Ion Exchange

The Short History

When a salt is placed in aqueous solution, it no longer exists as a salt but as charged particles called ions. Sodium chloride (NaCl), for instance, becomes Na+ (a cation) and Cl- (an anion) and become associated with the oppositely charged ions of the water itself: H+ (cation) and OH- (anion). Mixed salts such as CaCl2 and NaNO3 dissociate not only into their respective ions associated with the water ions but form associations with ions from the other salt as well. Thus CaCl2 and NaNO3 exchange partners to form Na+ + Cl- and Ca++ + NO3- partnerships. This is known as “ion exchange”. However, since all four salts are highly soluble, separating them into new “salt” components is very difficult other than through complex steps of crystallization.

The ion exchange process has been around in nature since the beginning of time although not observed and quantified until the mid 1800s when sodium forms of naturally occurring zeolites were shown to convert (exchange) ammonium salts in solution to their sodium specie after passing through a bed of the zeolite (which retained the ammonium ions). This process was also shown to be reversible by passing a strong solution of sodium salt through the zeolite, thus stripping the ammonium ion and restoring the bed to its sodium form. It was also demonstrated that “hard” water containing calcium (Ca++) and magnesium (Mg++) ions could be made “soft” by exchanging the hard water ions for sodium. This gave birth to the “softening” process which was put into commercial application around the turn of the 20th century. Regeneration is with common salt.

Utilizing a cation exchanger to replace all cations in solution with hydrogen ion (H+) and an anion exchanger to replace all anions with hydroxide (OH-) was not realized until the development of the modern styrene/di-vinyl benzene resins in the mid 1940s. It was now possible to substitute H+ and OH- ions (H+ + OH- = water) for salt ions and effectively de-mineralize water which had only been possible through multi-step distillation before.

De-mineralization via ion exchange has become the work horse of the high purity water industry. Regeneration is with strong acid and caustic respectively.