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 convention $å B$ is taken to be $1.5 mol kg −1$ at all temperatures and for all compositions of the solutions.