 As the concentration decreases so the second term becomes less important until, at very low concentrations,the Debye-Hückel equation gives a satisfactory account of the activity coefficient.Leroy A. Bromley observed that experimental values of formula_3 were often approximately proportional to ionic strength.Accordingly, he developed the equation, for a salt of general formula formula_4
At 25 °C "Aγ" is equal to 0.511 and ρ is equal to one.