The @[email protected] 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 @[email protected] 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 @[email protected] activity @[email protected] of the chloride ion in dilute aqueous solutions (\(I < 0.10\ \text{mol kg}^{-1}\)) can be estimated by means of the @[email protected]–Hückel equation: \[-\log _{10}\gamma _{\text{B}}=\frac{z_{\text{B}}^{2}\ A\ \sqrt{I}}{1 + \mathring{a}\ B\ \sqrt{I}}\] where \(I\) is @[email protected], \(z\) is the @[email protected] of the ion, \(\mathring{a}\) is ion size parameter and \(A\) and \(B\) are temperature-dependent constants. According to the @[email protected] \(\mathring{a}\ B\) is taken to be \(1.5\ \sqrt{\text{mol kg}^{-1}}\) at all temperatures and for all compositions of the solutions.
See also:
PAC, 1984, 56, 567. (Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Recommendations 1983)) on page 569 [Terms] [Paper]