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Redox state of species in GSS


ept78
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Hello, 

I am new in GWB, so probably this should  be an easy question to answer for an expert. 

I am trying to enter a set of analysis in GSS, however, I am confused by the fact that for eg heavy metals i need to know the redox state boforehand. For example, my analysis contains  values for Fe and As (not for ferrous/ferric or As speciation). Can I select a "total" redox state if i dont know the speciations? I am also curious about the the option of eg inserting Fe+++ and then make "right click on analyte --> As --> select eg "Fe". Does it solve my problem?

Thank you in advance

Evans

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Hello Evans,

If you don't know the specific concentration for each valence state of your iron, then typically you should pick the valence mostly likely in abundance for your fluid sample to represent the thermodynamic component. In most cases for Fe, that would be the basis species Fe++ for an aqueous fluid. You can also enter a total iron concentration into the spreadsheet as a user defined analyte. Note that user defined analytes will not be used in calculations or simulation. For more information regarding user analytes, please see section 3.3.4 in the GWB Essentials User's Guide.

The "As..." designation does not refer to oxidation state but it is typically used to define the concentration of molecular units such as H2S when the analytical concentration is given as the elemental mass of sulfur. 

Best regards,
Jia Wang

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Hello Evans,

I just want to clarify my previous post and provide more information on redox couples in GSS. The selection of either a basis or redox species also specifies whether redox equilibrium or disequilibrium model is used for calculations in species distribution. If you select the basis species (e.g. Fe++ in thermo.tdat), the redox couple for Fe2+/Fe3+ is automatically enabled unless you manually disable it in Redox Couples dialog. If you include a constraint on the oxidation state, such as Eh, pe, or O2(aq), then the program will figure the distribution of mass from the total you specified accounting for equilibrium among the different oxidation states (e.g. Fe2+ and Fe3+). If you select a redox species to be added to the spreadsheet, the redox coupling reaction is disabled. So if you know the oxidation state of your element from an analytical measurement, then you can pick that redox species for the spreadsheet and disable the redox couple.

Hope this helps,
Jia Wang

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