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There are two general types of ion exchange resins: those that exchange positive ions, called cation resins, and those that exchange negative ions, called anion resins. A cation is an ion with a positive charge. Common cations include Ca+2, Mg+2, Fe+2, and H+1. A cation resin is one that exchanges positive ions. An anion is an ion with a negative charge. Common anions include Cl-1, SO4-2, and OH-1 . An anion resin is one that exchanges negative ions. Chemically, both types are similar and belong to a group of compounds called polymers, which are extremely large molecules that are formed by the combination of many molecules of one or two compounds in a repeating structure that produces long chains. A demineralizer is a vessel, usually with a volume of several cubic feet, that contains the resin. A demineralizer may contain an intimate mixture of both cation and anion resin and is called a mixed bed. Two-bed demineralizers have two vessels, the first containing cation resin followed by a separate vessel containing anion resin.

Physically, ion exchange resins are formed in the shape of very small beads, called resin beads, with an average diameter of about 0.5 millimeters. Wet resin has the appearance of damp, transparent, amber sand and is insoluble in water, acids, and bases. The ratio is normally 2 parts cation resin to 3 parts anion resin.

Water softening

In this application, ion-exchange resins are used to replace the magnesium and calcium ions found in hard water with sodium ions.

When the resin is fresh, it contains sodium ions at it active sites. When in contact with a solution containing magnesium and calcium ions (but a low concentration of sodium ions), the magnesium and calcium ions preferentially migrate out of solution to the active sites on the resin, being replaced in solution by sodium ions. This process reaches equilibrium with a much lower concentration of magnesium and calcium ions in solution than was started with.

The resin can be recharged by washing it with a solution containing a high concentration of sodium ions (e.g. it has large amounts of common salt (NaCl) dissolved in it). The calcium and magnesium ions off the resin, being replaced by sodium ions from solution until a new equilibrium is reached.

This is the method of operation used in dishwashers that require the use of ‘dishwasher salt’. The salt is used to recharge an ion-exchange resin which itself is used to soften the water so that limescale deposits are not left on the cooking and eating utensils being washed.

Water Purification

In this application, ion-exchange resins are used to remove poisonous (e.g. copper and heavy metal (e.g. lead or cadmium) ions from solution, replacing them with more innocuous ions such as sodium and potassium.

Few ion-exchange resins remove chlorine or organic contaminants from water – this is usually done by using an activated charcoal filter mixed with the resin. There are some ion-exchange resins that do remove organic ions, such as MIEX (magnetic ion-exchange) resins. Domestic water purification resin is not usually recharged – the resin is discarded when it can no longer be used.

Production of high water purity water

Water of the highest purity is required for electronics, scientific experiments, production of superconductors, and nuclear industry, among others. Such water is produced using ion-exchange processes or combinations of membrane and ion-exchange methods. Cations are replaced with hydrogen ions using cation-exchange resins; anions are replaced with hydroxyl using anion-exchange resins. The hydrogen ions and hydroxyls recombine producing water molecules. Thus, no ions remain in the produced water. The purification process is usually performed in several steps with “mixed bed ion-exchange columns” at the end of the technological chain.