The activity coefficients of elctrolytes with particular reference to aqueous mixtures of 2:2 With 1:1 electrolytes

Abstract

Müller’s method for the exact solution of the Poisson-Boltzm an equation (P.B.M.) has been used to produce tables for general application, for the potential, for the activity coefficients and for the osmotic coefficients of 2:2 electrolytes in aqueous solutions, both alone and in the presence of strong 1:1 electrolyte. The activity coefficients for 2:2 electrolytes were found to be equivalent to those derived by Bjerrum ’s method of ionic association up to values of s/a = z 2 e 2 / ϵkTa of about 9. Beyond this value the Poisson-Boltzmann-Müller values of the activity coefficients are significantly higher (about 10% at s/a = 11) and it is shown that the Bjerrum method is better in this range. The P.B.M. calculations show that in the presence of 1:1 electrolyte the Bjerrum treatment of ion association does not give a constant dissociation constant for a 2:2 electrolyte—even at infinite dilution of the latter—and this makes the Bjerrum method less useful when dealing with mixtures of 2:2 and 1:1 electrolytes. The P.B.M. method has been applied to the activity coefficients at CaSO 4 in NaCl solutions (up to 6 mol/1 strength) in the temperature range from 0 to 95 °C ( s/a from 7.85 to 9.05). The results shows very clearly the advantage of using the Kirkwood-Glueckauf approximation rather than the Debye-Hückel term (both in combination with the Müller extension term) when dealing with highly charged ions in solutions of high concentrations.