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Ion exchange is the main process used to control purity and pH of boiler feed water. Ions are atoms or molecules that have split up in water and lost or gained an electron and have thus acquired an electrical charge. Ion exchange is a reversible chemical reaction, in which ions present in a solution are exchanged for a similarly charged ion attached to an immobile solid particle of ion exchange material. This process involves treating the feed water with an exchange resin. Because ions have positive or negative charges, there are different types of exchange resins. Cation resins exchange positively charged functional groups for undesirable positive ions. Anion resins exchange negatively charged functional groups for any undesirable negative ions. Ion exchangers are generally classified into two groups: Single bed ion exchangers and mixed bed ion exchangers.