Ion Exchange

Ion exchange separates charged molecules by reversibly exchanging them with mobile, active ions associated with the oppositely charged stationary phase. The stationary phase is a porous, insoluble crosslinked polymer with ionic functional groups. Mobile ions from the polymer migrate into the solution and are replaced by similarly charged ions until equilibrium is reached, similar to principles governing partition and distribution coefficients in extraction methods.

Cation exchangers contain anionic polymers with active cations that exchange positively charged ions from the solution. Anion exchangers contain cationic polymers with active anions that exchange negatively charged ions. Both types are classified as weak or strong based on their functional groups, such as sulfonic acid groups in strong acid cation exchangers or quaternary ammonium groups in strong anion exchangers.

Ion-exchange polymers are high-molecular-weight, chemically stable, and crosslinked to be porous and insoluble. The extent of crosslinking and the number of ion-exchange functional groups directly increase the ability to exchange ions. These properties allow the polymers to maintain structural integrity while providing numerous sites for reversible ion interactions.

Ion exchangers are classified as weak or strong based on their functional groups. Strong acid cation exchangers contain sulfonic acid groups, while weak acid cation exchangers contain carboxylic acid groups. Strong anion exchangers contain quaternary ammonium groups, whereas weak anion exchangers are functionalized with substituted amines.

The selectivity coefficient expresses the preference of an ion exchanger for one ion over another during exchange. It is derived by applying the mass-action law to the ion-exchange equilibrium, allowing prediction of which ions will be preferentially exchanged and retained by the stationary phase under specific conditions.

Exhausted cationic exchangers are regenerated by washing with an excess of acidic solution, which replaces bound ions with fresh active cations. Exhausted anionic exchangers are regenerated with an excess of basic solution to restore active anions. This regeneration process restores the exchanger's capacity for future separations.

Ion exchange can soften and deionize water by exchanging hardness ions and dissolved salts with active ions from the polymer. It is also used to purify solutions by selectively removing or concentrating charged species. These applications leverage the reversible nature of ion exchange and the selectivity of different exchanger types.