Computer Science
Scientific paper
Dec 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54.3265m&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 12, pp.3265-3282
Computer Science
3
Scientific paper
A model is presented which is used to calculate the effect of pressure on activity coefficients of aqueous solutes in the system Na-Ca-Cl-SO 4 -H 2 O to 200°C. Literature data for the density and compressibility of aqueous binary solutions of Na 2 SO 4 and CaCl 2 to 200°C are used to calculate the first and second pressure derivatives of Pitzer's ion interaction model parameters, as well as the standard molal compressibility and volume of these two salts. Empirical correlations between the apparent molal volume and compressibility of the aqueous electrolytes are used to guide the choice of the temperature dependent expressions used for the numerical representation of the derivatives of Pitzer's parameters with respect to pressure. For sodium sulfate solutions, such correlations are used to extrapolate compressibilities to 200°C. The change in the thermodynamic properties of the-CaSO 0 4 ion pair with pressure is taken into account by the variation of its dissociation constant. The volumetric properties (partial molal volumes and compressibilities) of multicomponent solutions in the Na-Ca-Cl-SO 4 -H 2 O system can be predicted from the information generated here and the volumetric equations of and (1982) for NaCl. This model is then combined with the high temperature model of (1988) of the same system in order to calculate activity coefficients at high pressures to 200°C. The resulting model is validated by comparing calculated and measured solubilities of anhydrite and gypsum in pure water and in NaCl solutions up to 6 M. The agreement between the calculated and measured solubilities of the calcium sulfates is typically better than 10% up to 200°C and 1 kbar. The relevance of temperature and pressure corrections to the activity coefficients of aqueous solutes is discussed in regard to the assumed accuracy with which geochemical models are able to calculate mineral solubilities.
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