The @C01032@ 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 @E01945@ 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 @M03969@ activity @C01124@ of the chloride ion in dilute aqueous solutions (\(I < 0.10\ \text{mol kg}^{-1}\)) can be estimated by means of the @D01533@–Hückel equation: \[-\log _{10}\gamma _{\text{B}}=\frac{z_{\text{B}}^{2}\ A\ \sqrt{I}}{1 + \mathring{a}\ B\ \sqrt{I}}\] where \(I\) is @I03180@, \(z\) is the @C00993@ of the ion, \(\mathring{a}\) is ion size parameter and \(A\) and \(B\) are temperature-dependent constants. According to the @B00617@ \(\mathring{a}\ B\) is taken to be \(1.5\ \sqrt{\text{mol kg}^{-1}}\) at all temperatures and for all compositions of the solutions.