Bulk solution activity correction

Activity and concentration (here expressed on the molality scale, i.e. mol/kg water or m in short) of any dissolved species differ from each other as soon as the ionic strength Im in solution exceeds a certain threshold. Thus, activity coefficients must be introduced. For very dilute systems (Im < 10-3m), Debye and Hückel developed a theory (Debye & Hückel, 1923). Later on, many (mostly empirical) extensions were proposed with the Davies equation (Davies, 1962) being the most popular one. Since an upper application limit of 0.5 m hampers its use for truly saline solutions,  more elaborate models come into play. Of which the most prominent ones are the Pitzer virial coefficient method (Pitzer, 1999) and the Brønstedt–Guggenheim–Scatchard approach (Brønsted, 1922,Guggenheim, 1935, Scatschard, 1936). The latter has been further developed to the specific ion interaction theory (SIT). For details see discussion and further references in e.g. Grenthe et al. (2013).

The SIT approach was adopted for the OECD/NEA Thermochemical Data Base (NEA TDB). The reason for choosing the SIT was the linear formulation of the model and the possibility to estimate ion interaction coefficients from various correlation laws. Moreover, because of its simplicity in comparison to e.g. the Pitzer virial method it promises a large potential for many real-world applications. The main purpose of the SIT within the NEA TDB is the extrapolation of equilibrium and formation constants to zero ionic strength. While the use of the SIT model had been hindered for a long time by a lack of implementation in commonly used geochemical speciation codes so far, the concept was introduced into the PHREEQC program (Parkhurst and Appelo, 2013). PHREEQC also provides an appropriate SIT database being an extension of the NEA compilation. Nevertheless, this database is still limited in scope – and also in quality due to excessive use of extrapolations and analogies, sometimes in a multi-step way. Thus it is strongly advised to critically evaluate SIT coefficients before application.

Within this study, the Davis approach was used for ionic strength correction so far. However, within the next years the SIT or the Pitzer approach will be implemented in the smart Kd-concept for saline solutions. Therefore, test cases will be calculated and both acitivity coefficient corrections compared. Once one approach will be chosen databases will be updated and extended in cooperation with other research projects (THEREDA, VESPA, Edukem, SEM2).

 

References:

Brønsted, J.N., Studies on solubility IV. The principle of the specific interaction of ions. J. Am. Chem. Soc. 44, pg. 877–898, (1922).

Debye, P.; Hückel, E., "Zur Theorie der Electrolyte". Phys. Z. 24, 185 p., (1923).

Ciavatta, L., The specific interaction theory in evaluating ionic equilibria, Ann. Chim. (Rome), 70, pg. 551-567, (1980).

Guggenheim, E. A., The specific thermodynamic properties of aqueous solutions of strong electrolytes, Philos. Mag., 19, pg. 588-643, (1935).

Grenthe, I., Plyasunov, V., Spahiu, K., Estimation of medium effects on thermodynamic data, Chapter IX, Modelling aquatic chemistry, OECD Publications, 724 p., (1997).

Parkhurst, D.L., and Appelo, C.A.J., Description of input and examples for PHREEQC version 3 - A computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations: U.S. Geological Survey Techniques and Methods, book 6, chap. A43, 497 p., (2013). Available only at http://pubs.usgs.gov/tm/06/a43.

Pitzer, K.S., Activity coefficients in electrolyte solutions (2nd ed.). C.R.C. Press. Chapter 3: Pitzer, K.S. Ion interaction approach: theory and data correlation, pg. 75–153, (1991) .

Scatchard, G., Concentrated solutions of strong electrolytes, Chem. Rev., 19, pg. 309-327, (1936).