The deionization process removes all ions from a solution. Cation exchangers remove all cations (positively charged ions such as Sodium, Calcium, and Magnesium) and anion exchanger remove all anions (negatively charged ions such as sulfate, bicarbonate, and chloride). The cation exchanger is operated in front of the anion exchanger and converts the salts to acids.

CaSO4 + 2R-H CaR2 + H2SO4 NaCl + R-H R-Na + HCl The anion exchanger removes the acid from solution: H2SO4 + 2R-OH R2SO4 + H2O HCl + R-OH RCl +H2O

The result is water without ions. Some Sodium will leak from the Cation exchanger and this is what determines the purity of the water leaving the system.