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.
Counterions are the mobile ions in ion exchange polymers and colloids. Ion exchange resins are polymers with a net negative or positive charge. Cation exchange resins consist of an anionic polymer with countercations, typically Na+. The resin has a higher affinity for highly charged countercations, e.g., by Ca2+ in the case of water softening. Complementarily, 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]+.
Many cationic organometallic complexes are isolated with inert, noncoordinating counterions. Ferrocenium tetrafluoroborate is one such example.
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. For measurements in nonaqueous solutions, salts composed of both lipophilic cations and anions are employed, e.g., tetrabutylammonium hexafluorophosphate. Even in such cases potentials are influenced by ion-pairing, an effect that is accentuated in solvents of low dielectric constant.
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.
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