The Specific Ion interaction Theory (SIT) was adopted by the
OECD-NEA thermodynamic database project, and described in its published reviews and books,
see list of references.
In the SIT the activity coefficient
of an ion i with electric charge Zi is given by:
|
log γi = |
−A Zi2
(√I /
(1+B √I
))
+ ∑ ε(i,j) mj
|
where I is the ionic strength;
A is the Debye-Hückel
slope (a solvent parameter); B is a temperature-dependent parameter:
t (°C) |
A |
B |
0 | 0.491 | 1.48 |
25 | 0.509 | 1.50 |
50 | 0.534 | 1.52 |
100 | 0.600 | 1.56 |
and ε(i,j) is the specific interaction parameters
between the ion i and another ion j of opposite charge sign.
The summation is taken over all ions j such that Zi×Zj > 0
(opposite electric charge sign). Often the ε-values are assumed to be independent of
the ionic strength. A fundamental principle of the SIT is that
ε(i,j) = ε(j,i),
that is, the specific ion interactions are symmetrical.
ε-values in SPANA (and
SED/PREDOM):
Default values are first assigned to all ions:
ε(MZ+,XY−)
= 0.15 + 0.15 (Z + Y).
For example:
ε(K+,MoO42−) = 0, and
ε(H+,OH−) = 0.15.
For interactions with Na+, Cl−
and ClO4− the
estimation method proposed by Hummel (2009) is used:
ε(Na+,XY−)
= 0.05 Y.
ε(MZ+,Cl−)
= −0.05 + (0.1 Z).
ε(MZ+,ClO4−)
= 0.2 Z.
For example,
ε(Sr2+,Cl−) = 0.15, and
ε(Na+,MoO42−) = −0.1.
ε-values
are finally searched in a file named:
SIT-coefficients.dta
this file is distributed and stored in the installation folder.
If you wish to add or change ε-coefficients you have to
create a copy of the file (with the same name) and store it either in your user directory
(system dependent, shown with the Help / About menu), or in the
working directory (where your input data file is located). Any
ε-values in the working directory will
superseede those in the user directory, and the data in the user directory will
superseede those in the installation directory. Any ε-values
in these files will superseede the default values.
The activity of water (H2O) is given by
|
log aH2O = |
− Φ ∑mk /
(ln(10) 55.508)
|
where k are all solute species in the aqeuous solution, and Φ
is the osmotic coefficient, calculated using the SIT as follows:
|
Φ = |
1 − (2/3) (ln(10)/∑mk)
A I3/2
σ(B √I)
+ (ln(10)/∑mk)
∑i ∑j
ε(i,j) mi mj
|
where i are cations or neutral species and j are anions,
and
|
σ(x) = |
(3/x3) ((1+x) − 1/(1+x) − 2 ln(1+x))
|
References
-
Grenthe, I., Fuger, J., Konings, R.J.M., Lemire, R.J., Muller, A.B.,
Nguyen-Trung, C., Wanner, H. (1992) Chemical Thermodynamics of Uranium, 715 p.
Amsterdam, The Netherlands: North-Holland, Elsevier Sci. Publ. B.V.
-
Grenthe, I., Plyasunov, A.V., Spahiu, K. (1997)
Estimations of medium effects on thermodynamic data. In: Modelling In Aquatic Chemistry
(I. Grenthe & I. Puigdomenech, eds.), pp. 325-426. Paris, France:
OECD Nuclear Energy Agency (NEA).
-
Rand, M.H., Fuger, J., Grenthe, I., Neck, V., Rai, D. (2008)
Chemical Thermodynamics of Thorium, 900 p. Paris, France: OECD Publishing.
-
Hummel, W. (2009) Ionic strength corrections and estimation of SIT ion interaction
coefficients, Report PSI-TM-44-09-01. Paul Scherrer Institut, Switzerland.
Back to activity coefficient models