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[OLD] Problems with substituting solid phase thermo data


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From: Davison V Vivit

Subject: Problems with Substituting Solid Phase Thermodynamic Data for Use with ACT2

Prior to running ACT2, I substituted the dataset for Cr2O3 with a dataset for amorphous Cr(OH)3. I encountered the following problems:

1) the program would not execute without a molar volume value

2) the program would not execute with a formula weight value identical to the value found in the dataset for aqueous Cr(OH)3

I plugged in an estimated value for the molar volume for amorphous Cr(OH)3 and a formula weight that was nearly but not exactly the same as for aqueous Cr(OH)3, and the program executed yielding a solubility diagram for the amorphous Cr(OH)3 solid phase with associated Cr(III) aqueous phase species at 25 C (see attachment).

Are these error observations correct? Is the molar volume needed to construct the diagram? How is the molar volume used in the computations? How does one arrive at a valid estimate of molar volume for a desired solid phase?


From: William F. McKenzie

Subject: Problems with Substituting Solid Phase Thermodynamic Data for Use with ACT2

WRT to log K = 500 = this is a flag that tells the program that there are not data at this temperature. WRT to molar volumes, you might ask yourself how the calculations are carried out (perhaps do one by hand for one of your boundaries in your activity diagram); and then you could answer your question.


From: Craig Bethke

Subject: Problems with Substituting Solid Phase Thermodynamic Data for Use with ACT2

You do not need to specify the molar volume of a mineral in the thermo dataset. If you do not know it, just enter a zero. This will prevent you from doing certain things in React, such as constraining a mineral's volume, calculating porosity, and so on. It will, however, have no effect on an Act2 calculation. Regarding the molar weight question, you should be able to enter the value for the Cr(OH)3 aqueous species. I tried this just now and the program ran without problem. Perhaps the reaction coefficients for the amorphous phase are not exactly correct? Finally, a log K of 500 indicates a lack of data. I think you will be less confused if you look at Appendix 3 of the GWB User's Guide, which discusses the thermo dataset and how to modify it.

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