Counterion

Polystyrene sulfonate, a cation exchange resin, is typically supplied with Na⁺ as the counterion.

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 (from gegen which means counter in German) 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.

Interfacial chemistry

Counterions are the mobile ions in ion exchange polymers and colloids.^[1]

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.

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".

• Lipophilic counteranions
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  Lithium tetrakis(pentafluorophenyl)borate is the lithium salt of a highly lipophilic tetraarylborate anion, often referred to as a weakly coordinating anion.^[1]

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  Tetraphenylborate is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.

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  Hexafluorophosphate is a common weakly coordinating anion.

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  As illustrated by the small counteranion tetrafluoroborate (BF₄⁻), lipophilic cations tend to be symmetric and singly charged.

1. ^ 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. 

• Lipophilic countercations
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  Bis(triphenylphosphine)iminium chloride is the chloride salt of a bulky lipophilic phosphonium cation [Ph₃PNPPh₃]⁺.

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  Tetraphenylphosphonium chloride (C₆H₅)₄PCl, abbreviated Ph₄PCl or PPh₄Cl is the chloride of a symmetrical phosphonium cation that is often used in organometallic chemistry. The arsonium salt is also well known.

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  The bromide salt of tetrabutylammonium, one of the most common counter cations. Many analogous "quat salts" are known.

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  Alkali metal cations bound by crown ethers are common lipophilic countercations, as illustrated by [Li(12-crown-4)₂]⁺.

Electrochemistry

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.^[2]

References