Suppose a solution containing Na' ions is passed through hydrogen resin. From the relative affinities given earlier, Na+ ions are attracted to the resin more strongly than H+ ions. Thus, Na+ ions will displace H+ ions from the resin or, in other words, NA ions and Fl+ ions exchange place between resin and solution. The process can be described by the following equilibrium reaction.

In most practical situations, a solution containing impurities at low concentrations is passed through a large amount of resin. By LeChatelier's Principle, the equilibrium of Reaction (4-1) is forced far to the right. The equilibrium is displaced so far that, for practical purposes, all Na' ions are removed from solution and replaced by H+ ions. As a result, the solution will be acidic because of the excess of H+ ions.

If a solution containing Cl^- ions is passed through hydroxyl resin, the Cl^- ions will be removed according to the following reaction.

Again, for a dilute solution and a large amount of resin, the removal of Cl^- ions is essentially 100 percent complete. In this case, the final solution will be basic because of the excess of OH^- ions.

Consider a situation in which the entering impurities are calcium sulfate (CaS0₄) and sodium chloride (NaCI), and the ion exchanger is a mixture of both hydrogen and hydroxyl resins (mixed-bed).

In the reaction with NaCl, both Na+ and Cl^- ions are removed from solution and replaced by H+ and OR ions, respectively (the CaS0₄ reaction has the same result). If the initial solution contained only NaCl, then the concentrations of Na+ and Cl^- ions were equal. Because both are removed with 100 percent efficiency, the concentrations of H+ and OH ions added to the solution are equal; thus, the solution is neutral. In solution, H+ and OH^- ions must obey the relationship for the ionization of water (refer to Module 1).

Because of the very small value of the dissociation constant K_w, the great majority of H+ and OR ions supplied by the resin must combine to form water by the following reaction.

By this process, the original NaCI solution becomes pure water.

The preceding examples involve hydrogen and hydroxyl resins. The use of other resins, especially cation resins, is very common. For instance, suppose a solution containing Na+ ions is passed through a lithium resin. Again referring to the relative affinities, Na+ is attracted to the resin more strongly than is Li+; thus, Na+ ions will displace Li+ from the resin.