A reversible chemical reaction between a solid (ion exchanger) and a fluid (usually a water solution) by means of which ions may be interchanged from one substance to another. The superficial physical structure of the solid is not affected. The customary procedure is to pass the fluid through a bed of the solid, which is granular and porous and has only a limited capacity for exchange. The process is essentially a batch type in which the ion exchanger, upon nearing depletion, is regenerated by inexpensive brines, carbonate solutions, etc. Ion exchange occurs extensively in soils. Ion exchange resins are synthetic resins containing active groups (usually sulfonic, carboxylic, phenol, or substituted amino groups) that give the resin the property of combining with or exchanging ions between the resin and a solution. Thus, a resin with active sulfonic groups can be converted to the sodium form and will then exchange its sodium ions with the calcium ions present in hard water. Some specific applications of ion exchange: water softening, milk softening (substitution of sodium ions for calcium ions in milk), removal of iron from wine (substitution of hydrogen ions), recovery of chromate from plating solutions uranium from acid solutions, streptomycin from broths, removal of formic acid from formaldehyde solutions, demineralization of sugar solutions, recovery of valuable metals from wastes, recovery of nicotine from tobacco-dryer gases, catalysis of reaction between butyl alcohol and fatty acids, recovery and separation of radioactive isotopes from atomic fission, chromatography, establishment of mass micro standards, in cigarette filters to remove polonium from smoke.See Zeolite.
