Reduced metal not visible in my Pourbaix diagram

[Akhilesh]

Hi, I am trying to create Pourbaix diagram for Fe2+ I am reducing. The drawing doesn't show solid Fe even at the Eh value of -1. I tried going further down in terms of Eh (up to -10), it still doesn't show any Fe(s). 

Anyone has any inputs? 

Thanks in advance!

 

Fe combined.ac2

[Jia Wang]

Hello,

A few suggestions to help you start troubleshooting.

The software can only consider reactions that are present within the thermodynamic database loaded into the calculation. The script attached uses thermo.tdat, the default database used in the GWB and is a good quality dataset published by Lawrence Livermore National Lab but it doesn't contain the solid Fe(s) reaction. You can add in the desired reaction from literature or take a look in other databases that might contain Fe(s). If appropriate, you can copy over the reaction to thermo.tdat and save the new dataset under a new name or switch over to another dataset entirely.

You should also check the Log Ks for reactions in the database and see if they are appropriate to model conditions of the experiment that you are conducting. Reference information regarding each dataset installed with the GWB can be found when you open the dataset in TEdit and go to the Header pane. For more information on TEdit, the GWB thermodynamic and surface dataset editor, please see chapter 9 of the GWB Essentials User Guide.

GWB programs consider all reactions available in the thermodynamic dataset with the given composition of your system. In some cases, when performing a calculation, you will need to suppress more stable minerals so that meta-stable phases that are observed are allowed to appear. The suppress option is under Config -> Suppress. You might want to experiment with trial and error to see what reduced Fe species that are appearing that might be more stable but not necessarily expected in your calculations and suppress them.

Also note that the program by default denotes the water stability limits using dashed lines and truncates the diagram to show the region in which the water is stable. Water is not stable outside of these boundaries. You can turn off the truncation to extend the diagram over the full plot by going to Format -> Quick toggle and uncheck water stability limit.

Hope this helps,
Jia Wang
Aqueous Solutions LLC