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 (\(\pu{I \lt 0.10 mol kg-1}\)) can be estimated by means of the Debye–Hückel equation: \[-\log_{10}\gamma_{\rm{B}}=\frac{z_{\rm{B}}^{2}\ A\ \sqrt{I}}{1 + \mathring{a}\ B\ \sqrt{I}}\] where \(I\) is ionic strength, \(z\) is the charge number of the ion, \(\mathring{a}\) is ion size parameter and \(A\) and \(B\) are temperature-dependent constants. According to the Bates–Guggenheim convention \(\mathring{a}\ B\) is taken to be \(1.5 \sqrt{\pu{mol kg-1}}\) at all temperatures and for all compositions of the solutions.