The chemical potential or activity of ions cannot be determined on a purely thermodynamic basis. This is due to the fact that the effects of an ion cannot be separated from the effects of the accompanying counter-ion, or in other terms, the electrochemical potential of the ion cannot be separated into the chemical and the electrical component. Such a separation must necessarily be based on a non-thermodynamic convention. The present convention is based on the assumption that the molal activity coefficient of the chloride ion in dilute aqueous solutions () can be estimated by means of the Debye–Hückel equation: where is ionic strength, is the charge number of the ion, is ion size parameter and and are temperature-dependent constants. According to the Bates–Guggenheim convention is taken to be at all temperatures and for all compositions of the solutions.