ion exchange

the exchange of ions in electrolytic solutions (homogeneous ion exchange). When dilute electrolytic solutions, such as NaCl and KNO₃, are mixed, Na⁺, K⁺, NO₃⁻, and Cl⁻ ions are present in the mixture. The state of equilibrium for this case is expressed by the equation NaCl + KNO₃ ⇄ NaNO₃ + KCl (double-exchange reaction). If one of the substances produced during the interaction is less dissociated than the others, the equilibrium shifts toward the formation of the poorly dissociated substance. The equilibrium also shifts toward the formation of a volatile or poorly soluble product (if it is precipitated from the particular phase) according to the reactions

H₂SO₄ + 2KCN ⇄ K₂SO₄ + 2HCN ↑

NaCl + AgNO₃ ⇄ NaNO₃ + AgCl↓ I

The crystallization of a salt (ion combination), which has lower solubility, is the first process to take place upon evaporation of an equilibrium solution. The selectivity of crystallization may also be caused by the addition of organic solvents, such as alcohol, acetone, or dioxane.

During heterogeneous ion exchange (ion-exchange sorption), exchange takes place between ions that are in solution and those that are on the surface of the solid phase (the ion exchanger). When an ion exchanger saturated with one ion (such as H⁺) comes into contact with a solution containing other ions (for example, Na⁺ and Ca²⁺), ion exchange occurs between the ion exchanger and the solution: the concentration of Na⁺ and Ca²⁺ in the solution is reduced, and an equivalent quantity of H⁺ ions appears.

Heterogeneous ion exchange takes place during sorption of certain minerals (aluminosilicates, metallic hydroxides, and zeolites) from electrolytic solutions in the cells and membranes of living organisms and in synthetic ion-exchange sorbents. This type of ion exchange is widely used for the desalinization of water used to feed high-pressure steam boilers, as well as in hydrometallurgy and the chemical and pharmaceutical industries.