Ion Exchange, in chemistry, method of exchanging ions in a solution with ions of the same charge in certain insoluble substances. By this means chemicals can be removed from a solution that contains large amounts of other chemicals. This is done by passing the solution through porous solid materials, usually minerals of the zeolite group or specially prepared synthetic resins (plastics) containing large, complex molecules. Certain ions in the solution replace ions or groups of ions in the resin or zeolite, from which they can then be eluted, or washed out. By controlling the acidity, strength, and composition of the solution and the nature of the resin, ions in solution are more or less selectively exchanged for the labile (exchangeable) ions that are in the resin.

Hardness in water, caused by calcium and magnesium ions, which form insoluble compounds, is removed by ion exchange. The water is filtered through an artificial zeolite, such as Permutite, and the sodium in the zeolite replaces the undesirable ions that are in the water. When the zeolite is saturated with these metallic ions, it is washed with salt solution, which restores the sodium.

Ion-exchange methods have been used to great advantage in the hitherto almost impossible separation of the various rare earth elements and in the separation of plutonium from uranium and fission products in research on nuclear fuels. Ion-exchange techniques contributed to the first identification and separation of the rare earth element promethium, as well as several of the transuranium elements. The element hafnium, used in the manufacture of tungsten filaments, has been separated from zirconium by ion-exchange methods more successfully than by any other chemical-separation methods.

Synthetic ion-exchange resins are employed extensively as filters and conditioners for automobile radiators and other cooling systems. They are also widely used in the pharmaceutical field as antacids for the treatment of peptic ulcers, as intestinal adsorbents for diarrhea control, and as sodium reduction agents in the treatment of migraine headache, heart disease, and edema. In other applications the resins are used for the removal of clot-producing calcium from whole blood in the preparation of blood plasma and for the isolation of antibiotics.