Ion Exchange

Ion exchange separates charged molecules from a solution by reversibly exchanging them with mobile, or 'active', ions associated with the oppositely charged stationary phase.

The stationary phase is usually a porous but insoluble solid, such as crosslinked organic polymers with high molecular weights that have a large number of monomers bearing ionic functional groups.

Anionic polymers with active cations are cation exchangers, whereas cationic polymers with active anions are anion exchangers.

The cationic and anionic exchangers are further classified as weak and strong based on their functional groups.

The mobile ions attached to the ion exchangers migrate into the solution and are replaced by the similarly charged ions from the solution until equilibrium is reached.

Applying the mass-action law to the ion-exchange equilibrium expresses the selectivity coefficient for the reaction.

Exhausted cationic and anionic exchangers are regenerated by washing them with an excess of acidic or basic solutions, respectively.