionic substance is dissolved in water, the ions are freedto a considerable extentfrom the restraints that hold them within the rigid array of the crystal, and they move about in the solution with relative freedom. Certain insoluble materials bearing positive or negative charges on their surfaces react with ionic solutions to remove various ions selectively, replacing them with ions of other kinds. Such processes are called ion-exchange reactions. They are used in a variety of ways to remove ions from solution and to separate ions of various kinds from one another. Such separations are widely utilized in the scientific laboratory to effect purifications and to aid in the analysis of unknown mixtures. Ion-exchange materials such as zeolites are also employed commercially to purify water (among other uses) and medically to serve as artificial kidneys and for other purposes. Additional reading Friedrich Helfferich, Ion Exchange (1962; originally published in German, 1959), is a standard early reference work on the theory of ion exchange. Robert Kunin, Ion Exchange Resins, 2nd ed. (1958, reprinted 1990), provides a short practical guide to ion-exchange resins and their use. William Rieman III and Harold F. Walton, Ion Exchange in Analytical Chemistry (1970), discusses the theory and types of ion exchangers. Further sources include M. Douglas Levan et al. (eds.), Adsorption and Ion Exchange: Fundamentals and Applications (1988); Lorenzo Liberti and John R. Millar (eds.), Fundamentals and Applications of Ion Exchange (1985); and A. Dyer, M.J. Hudson, and P.A. Williams (eds.), Ion Exchange Processes: Advances and Applications (1993). Ion Exchange and Solvent Extraction (irregular), contains current publications in these fields.Information on the increasingly important areas of ion chromatography and membrane separations can be found in Hamish Small, Ion Chromatography (1989); Robert E. Smith, Ion Chromatography Applications (1988); and D.S. Flett (ed.), Ion Exchange Membranes (1983). Details on separations of biological materials by ion exchange methods can be found in books on the separations of the biological components of intereste.g., Shuichi Yamamoto, Kazuhiro Nakanishi, and Ryuichi Matsuno, Ion-Exchange Chromatography of Proteins (1988). Harold F. Walton The Editors of the Encyclopdia Britannica