Calc.  SED & PREDOM / Ionic Strength

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.:

log_gamma_1:1

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.:

log_gamma_1:2

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+ γCl2

and

log γ±(CaCl2) = (log γCa2+ + 2 log γCl) / 3.


Higher Concentrations: the performance of these equations deteriorates quickly above I ≈ 0.3M:

log_gamma_1-1 high conc.

log_gamma_1-2 high conc.


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).

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