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, also known as sodium chloride) the sodium ion (positively charged) is the counterion for the chloride ion (negatively charged) and vice versa.
A counterion will be more commonly referred to as an anion or a cation, depending on whether it is negatively or positively charged. Thus, the counterion to an anion will be a cation, and vice versa.
Interfacial chemistry.
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 Ca2+ (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 couteranion.
Counterions are used in phase-transfer catalysis. In a typical application lipophilic countercation such as benzalkonium solubilizes reagents in organic solvents.
Solution chemistry.
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".
Many cationic organometallic complexes are isolated with inert, noncoordinating counterions. Ferrocenium tetrafluoroborate is one such example.
Electrochemistry.
In order to achieve high ionic conductivity, electrochemical measurements are conducted in the presence of excess electrolyte. In water the electrolyte is often a simple salt such as potassium chloride. For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g., tetrabutylammonium hexafluorophosphate. Even in such cases potentials are influenced by ion-pairing, an effect that is accentuated in solvents of low dielectric constant.
Counterion stability.
For many applications, the counterion simply provides charge and lipophilicity that allows manipulation of its partner ion. The counterion is expected to be chemically inert. For counteranions, inertness is expressed in terms of low Lewis basicity. The counterions are ideally rugged and unreactive. For quaternary ammonium and phosphonium countercations, inertness is related to their resistance of degradation by strong bases and strong nucleophiles.