Counterion
Encyclopedia
A counterion is the ion
that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.
In a charged
transition metal complex, a simple (i.e. non-coordinated) ionic species accompanying the complex is termed the counterion.
s like cetyl trimethylammonium bromide form micelle
s in solution. The bromine counterions are retained around the micelles even in the presence of potassium chloride. Selective counterion condensation
is observed with cetyl trimethylammonium chloride when the condensed chlorine ions are replaced by bromine on addition of potassium bromide.
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 is countered by the chloride
ion and vice versa. In most situations in chemistry, an ion has a counter ion, exceptions being in ion beam
s, mass spectrometry
and situations where electrons counter the charge of the ion (e.g. the Na/NH3 system
, and plasmas
).
ion ClO4-. The reason for this is that perchlorates are typically highly soluble materials.
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.
In a charged
Charge (physics)
In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...
transition metal complex, a simple (i.e. non-coordinated) ionic species accompanying the complex is termed the counterion.
Examples
SurfactantSurfactant
Surfactants are compounds that lower the surface tension of a liquid, the interfacial tension between two liquids, or that between a liquid and a solid...
s like cetyl trimethylammonium bromide form micelle
Micelle
A micelle is an aggregate of surfactant molecules dispersed in a liquid colloid. A typical micelle in aqueous solution forms an aggregate with the hydrophilic "head" regions in contact with surrounding solvent, sequestering the hydrophobic single tail regions in the micelle centre. This phase is...
s in solution. The bromine counterions are retained around the micelles even in the presence of potassium chloride. Selective counterion condensation
Counterion condensation
The counterion condensation phenomenon is commonly described by Manning’stheory , which assumes that counterions can condenseonto the polyions until the charged density between neighboring monomer charges...
is observed with cetyl trimethylammonium chloride when the condensed chlorine ions are replaced by bromine on addition of potassium bromide.
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.
Neutrally charged species
A counter ion is a mobile ion, the presence of which allows the formation of an overall neutrally charged species. For example, in the (neutral) species NaCl the sodiumSodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ion is countered by the chloride
Chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...
ion and vice versa. In most situations in chemistry, an ion has a counter ion, exceptions being in ion beam
Ion beam
An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing and other industries. A variety of ion beam sources exist, some derived from the mercury vapor thrusters developed by NASA in the 1960s.-Ion beam etching or sputtering:One type...
s, mass spectrometry
Mass spectrometry
Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of charged particles.It is used for determining masses of particles, for determining the elemental composition of a sample or molecule, and for elucidating the chemical structures of molecules, such as peptides and...
and situations where electrons counter the charge of the ion (e.g. the Na/NH3 system
Electride
An electride is an ionic compound in which an electron is the anion. The first electrides to be studied in depth were solutions of alkali metals in ammonia. When sodium metal dissolves in ammonia, the result is a blue solution consisting of [Na6]+ and solvated electrons. Such solutions are powerful...
, and plasmas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
).
Electrochemistry
Often the term is used to indicate an ion that is present in a solution but that is not the focus of the attention. For example when studying the electrochemistry of Co2+ one might dissolve the nitrate Co(NO3)2. The solution will then contain the counter ion NO3- that does not participate in the electrochemical reaction of interest. A very common counter ion is the perchloratePerchlorate
Perchlorates are the salts derived from perchloric acid . They occur both naturally and through manufacturing. They have been used as a medicine for more than 50 years to treat thyroid gland disorders. They are used extensively within the pyrotechnics industry, and ammonium perchlorate is also a...
ion ClO4-. The reason for this is that perchlorates are typically highly soluble materials.