FUGACITY AND ACTIVITY OF MOLECULAR SPECIES IN SUPERCRITICAL FLUIDS

Division of Geochemistry Department of Chemistry Arizona State University Tempe, Arizona 85281 

Fraser (ed.J, Thermodynamics in Geology, 161-181. All Rights Reserved, Copyright 1977 by D. Reidel Publishing Company, Dordrecht-Holland. 

This chapter deals exclusively with molecular fluids in the supercritical region. The term fluid is preferred over gas or vapor because for pressure-temperature conditions close to any reasonable geotherm, the density of mixtures of C02f H2O, etc. is much closer to a liquid than a gas. The critical temperature for those mixtures is usually not above about 400 C. Ionized species are neglected for two reasons the equations of state used cannot adequately describe ionic fluids and the degree of dissociation of aqueous electrolytes is small at temperatures above about 500 C. The methods presented in this chapter for the calculation of activity and fugacity coefficients are thus restricted to temperatures above about 500 C for most geologically important fluids. 

Relationships and abbreviations used in this chapter are given below. Derivations of the relationships can be found in standard textbooks on thermodynaunics. 

The partial molar free energy or chemical potential (y.) of a substance can be calculated at P and T from  

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Holloway J. R. (1977). Fugacity and activity of molecular species in supercritical fluids. In Thermodynamics in Geology, D. G. Fraser, ed. Reidel, Dordrecht. 

FUGACITY AND ACTIVITY OF MOLECULAR SPECIES IN SUPERCRITICAL FLUIDS... 

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