The practical activity coefficient of electrolyte B is given by \[\gamma _{\pm }=\frac{a_{\pm }}{\sqrt{\nu _{\text{+}}^{\nu _{\text{+}}}\ \nu _{\text{-}}^{\nu _{\text{-}}}}\ \frac{m_{\text{B}}}{m^{\text{+}}}}\] where a± is the mean activity of B in solution, m B is the molality of B, m + = 1 mol kg-1, ν + is the number of cations and ν- the number of anions in the chosen group B which is taken as the electrolyte. \[\nu = \nu _{\text{+}}+\nu _{\text{-}}\]