Counterions are the mobile ions in ion exchange polymers and colloids. Ion-exchange resins are polymers with a net negative or positive charge. Cation-exchange resins consist of an anionic polymer with countercations, typically Na (sodium). The resin has a higher affinity for highly charged countercations, for example by Ca (calcium) in the case of water softening. Correspondingly, anion-exchange resins are typically provided in the form of chloride Cl, which is a highly mobile counteranion.
Counterions are used in phase-transfer catalysis. In a typical application lipophilic countercation such as benzalkonium solubilizes reagents in organic solvents.