Demineralised water is free from all soluble mineral salts. It does not contain any anions or cations. Demineralised water is obtained by passing water successively through a cation exchange (in the H+ form) and an anion exchange (in the OH– form) resin.
During the cation exchange process, H+ exchanges for Na+, Mg2+, Ca2+, and other cations present in water.
In the anion exchange process, OH– exchanges for anions such as
OH– ions liberated in reaction (2) neutralize H+ ions liberated in reaction (1), thereby forming water.