Debye–Hückel equation

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 (I < 0.10 mol kg −1) can be estimated by means of the Debye–Hückel equation:
− log 10 γ B = z B 2 A I 1 + å B I
where I is ionic strength, z is the charge number of the ion, å is ion size parameter and A and B are temperature-dependent constants. According to the Bates–Guggenheim conventionå B is taken to be 1.5 mol kg −1 at all temperatures and for all compositions of the solutions.
See also: pH
PAC, 1984, 56, 567 (Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Recommendations 1983)) on page 569