Ionization of water:

In the reaction along with NaCl, Na⁺ and Cl^- ions both are removed from solution and replaced through H⁺ and OH^- ions, respectively (the CaSO₄ reaction has the similar result).  The concentrations of Na⁺ and Cl^- ions were equal if the initial solution contained just NaCl.  Since both are removed with 100 % efficiency, the concentrations of H⁺ and OH^- ions added to the solution are equivalent; therefore, the solution is neutral.  Within solution, H⁺ and OH^- ions must obey the relationship for the ionization of water.

K_w = 10^-14 = [H⁺] [OH^-]

Because of the extremely small value of the dissociation constant K_w, the great majority of H⁺ and OH^- ions supplied through the resin must merge to form water through the following reaction.

H⁺ + OH^- →H2O

Through this process, the original NaCl solution becomes pure water.

The preceding instance involves hydrogen and hydroxyl resins. The use of other resins, especially cation resins, is extremely general.  For instance, assume a solution holding Na⁺ ions is passed by a lithium resin. Again referring to the associative affinities, Na⁺ is attracted to the resin more strongly than is Li⁺; therefore, Na⁺ ions will displace Li⁺ from the resin.