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{{Short description|Ion which negates another oppositely-charged ion in an ionic molecule}}
], a ], is typically supplied with Na<sup>+</sup> as the counterion.]] ], a ], is typically supplied with {{chem2|Na+}} as the counterion.]]
⚫ A '''counterion''' (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ] that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium ] is the counterion for the chlorine ] and vice versa.


⚫ In ], a '''counterion''' (sometimes written as "'''counter ion'''", pronounced as such) is the ] that accompanies an ] in order to maintain ] neutrality. In ] (NaCl, also known as sodium chloride) the ] ion (positively charged) is the counterion for the ] ion (negatively charged) and vice versa.
⚫ <!-- not very helpful to typical reader: Counterions are generally vaguely defined in biological systems. Depending on their charge, proteins are associated with a variety of smaller anions and cations. In plant cells, the anion malate is often accumulated in the vacuole to decrease water potential and drive cell expansion. To maintain neutrality, K<sup>+</sup> ions are often accumulated as the counterion. Ion permeation through hydrophobic cell walls is mediated by ion transport channels. Nucleic acids are anionic, the corresponding cations are often protonated ]s.-->

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

⚫ In ], counterions are generally vaguely defined. Depending on their charge, ]s are associated with a variety of smaller anions and cations. In ]s, the anion ] is often accumulated in the ] to decrease water potential and drive cell expansion. To maintain neutrality, {{chem2|K+}} ions are often accumulated as the counterion. Ion ] through hydrophobic cell walls is mediated by ]. ]s are anionic, the corresponding cations are often protonated ]s.


==Interfacial chemistry== ==Interfacial chemistry==
Counterions are the ]s in ion exchange polymers and ]s.<ref>http://goldbook.iupac.org/C01371.html</ref> ]s are polymers with a net negative or positive charge. Cation exchange resins consist of an anionic polymer with countercations, typically ]. The resin has a higher affinity for highly charged countercations, e.g., by Ca<sup>2+</sup> in the case of ]. Complementarily, anion exchange resins are typically provided in the form of chloride, which is a highly mobile couteranion. Counterions are the mobile ions in ion exchange polymers and ]s.<ref>{{GoldBookRef |title=counter-ions |file=C01371 }}</ref> ]s are polymers with a net negative or positive charge. Cation-exchange resins consist of an anionic polymer with countercations, typically ] (sodium). The resin has a higher affinity for highly charged countercations, for example by Ca<sup>2+</sup> (calcium) in the case of ]. Correspondingly, anion-exchange resins are typically provided in the form of chloride ], which is a highly mobile counteranion.


Counterions are used in ]. In a typical application lipophilic countercation such as ] solubilizes reagents in organic solvents. Counterions are used in ]. In a typical application lipophilic countercation such as ] solubilizes reagents in organic solvents.


==Solution chemistry== ==Solution chemistry==
Solubility of salts in ]s 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 ]s, called "quat salts". 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 ]s, called "quat salts".


<gallery caption="Lipophilic counteranions" widths="180px" heights="120px" perrow="4"> <gallery caption="Lipophilic counteranions" widths="180px" heights="120px" perrow="4">
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File:Tetraphenylborate.png|] is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent. File:Tetraphenylborate.png|] is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
File:PF6.png|] is a common ]. File:PF6.png|] is a common ].
File:Tetrafluoroborate-ion-3D-balls.png|As illustrated by the small counteranion ] (BF<sub>4</sub><sup>−</sup>), lipophilic cations tend to be symmetric and singly charged. File:Tetrafluoroborate-ion-3D-balls.png|As illustrated by the small counteranion ] ({{chem|BF|4|-}}), lipophilic cations tend to be symmetric and singly charged.
</gallery> </gallery>


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==Electrochemistry== ==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 ]. For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g., ]. Even in such cases potentials are influenced by ], an effect that is accentuated in solvents of low ].<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> 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 ]. For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g., ]. Even in such cases potentials are influenced by ], an effect that is accentuated in solvents of low ].<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== ==Counterion stability==
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{{Reflist}} {{Reflist}}


] ]
] ]

Latest revision as of 19:26, 27 October 2022

Ion which negates another oppositely-charged ion in an ionic molecule
Polystyrene sulfonate, a cation-exchange resin, is typically supplied with Na as the counterion.

In chemistry, a counterion (sometimes written as "counter ion", pronounced as such) 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.

In biochemistry, counterions are generally vaguely defined. Depending on their charge, proteins are associated with a variety of smaller anions and cations. In plant cells, the anion malate is often accumulated in the vacuole to decrease water potential and drive cell expansion. To maintain neutrality, K ions are often accumulated as the counterion. Ion permeation through hydrophobic cell walls is mediated by ion transport channels. Nucleic acids are anionic, the corresponding cations are often protonated polyamines.

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 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.

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.

References

  1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "counter-ions". doi:10.1351/goldbook.C01371
  2. I. Krossing and I. Raabe (2004). "Noncoordinating Anions - Fact or Fiction? A Survey of Likely Candidates". Angewandte Chemie International Edition. 43 (16): 2066–2090. doi:10.1002/anie.200300620. PMID 15083452.
  3. 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
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