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I'm wondering if you could provide some information about the water activity calculation in Debye-Huckel/Bdot thermodynamic datasets.  How exactly are the coefficients in the header of the file derived and utilized?  I have scanned the reference manual and the Bethke (2010) book, but cannot find this information in either. Any relevant reference would be greatly appreciated.



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Hello Ben,

The water activity in the Bdot method is calculated using the mole number of water, stoichiometric or actual ionic strength, and osmotic coefficient. The equation to calculating the osmotic coefficient is given as power series in equation 8.8 in section 8.1.2 'B-dot model' of the Geochemical and Biogeochemical Reaction Modeling text. The water activity coefficients given in the thermodynamic dataset is used in this equation to calculate water activity. The text does not discuss the derivation of the values given but a good place to start might the papers and other text books referenced in this section.

Denbigh, K., 1971, The Principles of Chemical Equilibrium, 3rd ed. Cambridge University Press.

Pitzer, K. S. and L. Brewer, 1961, Revised edition of Thermodynamics, by G. N. Lewis and M. Randall, 2nd ed. MacGraw-Hill, New York.

Helgeson, H.C., 1969, Thermodynamics of hydrothermal systems at elevated temperatures and pressures. American Journal of Science 267, 729–804.

Hope this helps,
Jia Wang

Edited by Jia Wang
corrected reference paper citation
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