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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
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
(NaCl, also known as sodium chloride) the
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable iso ...
ion (positively charged) is the counterion for the
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride salts ...
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 convent ...
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 and ...
, 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 residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s are associated with a variety of smaller anions and cations. In
plant cell
Plant cells are the cells present in green plants, photosynthetic eukaryotes of the kingdom Plantae. Their distinctive features include primary cell walls containing cellulose, hemicelluloses and pectin, the presence of plastids with the capabi ...
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 (L ...
is often accumulated in the
vacuole
A vacuole () is a membrane-bound organelle which is present in plant and fungal cells and some protist, animal, and bacterial cells. Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic mo ...
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 permeate (a fluid such as a liquid, gas, or vapor) through a solid. It is directly related to the concentration gradient of the permeate, a material's intrinsic ...
through hydrophobic cell walls is mediated by
ion transport channels.
Nucleic acid
Nucleic acids are biopolymers, macromolecules, essential to all known forms of life. They are composed of nucleotides, which are the monomers made of three components: a 5-carbon sugar, a phosphate group and a nitrogenous base. The two main cl ...
s are anionic, the corresponding cations are often protonated
polyamine
A polyamine is an organic compound having more than two amino groups. Alkyl polyamines occur naturally, but some are synthetic. Alkylpolyamines are colorless, hygroscopic, and water soluble. Near neutral pH, they exist as the ammonium derivatives. ...
s.
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 extend ...
s.
Ion-exchange resin
An ion-exchange resin or ion-exchange polymer is a resin or polymer that acts as a medium for ion exchange. It is an insoluble matrix (or support structure) normally in the form of small (0.25–1.43 mm radius) microbeads, usually white or ye ...
s 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 chemistry, a phase-transfer catalyst or PTC is a catalyst that facilitates the transition of a reactant from one phase into another phase where reaction occurs. Phase-transfer catalysis is a special form of heterogeneous catalysis. Ionic rea ...
. In a typical application lipophilic countercation such as
benzalkonium
Benzalkonium chloride (BZK, BKC, BAK, BAC), also known as alkyldimethylbenzylammonium chloride (ADBAC) and by the trade name Zephiran, is a type of Surfactant#Cationic head groups, cationic surfactant. It is an organic salt (chemistry), salt class ...
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 chemistry, quaternary ammonium cations, also known as quats, are positively charged polyatomic ions of the structure , R being an alkyl group or an aryl group. Unlike the ammonium ion () and the primary, secondary, or tertiary ammonium cations ...
s, called "quat salts".
File:Lithium-tetrakis(pentafluorophenyl)borate-2D-skeletal.png, Lithium tetrakis(pentafluorophenyl)borate
Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of the weakly coordinating anion (B(C6F5)4)−. Because of its weakly coordinating abilities, lithium tetrakis(pentafluorophenyl)borate makes it commercially valuable in the salt form ...
is the lithium salt of a highly lipophilic tetraarylborate anion, often referred to as a weakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
.
File:Tetraphenylborate.png, Tetraphenylborate
Tetraphenylborate (IUPAC name: Tetraphenylboranuide) is an organoboron anion consisting of a central boron atom with four phenyl groups. Salts of tetraphenylborate uncouple oxidative phosphorylation
Oxidative phosphorylation (UK , US ) or e ...
is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
File:PF6.png, Hexafluorophosphate
Hexafluorophosphate is an anion with chemical formula of . It is an octahedral species that imparts no color to its salts. is isoelectronic with sulfur hexafluoride, , and the hexafluorosilicate dianion, , and hexafluoroantimonate . In this anio ...
is a common weakly coordinating anion
Anions that interact weakly with cations are termed non-coordinating anions, although a more accurate term is weakly coordinating anion. Non-coordinating anions are useful in studying the reactivity of electrophilic cations. They are commonly found ...
.
File:Tetrafluoroborate-ion-3D-balls.png, As illustrated by the small counteranion tetrafluoroborate
Tetrafluoroborate is the anion . This tetrahedral species is isoelectronic with tetrafluoroberyllate (), tetrafluoromethane (CF4), and tetrafluoroammonium () and is valence isoelectronic with many stable and important species including the perchl ...
(), lipophilic cations tend to be symmetric and singly charged.
File:PPNCl.png, Bis(triphenylphosphine)iminium chloride
Bis(triphenylphosphine)iminium chloride is the chemical compound with the formula , often abbreviated , where Ph is phenyl , or even abbreviated PNl or NPl or PPNCl or PNPCl, where PPN or PNP stands for . This colorless salt is a source of the c ...
is the chloride salt of a bulky lipophilic phosphonium cation 3PNPPh3">h3PNPPh3sup>+.
File:PPh4Cl.png, Tetraphenylphosphonium chloride
Tetraphenylphosphonium chloride is the chemical compound with the formula (C6H5)4PCl, abbreviated Ph4PCl or PPh4Cl. Tetraphenylphosphonium and especially tetraphenylarsonium salts were formerly of interest in gravimetric analysis of perchlorate an ...
(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
Tetrabutylammonium is a quaternary ammonium cation with the formula (C4H9)4sup>+. It is used in the research laboratory to prepare lipophilic salts of inorganic anions. Relative to tetraethylammonium derivatives, tetrabutylammonium salts are more ...
, 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
In organic chemistry, crown ethers are cyclic chemical compounds that consist of a ring containing several ether groups (). The most common crown ethers are cyclic oligomers of ethylene oxide, the repeating unit being ethyleneoxy, i.e., . Impor ...
s 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
Ferrocenium tetrafluoroborate is an organometallic compound with the formula e(C5H5)2F4. This salt is composed of the cation e(C5H5)2sup>+ and the tetrafluoroborate anion (). The related hexafluorophosphate is also a popular reagent with simil ...
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 salt ...
. For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g.,
tetrabutylammonium hexafluorophosphate
Tetrabutylammonium hexafluorophosphate is a salt with the formula NBu4PF6. It is a white powder that is used as an electrolyte in nonaqueous electrochemistry. It is highly soluble in polar organic solvents such as acetone and acetonitrile.
The s ...
. 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 insulat ...
.
[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