Anion exchange is the exchange or replacement of one anion by another on the positively charged exchange complex. The exchange of ions of the same charge takes place between a solution (usually aqueous) and a solid (ion exchange resin) in contact with it. The type of exchange, whether cation or anion, is determined by functional groups added to the resin. Ion exchange resins are ideally suited to protein purification, antibody isolation and peptide fractionation. They are also used in a traditional inorganic ion exchange. Anion exchange resins have ammonium ions in the framework of the resin as the ion exchange site. Cation exchange resins have sulphonic or carboxylic acid groups as the exchange site.
An anion exchange resin is used in softening water. When water is passed through anion exchange resins, the electronegative ions present in water (like Cl-, SO42-) are held by the ammonium ions in the resin. The outgoing water is devoid of electronegative ions, and, if subsequently passed through a cation exchange resin, the water practically contains no ions and can be used in place of distilled water. Treating with moderately concentrated solution of caustic soda can regenerate the resin.
The reaction processes in resins occur widely in nature, especially in the absorption and retention of water-soluble fertilizers b soil. The order of adsorption of anions is H2PO4- ; SO42-; NO3-= Cl-.
