Other
Scientific paper
Mar 1990
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990gecoa..54..727o&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 54, Issue 3, pp.727-738
Other
17
Scientific paper
Calculation of accurate mineral solubilities in hydrothermal solutions requires dissociation constants for the predominant aqueous complexes in solution, many of which may be present in appreciable concentrations only at high temperatures. Consideration of dissociation constants representing the standard molal Gibbs free energy changes resulting from the electrostatic interaction of aqueous species indicates that triple ions such as NaCl 2 - and Na 2 Cl + may be major species in alkali metal halide solutions at high temperatures and low pressures where the dielectric constant of H 2 O is 15. Molal triple ion dissociation constants were computed for temperatures from 400° to 800°C at pressures from 500 to 4000 bars using equations given by and (1933b), together with values of the effective electrostatic radii of aqueous ions generated from algorithms developed by and (1988) and et al. (1990). The logarithms of the calculated second stepwise dissociation constants (log K 2 ) for 16 alkali metal and hydrogen halides typically decrease at 500 bars from ~0 at 400° to ~ -3 at 700°C, but they increase with increasing pressure from ~ -3 at 500 bars to ~0 at 3 kb and 700°C. Distribution of species calculations using these log K 2 values and dissociation constants for 1:1 ion pairs ( and , 1988), together with activity coefficients generated by adopting the revised HKF ( , and , 1981) equation of state ( and , 1988) to calculate extended term parameters in the Hückel equation ( et al., 1981) indicate that triple ions are the predominant species in low-pressure supercritical aqueous solutions at concentrations 0.8 m. Under these circumstances, polynuclear and mixed ligand complexes in natural electrolyte solutions such as KNaCl + , MgKCl 2 + , ZnNaCl 2 + , FeKCl 2 + , Na 2 ClSO 4 - , etc. may have a substantial effect on the hydrothermal solubilities of minerals at high temperatures and low pressures where many geochemical processes occur.
Helgeson Harold C.
Oelkers Eric H.
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