Ion-exchange resin

An ion-exchange resin is an insoluble polymer bearing charged groups that exchange mobile counterions with ions in a surrounding solution. Its selectivity and capacity depend on the polymer structure and solution conditions.

Cation-exchange resin: A resin with negatively charged groups that exchanges positively charged ions. Its fixed negative groups retain cations while mobile counterions are exchanged.
Cross-linked polymer: A polymer whose chains are connected by covalent links into a network. Cross-links keep the resin insoluble while allowing its network to swell.
Water softening: The removal of calcium and magnesium ions from water, usually by ion exchange. Sodium-form cation resin replaces hardness ions with sodium.
Adsorption: The accumulation of molecules or ions at the surface of a solid or liquid. Adsorption captures species at surfaces, whereas ion exchange replaces resin counterions.
Anion-exchange resin: A resin with positively charged groups that exchanges negatively charged ions. Its fixed positive groups retain anions while mobile counterions are exchanged.
Polystyrene: A synthetic polymer made from styrene monomers. Many commercial resins use cross-linked polystyrene as their structural matrix.
Deionization: The removal of dissolved ions from water to produce low-conductivity water. Cation and anion resins remove dissolved salts in successive exchange steps.
Reverse osmosis: A membrane process that removes dissolved substances by pressure-driven water transport. Unlike ion exchange, reverse osmosis separates through a membrane rather than exchanging ions.
Ion-exchange capacity: The quantity of exchangeable ionic charge a material can hold per unit mass or volume. Capacity sets the maximum ionic load a resin can exchange.
Functional group: A group of atoms that gives a molecule characteristic chemical properties and reactivity. Ion-exchange sites are functional groups attached to the polymer matrix.

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