The following graphs show comparisons between calculated and
measured mean ionic activity coeficients at 25°C.
1:1 electrolytes:
measured γ± data at
25°C for NaCl compared with results obtained using both the Davies eqn.
and the eqns. by Helgeson et al.:
Note that the Davies eqn. should not be used for ionic strengths
> 0.1 M. It should also be noted that the paramaters for the eqns.
by Helgeson et al. were chosen to fit the NaCl data over
a wide range of temperatures and concentrations. It is not surprising that
the model fits the data so well.
1:2 and 2:1 electrolytes:
γ± data at
25°C for CaCl2 and
Na2SO4 compared with results obtained using both
the Davies eqn. and the eqns. by Helgeson et al.:
Note that the mean activity coefficients for an unsymmetric electrolyte
XνXYνY is:
γ±ν =
γXνX
γYνY
For a 1:2
(or 2:1) electrolyte,
for example CaCl2:
γ±3 =
γCa2+
γCl−2
and
log γ±(CaCl2) =
(log γCa2+
+ 2 log γCl−) / 3.
Higher Concentrations: the performance of these equations deteriorates
quickly above I ≈ 0.3M:
The experimental γ± data for NaCl, CaCl2 and
Na2SO4 may be found in the appendices of:
R.A. Robinson and R.H. Stokes (1970)
Electrolyte Solutions, 2nd edn., London: Butterworth
& Co., 571 pp.
(reprinted in 2002 by Dover Publications, Inc., USA).
Back