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.