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A counterion is the ion that accompanies an ionic species in order to maintain electric neutrality. In table salt the sodium cation is the counterion for the chlorine anion and vice versa.
In a charged transition metal complex, a simple (i.e. non-coordinated) ionic species accompanying the complex is termed the counterion.
Surfactants like cetyl trimethylammonium bromide form micelles in solution. The bromine counterions are retained around the micelles even in the presence of potassium chloride. Selective counterion condensation is observed with cetyl trimethylammonium chloride when the condensed chlorine ions are replaced by bromine on addition of potassium bromide.1
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.
A counter ion is a mobile ion,2 the presence of which allows the formation of an overall neutrally charged species. For example, in the (neutral) species NaCl the sodium ion is countered by the chloride ion and vice versa. In most situations in chemistry, an ion has a counter ion, exceptions being in ion beams, mass spectrometry and situations where electrons counter the charge of the ion (e.g. the Na/NH3 system, and plasmas).
Often the term is used to indicate an ion that is present in a solution but that is not the focus of the attention. For example when studying the electrochemistry of Co2+ one might dissolve the nitrate Co(NO3)2. The solution will then contain the counter ion NO3- that does not participate in the electrochemical reaction of interest. A very common counter ion is the perchlorate ion ClO4-. The reason for this is that perclorates are typically highly soluble materials.
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