# Counterion

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{{Short description|Ion which negates another oppositely-charged ion in an ionic molecule}}
[[image:Polystyrolsulfonat.svg|thumb|160px|right|[Polystyrene sulfonate](/source/Polystyrene_sulfonate), a [cation-exchange resin](/source/cation-exchange_resin), is typically supplied with {{chem2|Na+}} as the counterion.]]

In [chemistry](/source/chemistry), a '''counterion''' (sometimes written as "'''counter ion'''", pronounced as such) is the [ion](/source/ion) that accompanies an [ionic species](/source/Ionic_compound) in order to maintain [electric](/source/Electric_charge) neutrality. In [table salt](/source/table_salt) (NaCl, also known as sodium chloride) the [sodium](/source/sodium) ion (positively charged) is the counterion for the [chloride](/source/chloride) ion (negatively charged) and vice versa.

A counterion will be more commonly referred to as an [anion](/source/anion) or a [cation](/source/cation), depending on whether it is negatively or positively charged.  Thus, the counterion to an anion will be a cation, and vice versa.

In [biochemistry](/source/biochemistry), counterions are generally vaguely defined.  Depending on their charge, [protein](/source/protein)s are associated with a variety of smaller anions and cations.  In [plant cell](/source/plant_cell)s, the anion [malate](/source/malate) is often accumulated in the [vacuole](/source/vacuole) to decrease water potential and drive cell expansion. To maintain neutrality, {{chem2|K+}} ions are often accumulated as the counterion.  Ion [permeation](/source/permeation) through hydrophobic cell walls is mediated by [ion transport channels](/source/Ion_channel).  [Nucleic acid](/source/Nucleic_acid)s are anionic, the corresponding cations are often protonated [polyamine](/source/polyamine)s.

==Interfacial chemistry==
Counterions are the mobile ions in ion exchange polymers and [colloid](/source/colloid)s.<ref>{{GoldBookRef |title=counter-ions |file=C01371 }}</ref> [Ion-exchange resin](/source/Ion-exchange_resin)s are polymers with a net negative or positive charge. Cation-exchange resins consist of an anionic polymer with countercations, typically [Na<sup>+</sup>](/source/sodium) (sodium). The resin has a higher affinity for highly charged countercations, for example by Ca<sup>2+</sup> (calcium) in the case of [water softening](/source/water_softening). Correspondingly, anion-exchange resins are typically provided in the form of chloride [Cl<sup>−</sup>](/source/chlorine), which is a highly mobile counteranion.

Counterions are used in [phase-transfer catalysis](/source/phase-transfer_catalysis). In a typical application lipophilic countercation such as [benzalkonium](/source/Benzalkonium_chloride) 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 cation](/source/quaternary_ammonium_cation)s, called "quat salts".

<gallery caption="Lipophilic counteranions" widths="180px" heights="120px" perrow="4">
File:Lithium-tetrakis(pentafluorophenyl)borate-2D-skeletal.png|[Lithium tetrakis(pentafluorophenyl)borate](/source/Lithium_tetrakis(pentafluorophenyl)borate) is the lithium salt of a highly lipophilic tetraarylborate anion, often referred to as a [weakly coordinating anion](/source/weakly_coordinating_anion).<ref>{{cite journal | author = I. Krossing and I. Raabe | title = Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates | year = 2004 | journal = [Angewandte Chemie International Edition](/source/Angewandte_Chemie_International_Edition) | volume = 43 | issue = 16 | pages = 2066–2090 | doi = 10.1002/anie.200300620 | pmid = 15083452}}</ref> 
File:Tetraphenylborate.png|[Tetraphenylborate](/source/Tetraphenylborate) is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
File:PF6.png|[Hexafluorophosphate](/source/Hexafluorophosphate) is a common [weakly coordinating anion](/source/weakly_coordinating_anion).
File:Tetrafluoroborate-ion-3D-balls.png|As illustrated by the small counteranion [tetrafluoroborate](/source/tetrafluoroborate) ({{chem|BF|4|-}}), lipophilic cations tend to be symmetric and singly charged.
</gallery>

<gallery caption="Lipophilic countercations" widths="180px" heights="120px" perrow="4">
File:PPNCl.png|[Bis(triphenylphosphine)iminium chloride](/source/Bis(triphenylphosphine)iminium_chloride) is the chloride salt of a bulky lipophilic phosphonium cation [Ph<sub>3</sub>PNPPh<sub>3</sub>]<sup>+</sup>. 
File:PPh4Cl.png|[Tetraphenylphosphonium chloride](/source/Tetraphenylphosphonium_chloride) (C<sub>6</sub>H<sub>5</sub>)<sub>4</sub>PCl, abbreviated Ph<sub>4</sub>PCl or PPh<sub>4</sub>Cl is the chloride of a symmetrical phosphonium cation that is often used in [organometallic chemistry](/source/organometallic_chemistry).  The arsonium salt is also well known.
File:Tetrabutylammonium bromide.svg|The bromide salt of [tetrabutylammonium](/source/tetrabutylammonium_bromide), one of the most common counter cations.  Many analogous "quat salts" are known.
File:Bis(12-crown-4)lithium-cation-from-xtal-3D-balls-B.png|Alkali metal cations bound by [crown ether](/source/crown_ether)s are common lipophilic countercations, as illustrated by [Li(12-crown-4)<sub>2</sub>]<sup>+</sup>.
</gallery>

Many cationic organometallic complexes are isolated with inert, noncoordinating counterions.  [Ferrocenium tetrafluoroborate](/source/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](/source/potassium_chloride).  For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g., [tetrabutylammonium hexafluorophosphate](/source/tetrabutylammonium_hexafluorophosphate).  Even in such cases potentials are influenced by [ion-pairing](/source/ion-pairing), an effect that is accentuated in solvents of low [dielectric constant](/source/dielectric_constant).<ref>Geiger, W. E., Barrière, F., "Organometallic Electrochemistry Based on Electrolytes Containing Weakly-Coordinating Fluoroarylborate Anions", Acc. Chem. Res. 2010, 43, 1030. {{doi|10.1021/ar1000023}}</ref>

==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](/source/Lewis_base).  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.

==References==
{{Reflist}}

Category:Electrochemical concepts
Category:Salts

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Adapted from the Wikipedia article [Counterion](https://en.wikipedia.org/wiki/Counterion) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Counterion?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.