Ion exchange is a reversible chemical reaction in which dissolved ions are removed from solution and replaced with other ions of the same or similar electrical charge. The resin is not consumed as a chemical reactant; rather, it acts as a physical medium that facilitates ion exchange reactions.
The resin itself consists of organic polymers forming a hydrocarbon network. Distributed through this polymer matrix are ion exchange sites, where functional groups carrying either positively charged ions (cations) or negatively charged ions (anions) are permanently bound. These functional groups readily attract ions of the opposite charge.
Two families dominate industrial practice: cation resins carrying acidic functional groups such as sulfonic acid, and anion resins carrying basic groups such as quaternary ammonium. Selecting between them depends on which ion must be removed.