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Thermodynamical screening


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Dear GWB community,

i am just getting started with GWB working in the field of geomicrobiology with a microbiology and marine biology background. I want to generate "fields" of gibbs free energy under varying boundary conditions for a specific redox reaction. This includes pH, Eh, temperature and different concentrations of chemical species and gases.

What i did so far:

I used SpecE8 to determine the chemical species in the aqueous solution. After that i used Rxn to define half-reactions for active redox species and calculated the gibbs free energy for each based on the activities calculated with SpecE8. 

Now comes the tricky part. How can i perform a screening e.g. starting with changing environmental Eh?

In terms of screening pH and concentrations of chemical species, do i have to change them in SpecE8 first and redefine activities in Rxn afterwards? This would create a huge amount of work, especially if several other concentrations would need to be altered as well.

Thanks in advance!  

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Hello Jan,
It sounds like you already worked out a pretty robust method for calculating Gibbs free energy for your reactions. Unfortunately, there is not a way to screen pH or temperature in Rxn. You can however, scan over a range of pH, temperature, and Eh using the sliding feature in React. For detailed examples, please see sections 3.4 Polythermal reaction paths and 3.6 Sliding activity and fugacity in the Reaction Modeling User Guide.
Using Gtplot , you can display the simulation results. You can easily change the axes to the variables you're interested in. For example, if you are screening from pH 5-8, you might want to configure your plot to display pH on the x-axis and species activity for the y-axis. Retrieve the numerical results by going to 'Edit' --> 'Copy as' --> 'Spreadsheet'. You can place these activities into your Rxn calculations to calculate Gibbs free energy. Alternatively, you can paste the species activities into a spreadsheet and calculate delta_G using Gibb's free energy equation. 
You can also setup your reaction in Phase2, if you would like to screen across two variables at once to produce a map of species activities. You can setup sliding paths along the x-axis(e.g. pH) and y-axis(e.g. Eh) in Phase2 and calculate species activities. Similar to the results in Gtplot above, you can copy the results from P2plot and paste it into excel for each species activity of interest to calculate delta_G. For more information on Phase2, please see sections 7 Using Phase2 and 8 Using P2plot in the Reaction Modeling User Guide. 
Hope this helps,
Jia Wang
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