Estimation of medium effects on equilibrium constants in moderate and high ionic strength solutions at elevated temperatures by using specific interaction theory (SIT): Interaction coefficients involving Cl, OH- and Ac-up to 200°C and 400 bars
Geochemical Transactions
Estimation of medium effects on equilibrium constants in moderate and high ionic strength solutions at elevated temperatures by using specific interaction theory (SIT): Interaction coefficients involving Cl, OH- and Ac- up to 200°C and 400 bars Yongliang Xiong*
0 Address: Sandia National Laboratories, Carlsbad Programs Group, 4100 National Parks Highway , Carlsbad, NM 88220 , USA
The above interaction coefficients are tested against both experimental mean activity coefficients and equilibrium quotients. Predicted mean activity coefficients are in satisfactory agreement with experimental data. Predicted equilibrium quotients are in very good agreement with experimental values. Based upon its relatively rapid attainment of equilibrium and the ease of determining magnesium concentrations, this study also proposes that the solubility of brucite can be used as a pH (pcH) buffer/sensor for experimental systems in NaCl solutions up to 200°C by employing the predicted solubility quotients of brucite in conjunction with the dissociation quotients of water and the first hydrolysis quotients of Mg2+, all in NaCl solutions.
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Introduction
Knowledge of medium effects on thermodynamics in
concentrated solutions is fundamentally important to the
thermodynamic modeling in many fields ranging from
experimental systems in aqueous solutions to
hydrothermal ore deposits of the natural systems. In two recent,
detailed reviews [1,2], several models which can handle
moderate to high ionic strength solutions are surveyed.
Those models surveyed include the Pitzer equations [3],
the Brønsted-Guggenheim-Scatchard specific interaction
theory (SIT) [4-6], the Bromley model [7], and the
Helgeson activity coefficient model [8]. In addition, although
not surveyed in the above two reviews, the commonly
used B dot equation [9] in geochemistry is valid to the
ionic strength of 1.0 m at most [10].
Because they have a large number of adjustable
parameters, the Pitzer equations are excellent in fitting the
experimental data in highly concentrated solutions as well as in
diluted solutions [11]. Therefore, the Pitzer equations can
accurately reproduce activity coefficients and other
thermodynamic properties at high ionic strength up to the
saturation of most salts.
The SIT model is most useful in the ionic strength range
up to 3.5–4.0 m [e.g, [12-15]], and successful applications
of the SIT model at 25°C in NaCl solutions up to the
saturation of halite have also been demonstrated [e.g., [16]].
The SIT model can be regarded as a simplified version of
the Pitzer formalism without consideration of triple
interactions and interactions between ions of the same charge
sign. Therefore, the Pitzer formalism is certainly superior
to the SIT model. The shortcoming of the SIT model is its
rather low accuracy in reproduction of mean activity
coefficients in comparison with Pitzer model [2]. However,
the error is usually less than 10% at ionic strength up to
6–10 m at 25°C [2].
The Bromley model is similar to the SIT model, but it
takes the concentration dependence of second virial
coefficients into consideration. Accordingly, the Bromley
model fits experimental data slightly better than the SIT
model does [2]. However, Wang et al. [2] also pointed out
that even though the Bromley model has a more
complicated analytical form than the SIT model, both the
Bromley and SIT models reproduce experimental data with
practically equal quality according to their extensive
evaluation.
As stated by Grenthe et al. [1] and Wang et al. [2], the
Helgeson activity coefficient model is actually a
oneparameter equation, and it has the same accuracy as that
of the SIT model. Nevertheless, the validity of the
assumptions of the Helgeson activity coefficient model is not
clear. Furthermore, the usage of different values of the ion
size parameters (aj) for different ions and electrolytes is
considered as an obvious drawback of the model, because
it creates difficulties in employing the model to mixtures
of electrolytes, and results in the violation of
cross-differential relations [1,2].
In investigations of systems where complex formation
takes place, a method of constant ionic medium is usually
adopted. As pointed out by Wang et al. [2], there are
difficulties in determination of activity coefficients of reaction
species in a constant ionic medium. Usually only a value
of equilibrium constant in a certain medium can be
determined, and the number of equilibrium constants
obtained is generally small. Second, the accuracy of
equilibrium constants is relatively low in comparison with
that of mean activity coefficients and osmotic coefficients.
Accordingly, owing to these two facts, it is sensible to use
an activity model with fewer parameters when dealing
with experimental equilibrium constants, as it is often
impractical to determine more than one or two empirical
parameters from a small number of such constants with
limited accuracy. The Pitzer equa (...truncated)