Ion Exchange

If an ionic solution is brought into contact with a solid having ions that are only weakly joined in its crystalline structure, it is possible for ions from the solution to interchange with those of the same charge in the solid. Electrical neutrality is maintained throughout this exchange; that is, the total number of positive charges equals the total number of negative charges in the solid and the solution at all times. What changes is the type of ion that then resides with the solid and in the solution. A solid that has loosely bound sodium ions, when placed in a solution of potassium chloride, will interchange some of its sodium ions for potassium ions. The result is that the solid and the solution each have sodium and potassium ions in some ratio determined by the inherent capacity of the solid to undergo the exchange process. Equation 1 illustrates this interchange between cations initially attached to a solid interacting with cations initially in solution.

Water solution is indicated by (aq) for aqueous.

The exchange continues until the ratio of each cation in the solid and the solution remains constant. For this example:

Because the solid in equation 1 exchanges cations with the solution, it is termed a cation exchange solid. Other solids with exchangeable negatively charged ions are called anion exchangers.

There are several naturally occurring materials that function as ion exchangers. Many synthetic ion exchange materials are also available. Many of these synthetic materials are tailor-made to serve a specific purpose and be selective in the type of ions with which they exchange. Zeolites are a naturally occurring class of minerals containing aluminum, silicon, oxygen and a loosely held cation from group 1 or group 2 of the periodic table (e.g., sodium or magnesium). When placed in a solution of an ionic compound, exchange occurs between the loosely held zeolite cation and the dissolved cation in water. Various clay and soil materials also possess ion exchange capabilities. Most often an ion exchange reaction uses a synthetic ion exchange material specifically designed to achieve the desired separation.

Synthetic ion exchangers are composed of a charged group attached to a rigid structural framework. One end

of the charged group is permanently fixed to the frame while a positive or negative charged portion loosely held at the other end attracts other ions in solution. Common materials for these ion exchange resins are styrene and divinylbenzene. Molecules of these organic substances can join together forming a divinylbenzene polymer consisting of long rows of styrene crosslinked, that is attached, by divinylbenzene.

The extent to which divinylbenzene is crosslinked affects the ability of the resin to undergo ion exchange with an ion in solution. Resins that are only slightly crosslinked may have sufficient open space to allow solution ions to pass through and avoid contact with the fixed, exchangeable groups. Resins that are too highly crosslinked may not have openings big enough for solution ions to penetrate. This prevents them from contact with the fixed exchangeable groups. Table 1 lists various chemical groups that can be joined to the resin framework for attracting ions in solution.

Cation resins often are prepared in their hydrogen ion form. In this state exchange occurs when the resulting product in solution is the acid corresponding to the dissolved solid. An example of this type of exchange is shown in equation 3 where a strong cation resin in the hydrogen form interacts with a sodium chloride salt solution to yield the sodium form of the resin and hydrochloric acid.

A similar exchange between dissolved sodium chloride and a strong anion resin in the hydroxide (basic) form yields dissolved sodium hydroxide, a strong base.

Figure 1. Structure of a synthetic ion exchange resin. Illustration by Hans & Cassidy. Courtesy of Gale Group.

Complete exchange of solution ions (cation or anion)—that is, complete absorption on the resin—can occur if the sample solution is poured slowly through a packed column of resin material. This allows the sample to come into contact continually with fresh resin; the exchange occurs until none of the original exchangeable ions remains. These ions then can be collected by running another solution through the column, a solution that removes, or elutes, the absorbed ions from the resin.