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In chemistry, a counterion (sometimes written as "counter ion", pronounced as such) is the ion that accompanies an Ionic compound, ionic species in order to maintain Electric charge, electric neutrality. In
table salt
In common usage, salt is a mineral composed primarily of sodium chloride (NaCl). When used in food, especially in granulated form, it is more formally called table salt. In the form of a natural crystalline mineral, salt is also known as ro ...
(NaCl, also known as sodium chloride) the
sodium
Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...
ion (positively charged) is the counterion for the
chloride
The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
ion (negatively charged) and vice versa.
A counterion will be more commonly referred to as an
anion
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
or a
cation
An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
, depending on whether it is negatively or positively charged. Thus, the counterion to an anion will be a cation, and vice versa.
In
biochemistry
Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
, counterions are generally vaguely defined. Depending on their charge,
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...
s are associated with a variety of smaller anions and cations. In
plant cell
Plant cells are the cells present in Viridiplantae, green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids ...
s, the anion
malate
Malic acid is an organic compound with the molecular formula . It is a dicarboxylic acid that is made by all living organisms, contributes to the sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms ( ...
is often accumulated in the
vacuole
A vacuole () is a membrane-bound organelle which is present in Plant cell, plant and Fungus, fungal Cell (biology), cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water ...
to decrease water potential and drive cell expansion. To maintain neutrality, ions are often accumulated as the counterion. Ion
permeation
In physics and engineering, permeation (also called imbuing) is the penetration of a wikt:permeate#English, permeate (a fluid such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, ...
through hydrophobic cell walls is mediated by
ion transport channels.
Nucleic acid
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
s are anionic, the corresponding cations are often protonated
polyamines.
Interfacial chemistry
Counterions are the mobile ions in ion exchange polymers and
colloid
A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...
s.
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
2+ (calcium) in the case of
water softening
Water softening is the removal of calcium, magnesium, and certain other metal cations in hard water. The resulting soft water requires less soap for the same cleaning effort, as soap is not wasted bonding with calcium ions. Soft water also extend ...
. 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 cation
In organic chemistry, quaternary ammonium cations, also known as quats, are positively-charged polyatomic ions of the structure , where R is an alkyl group, an aryl group or organyl group. Unlike the ammonium ion () and the primary, secondary, ...
s, called "quat salts".
File:Lithium-tetrakis(pentafluorophenyl)borate-2D-skeletal.png, Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of a highly lipophilic tetraarylborate anion, often referred to as a weakly coordinating anion.
File:Tetraphenylborate.png, Tetraphenylborate is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
File:PF6.png, Hexafluorophosphate is a common weakly coordinating anion.
File:Tetrafluoroborate-ion-3D-balls.png, As illustrated by the small counteranion tetrafluoroborate (), lipophilic cations tend to be symmetric and singly charged.
File:PPNCl.png, Bis(triphenylphosphine)iminium chloride is the chloride salt of a bulky lipophilic phosphonium cation 3PNPPh3">h3PNPPh3sup>+.
File:PPh4Cl.png, Tetraphenylphosphonium chloride (C6H5)4PCl, abbreviated Ph4PCl or PPh4Cl is the chloride of a symmetrical phosphonium cation that is often used in organometallic chemistry
Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
. The arsonium salt is also well known.
File:Tetrabutylammonium bromide.svg, The bromide salt of tetrabutylammonium, 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 ethers are common lipophilic countercations, as illustrated by 2">i(12-crown-4)2sup>+.
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
Potassium chloride (KCl, or potassium salt) is a metal halide salt composed of potassium and chlorine. It is odorless and has a white or colorless vitreous crystal appearance. The solid dissolves readily in water, and its solutions have a sa ...
. 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
The relative permittivity (in older texts, dielectric constant) is the permittivity of a material expressed as a ratio with the electric permittivity of a vacuum. A dielectric is an insulating material, and the dielectric constant of an insul ...
.
[Geiger, W. E., Barrière, F., "Organometallic Electrochemistry Based on Electrolytes Containing Weakly-Coordinating Fluoroarylborate Anions", Acc. Chem. Res. 2010, 43, 1030. ]
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
{{Reflist
Electrochemical concepts
Salts