counterion

A counterion is an ion of opposite charge that accompanies a given ion or charged species to maintain electrical neutrality. In salts, acids, and ionic complexes, the counterion balances the charge of the countercharged species. If a compound contains a cation X+, the counterion is an anion Y−, and vice versa.

In solid ionic compounds, counterions are bound within the lattice to neutralize the positively or negatively charged framework. In solution, counterions are mobile and can migrate in response to an electric field. For example, in sodium chloride, Na+ and Cl− balance each other; in ammonium nitrate, NH4+ is paired with NO3−.

Counterions play a key role in solution chemistry and biophysics. They influence solubility, conductivity, and electrostatic screening. In polyelectrolytes such as DNA, phosphorylation-bearing backbones are neutralized by counterions (often Na+, K+, Mg2+), with multivalent ions capable of condensing on the chain and reducing repulsion. The concept of counterion condensation helps explain changes in persistence length and inter-molecular interactions.

In coordination chemistry and ion exchange, counterions can be external to the coordination sphere or part of the complete salt used to balance charge. In ionic liquids and electrolytes, the many ions present act as counterions to each other, maintaining charge neutrality. The behavior of counterions affects crystallization, phase behavior, and electrochemical properties.