Davies equation

The Davies equation is an empirical extension of Debye–Hückel theory which can be used to calculate activity coefficients of electrolyte solutions at relatively high concentrations. The equation, originally published in 1938,[1] was refined by fitting to experimental data. The final form of the equation gives the mean molal activity coefficient, $f_{\pm}$, of an electrolyte which dissociates into ions having charges z1 and z2 as a function of ionic strength, I.
$-\log f_{\pm} = 0.5 z_1z_2\left(\frac{\sqrt I}{1+\sqrt{I}}-0.15 I \right)$