Comparisons between calculated and
measured mean ionic activity coeficients at 25°C
Note that for an unsymmetric electrolyte
Xνx
Yνy
the mean activity coefficient is:
γ
±
ν
=
γ
X
νx
γ
Y
νy
(where ν =
νx +
νy).
For example, for CaCl2:
γ±3 =
γCa
γCl2,
or log γ± =
(log γCa +
2 log γCl) / 3.
Moderate concentrations
Measured mean activity coefficients (γ±) at
25°C compared with results obtained using the
Davies eqn.,
the simplified HKF model,
and the SIT model.
The three models reproduce fairly well
the data for concentrations ≤ 0.1 molal (ionic strengths
≤ 0.3 molal for 1:2 or 2:1 electrolytes);
for 1:1 electrolytes perhaps even up to ≈ 0.2 molal.
|
|
1:1 electrolytes: NaCl, HCl and AgNO3
|
1:2 and 2:1 electrolytes: CaCl2 and
Na2SO4
|
|
|
Higher Concentrations
Measured mean activity coefficients (γ±) at
25°C compared with results obtained using the
Davies eqn.,
the simplified HKF model,
and the SIT model:
|
|
1:1 electrolytes: NaCl, HCl and AgNO3
|
1:2 and 2:1 electrolytes: CaCl2 and
Na2SO4
|
Conclusions
The Davies eqn.
should only be used at concentrations ≤ 0.1 molal
for most electrolytes.
The HKF model
uses a specific electrolyte paramater (bγ,MX) to fit experimental
data over a wide range of temperatures and concentrations. In Spana
this model is simplified by setting
the b-parameter to the value for NaCl at the temperature of interest.
It is therefore not surprising that the model fits so well the NaCl. This
model should be adequate for most electrolytes at concentrations
≤ 0.2 molal over a wide range of temperatures.
The SIT model
uses specific ion-interaction paramaters (ε) chosen
to fit experimental data 25°C. It is therefore not
surprising that the SIT model fits the data so well. The temperature dependence
of the (ε) parameters is in general not known, and
the SIT model
should not be used at temperatures ≠ 25°C.
The experimental γ± data show
in the figures 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).
Models for activity coefficients
