Counterion

A counterion will be more commonly referred to as an anion or a cation, depending on whether it is negatively or positively charged.

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.

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.

[3] For many applications, the counterion simply provides charge and lipophilicity that allows manipulation of its partner ion.