1. Any of a number of (usually organic) materials that are capable of exchanging the included ions with ions in a surrounding solution; used for deionizing water or for chromatography (technique of separating and analyzing the components of a mixture of liquids or gases) of organic molecules.
2. Synthetic organic substances of high molecular weight which are used to replace certain negative or positive ions that they encounter in solutions.
3. A polymeric resin that contains electrically charged fragments (fixed ions) permanently attached to the polymer backbone, electrical neutrality is achieved by attached mobile "counterions" in the solution phase the resin is immersed into.

A practical use of such resin is the removal of unwanted ions from a solution by replacing them with other ions; for example, a cation exchange resin containing fixed negative charges with attached mobile sodium ions can be used to remove "hardness" from water if the calcium and magnesium ions are more strongly attracted to the resin and therefore will replace the sodium ions.

Eventually all the sodium ions will go into solution and the ion-exchange process terminates; then, resin can be regenerated by soaking in a high concentration sodium salt solution.

Such a process can also be used to remove unwanted ions from polluted water streams.

More about ion-exchange resins

When immersed in a solution, the resins absorb the solution and swell; the degree of swelling is dependent on the polymeric structure and the total ion concentration of the solution.

Resins of suitable chemical compositions and physical properties may be synthesized at will for specific ion-exchange applications; thus, they comprise the bulk of synthetic ion-exchange materials used in the laboratory and industry.

In industrial and domestic applications, ion-exchange resins are used for the removal of calcium, magnesium, iron, and manganese salts from water (water softening), for purification of sugar, and for concentration of valuable elements; such as, gold, silver, and uranium from mineral ores.

In chemical analysis, ion-exchange resins are used for the separation or concentration of ionic substances, and in chemical synthesis, some ion-exchange resins have been used as effective catalysts, notably in esterification and hydrolysis reactions.

Types of ion-exchange resins

Two separate types of resins are commonly classed as ion-exchange resins, although their functions do not involve an interchange of ions.

These are the chelating resins and the electron-exchange resins.

• Chelating resins are styrene-divinylbenzene polymers to which iminodiacetate groups are introduced. This functional group forms complexes with all the metallic elements except the alkali metals, with stabilities that vary with the different metals; in analytical chemistry, they are used for the separation of trace amounts of metals.
• Electron-exchange resins accept or donate electrons to the surrounding solution and are used in oxidation-reduction reactions; examples include polymers prepared from hydroquinone, phenol, and formaldehyde.