transition elements speciation in GSS file

[Stanislav]

I am a new user of GWB and I am not sure what chemical species I should choose while organizing the spreadsheet. My samples were measured on ICPOES and the results are total masses (mg/l) of each element, not masses of each ion. When I try to add an analyte, software doesn't allow me to choose e.g. Mn total and asks me to choose between Mn2+ and MnO4-  and I want to use GWB to determine speciation. I right-clicked on each analyte and I chosen "As - Mn" but the charge imbalance is very high (30%) so I think something is wrong with the spreadsheet. Thank you for the help.

[Jia Wang]

Hello,

It's difficult to say what might be the exact problem here, but here are some information on GSS that might help.

The basis species analyte available in GSS contains all primary basis species and redox species in the thermodynamic dataset loaded.

If you select a primary basis species for that element and provide the constraints for the oxidation state, such as O2(aq), Eh, or pe, then the program will figure distribution of mass from the total concentration you specified between valence states. Since your units are in elemental concentrations, then selecting the "as element" unit is right. For example, Mn++ is a basis species in thermo.tdat, the default thermo dataset. Adding this analyte to your spreadsheet and providing the constraints for oxidation will automatically prompt the speciation calculation to account for the distribution of mass to other oxidation states, MnO4- and MnO4--.

If you select a redox species to be added to the spreadsheet, the redox coupling reaction is disabled. 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.

You can open and view the thermo dataset loaded into your GSS spreadsheet by going to the File menu -> View -> and select the file that ends with the extension .tdat. Please see section 2.4 Redox couples in the GWB Essentials Guide to see how redox couples are specified in the thermodynamic dataset. I would also suggest looking at section 7.3 Redox disequilibrium for a detailed example on decoupling redox couples. Section 7.2 gives an example for how the software calculates the equilibrium state of a fluid. This is a good starting point for you to learning the software.

If you would like further help with troubleshooting, please attach your input file and any custom thermodynamic dataset so we can take a closer look.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[Stanislav]

My understanding of your answer is that no matter if I pick Fe3+ or Fe2+ in GSS the program computes Fe speciation correctly based on that Fe3+ OR Fe2+ concentration and other parameters (T, pH, Eh, O2). And it applies to other elements as well. Am I right? My analytical instrument does not provide information about speciation, only total concentration.

[Jia Wang]

Hello Stanislav,

If you want the program to calculate the distribution of bulk mass across the redox states then you should pick the basis species and provide the appropriate parameters for the oxidation state.

For example, in thermo.tdat, Fe++ is the basis species and the Fe+++ is a redox species. If you only have a bulk concentration for iron, you would select Fe++. If you also provide a parameter for the oxidation state (like O2(aq) or Eh), then when you run a speciation calculation, the total concentration is going to be distributed between Fe++ and Fe+++ species.

Using the above example, if you add Fe+++ to the spreadsheet, the program will only distribute mass to just Fe+++ species during speciation. The reason for this is because the equilibrium reaction between Fe++ and Fe+++ is disabled or as we call it "decoupled" throughout the user guides. This occurs when you add a redox species to the spreadsheet.

You can open the thermodynamic dataset in the TEdit application to view all the entries available in the basis and redox species sections. If you are in a GWB application, you can always launch the thermo dataset loaded by going to "File" -> "View" and select the file that ends with the extension .tdat.

If you already have your data entered in GSS, you can launch a sample to SpecE8 to troubleshoot more closely. If you have added any redox species to your spreadsheet, the redox reaction should be automatically disabled when you go to "Config"-> "Redox Couples...".

The oxidation set by the pe, Eh, or O2(aq) concentration is going to affect all redox couples mass distribution unless they have been disabled. So if you have measurements for the separate pools of mass for Fe++ and Fe+++ species, that oxidation state parameters won't affect that pair's mass distribution.

Section 7.3 Redox disequilibrium in the Essentials Guide walks through a detailed example on decoupling redox reactions. For information on launching samples from GSS, please see section 3.5 Launching SpecE8 and React of the same user guide.

Hope this helps,
Jia