A counter ion is an ion, 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.Template:Physical-chemistry-stub
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