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Found 1 result

  1. c.trivelpiece

    LLNL log K's

    Hello, I'm looking for some help reconciling (or understanding) log K values for aqueous species with values I'm calculating from various references (Solutions, Minerals, and Equilibria - Garrels and Christ; Chemistry of Glasses - A. Paul). (As a disclaimer - my formal training is in nuclear science and I have little academic training in geochemistry, environmental chemistry or physical chemistry - and I'm hoping I'm just missing something about how these values are calculated.) For instance, the value for Zr++++ is set to log K (25°C) = -0.2385 in the thermo database, and there are no other log K's listed at other temperatures for this species (I need to simulate everything at 90°C). However, the delG0f is given as -125.350 kcal/mol (524.46 kJ/mol) in the header. Elsewhere (e.g., in Paul and in Garrels & Christ) the value is 142.0 kcal. That discrepancy aside, if I use the log K value given at 25°C in the database to back calculate the delG0f from the equation log K = -delG0f/2.303*R*T, I get a value of 1.3615 kJ/mol, which is significantly different from the other values listed. I have noticed this with a few other aqueous species; also, when I'm trying to add new species not found in the database using values I calculate, I think I'm getting skewed results in terms of activity diagrams and species not showing up or having extremely low activities when we are thinking they should be playing a larger role in the systems I'm investigating. This is especially true in terms of species that I'm adding to the database for which I'm using reference values for delG0f which yield log Ks that are significantly higher than those for other preexisting species in the database. What am I missing about how these values are determined in the file and the discrepancies between the delG0f values from the literature? Is is "ok" to back calculate the delG0f from the log K given in the database and then use that same equation to calculate values for log K at different temperatures? Please let me know when you have a chance...like I said, I feel like I'm missing a fundamental piece of knowledge here. Any help is very much appreciated.
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