Ion exchange

Ion exchange is the reversible replacement of ions held by a solid material with ions from a surrounding solution. It occurs when the material’s charged sites retain some ions while releasing others.

Ion-exchange resin: An insoluble polymer bearing charged groups that reversibly bind ions from a solution. Its fixed charged groups provide the exchange sites in a common engineered form of ion exchange.
Water softening: Treatment that reduces water hardness by removing calcium and magnesium ions. Sodium-form cation exchangers replace hardness ions with sodium in a standard softening process.
Ion: An atom or molecule with a net electric charge from losing or gaining electrons. Ion exchange transfers charged particles rather than neutral atoms or molecules.
Adsorption: The accumulation of atoms, ions, or molecules at the surface of a solid or liquid. Adsorption can retain solutes without the charge-for-charge replacement that defines ion exchange.
Cation-exchange resin: An ion-exchange material whose fixed negative charges bind and exchange positively charged ions. It releases bound cations as other cations enter from the surrounding solution.
Ion-exchange chromatography: A separation method that resolves molecules according to their reversible interactions with charged stationary-phase groups. Differences in charge and binding strength let exchange media separate ions and charged molecules.
Ion-exchange membrane: A membrane containing fixed charged groups that preferentially permits counterions to pass. Its fixed charges and selective transport extend exchange behavior to membrane separations.
Absorption: The uptake of a substance into the bulk of another material. Absorption places material within a bulk phase rather than exchanging it at charged sites.
Anion-exchange resin: An ion-exchange material whose fixed positive charges bind and exchange negatively charged ions. Its positively charged sites retain anions while exchanging them with solution anions.
Demineralization: Removal of dissolved mineral ions from water or another liquid. Cation and anion exchangers can remove dissolved salts in sequence.

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