Counterion
A counterion (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium cation is the counterion for the chlorine anion and vice versa.
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+. The resin has a higher affinity for highly charged countercations, e.g., by Ca2+ in the case of water softening. Complementarily, anion exchange resins are typically provided in the form of chloride, which is a highly mobile couteranion.
Counterions are used in phase-transfer catalysis. In a typical application lipophilic countercation such as benzalkonium solubilizes reagents in organic solvents.
Solubility of salts in organic solvents is a function of both the cation and the anion. The solubility of cations in organic solvents can be enhanced when the anion is lipophilic. Similarly, the solubility of anions in organic solvents is enhanced with lipophilic cations. The most common lipophilic cations are quaternary ammonium cations, called "quat salts".
Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of a highly lipophilic tetraarylborate anion, often referred to as a weakly coordinating anion.
Tetraphenylborate is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
Hexafluorophosphate is a common weakly coordinating anion.
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