A counterion (pronounced as two words, i.e. "counter" "ion", and sometimes written as two words) is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt (NaCl), the sodium cation is the counterion for the chlorine anion and vice versa. In a charged transition metal complex, a (i.e. non-coordinated) ion accompanying the complex is termed the counterion. In the older literature, the term gegenion is sometimes used.
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+ 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.
Ion exchange resins are polymers with a net negative or positive charge. Typically, cation exchange resins are supplied with sodium as the counterion. Because the resin is insoluble and imobile, the countercations are relatively easily exchanged, e.g., by Ca2+ in the case of water softening. Similarly, anion exchange resins are typically provided in the form of chloride, which is a highly mobile couteranion.
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".
Tetraphenylborate is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
As illustrated by the small counteranion tetrafluoroborate (BF4−), lipophilic cations tend to be symmetric and singly charged.
Bis(triphenylphosphine)iminium chloride is the chloride salt of a bulky lipophilic phosphonium cation [Ph3PNPPh3]+.
The bromide salt of tetrabutylammonium, one of the most common counter cations. Many analogous "quat salts" are known.
Alkali metal cations bound by crown ethers are common lipophilic countercations, as illustrated by [Li(12-crown-4)2]+.
Often the term is used to indicate an ion that is present in a solution but is not redox active. For example when studying the electrochemistry of ferrocenium (FeCp2+), the counteranion (typically BF4- or PF6-) has only a small effect on the redox potential. Redox potentials are however influenced by ion pairing, an effect that is accentuated in solvents of low dielectric constant.
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