Counterion
In
A counterion will be more commonly referred to as an
In
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. 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 Ca2+ (calcium) in the case of water softening. Correspondingly, anion-exchange resins are typically provided in the form of chloride Cl−, which is a highly mobile counteranion.
Counterions are used in
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".
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weakly coordinating anion.[2]
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Tetraphenylborate is less lipophilic than the perfluorinated derivative, but widely used as a precipitating agent.
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weakly coordinating anion.
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As illustrated by the small counteranion tetrafluoroborate (BF−
4), lipophilic cations tend to be symmetric and singly charged.
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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]+.
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
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
References
- PMID 15083452.