Eh-Ph diagram for Antimony

[tam]

Hello,

 

I'm sorry; I have the feeling this is going to be a very basic question. I am trying to make an Eh-pH diagram for Sb using Act2, but I don't see a antimony in the diagram species. Please help.

 

Thank you!

[Brian Farrell]

Hi Tam,

 

The default database included with GWB, thermo.dat, does not include any Antimony species. If you know the logKs for the relevant species and minerals, or can find them in the literature, you can create your own database by adding these to thermo.dat (See GWB Reference Manual, Appendix: Thermo Datasets). Alternatively, you can use the thermo.com.v8.r6+.dat or thermo_minteq.dat datasets, which include Sb. Take a good look at both of these to see which, if any, contains the species you need over the temperature range of interest to you. Keep in mind that your modeling results will only be as good as the thermodynamic data upon which they are based.

 

Hope this helps,

Brian

[tam]

Thank you, Brian. The thermo_minteq.dat has all the Sb species and minerals I need, but when I try to use it in an Eh-pH diagram, I get a warning "Reaction for H2(g) is missing; can not plot water limit". Do I need to add H2(g) to the database? I would have thought it would have that already? Sorry for these beginner questions and thanks for your help!

[Brian Farrell]

Tam,

 

I'm not sure why H2(aq) and H2(g) are not in the Minteq database. Using GWB8 I can still make an Eh-pH diagram, though without H2(g) loaded the stability in the top right and bottom left corners continue to the edges of the diagram instead of stopping. Depending on your version of GWB, you may need to turn the water limit option off to even generate a plot.

 

You can certainly add H2(aq) and H2(g) to themo_minteq.dat (I would make a copy and call it something else). Be sure to follow the instructions in the Appendix to the Reference Manual. If you're going to paste the data from thermo.dat (0 - 300 C), keep in mind that the principal temperatures are different in thermo_minteq.dat (0 - 100 C). Using thermo.dat, you can go to Rxn and write the reaction for H2(aq) + 0.5 O2(aq) = H2O . Specify the temperature as one of the new principal temperatures (ie 12.5 C) and Rxn will spit out the logK value for the reaction at that temperature. Repeat this for every temperature you need and add it to your new file. If you follow the way the reactions are written in thermo.dat, you'll need a reaction for H2(aq) added to the redox species, and for H2(g) added to the gases.

 

Remember to add to the counters for redox species and gases, and you'll want to change O2 (g) to O2(g) so that Act2 can read the upper stability limit.

 

Everyone is a beginner at some point. Hope this helps,

Brian

[geominer]

I have the same error message: "Reaction for H2(g) is missing; can not plot water limit" when trying to use Act2. This happens on all cases I have tried, including the youtube example of selenium.

I am using the thermo_minteq database. But I cannot find any appendix referred to in the reply above. I don't see any Appendix at all in the Reference manual.

What is the recommended procedure for adding the reaction of H2(g) to theormo_minteq.dat produce the correct Eh-pH diagram?

 

(I am using 2021 version of GWB and I realize this thread is quite old. So the documentation may have moved somewhere else.)

[geominer]

I was able to follow the direction in Brian's response to add H2 to a copy of the minteq database. I am able to see the water lines now in pe-pH chart.

 

The one piece I left unresolved is k values for the gas entry, H2(g) = H2(aq), at some of the intermediate temperatures, e.g. 12.5 C, 60 C etc. because I was not sure what reaction would work in this case. But I only need 25 C so I am not too worried about it.

[Jia Wang]

Hello,

I am glad to hear that you were able to copy the reaction for H2(aq) and H2(g) to thermo_minteq. If you like, you can use Rxn to calculate the log K values for the intermediate temperatures, similar to how it is done for H2(aq). You can load thermo.tdat into Rxn and select H2(g) in the "balance reaction for" section. The program by default populates the "in terms of" section with the basis species that reacts to form H2(g). Since you want the log Ks for the reaction H2(g) = H2(aq), swap H2(aq) for O2(aq) to rebalance the reaction. Then enter the desired temperature and run the calculation. You can find the log K at your desired temperature in the Results pane.

Hope this helps,
Jia Wang
Aqueous Solutions LLC