Entering iron activity in ACT2

[Gregg W. Jones]

Hi Jia - last week you responded to my question about how to enter the iron concentration into REACT.  You said I could enter the total iron concentration from my sample analysis as Fe2+ (the default species) and as long as I specified the redox condition using Eh or O2(aq), the program would determine the concentration of the Fe2+ and Fe3+ species (at least I think that's what you said).  Now I want to take what you said about iron into making Eh/pH diagrams in ACT 2.   I want to diagram iron from my water sample in the presence of my other analytes. So I enter the analytes into a gss file, entering my total iron concentration as Fe2+ (the default species), then calculate with Spec8, then choose species activity, and I get the activities of all my analytes.   Next step is to open ACT2 and enter in the activity of Fe2+ in the presence of my other analytes.  What concerns me here is that I don't see how the redox condition is specified because even though I specify Eh in the "on axes" area, I'm not actually entering a specific Eh. Maybe the redox condition doesn't need to be specified because the program looks at the Fe2+ activity I entered and shows the minerals that would form under the full range of Eh and pH?     I've attached my ACT2 file  

Fe in presence of all 07.23.24.ac2

[Jia Wang]

Hello Gregg,

In React, you provide the program with a list of concentrations and constraints, and the software then makes a series of numerical calculations to determine the distribution of mass at true equilibrium. Act2, and also Tact, is much simpler than React. Act2 takes the given activities, calculates equilibrium equations for reactions, and assembles them into a diagram. You can click and hover over the boundaries in the activity diagram to see the reaction that each equilibrium equation represents. If you want to view all the equations used to construct the diagram, you can click on "View Results."

You can also pick a reaction from your diagram, for example, 2 Fe++  + 3 H2O  = Hematite  + 2 e-  + 6 H+ , and launch an Rxn instance to balance that reaction. You would balance the reaction for Fe2+, swap Hematite for Fe3+, swap e− for O2(aq), and set the unit as Eh. Set the unit for H+ as pH. Then, provide the activities for Fe2+ and H2O, hit Run, and expand the view in the Results pane to view the resulting equilibrium equation. This is the same equilibrium equation considered in Act2.

If you are creating other types of activity diagrams at a specific Eh, you can add the Eh to the "in terms of" section instead of setting it as a range on one of the axes.

For more information on Rxn and Act2, please see sections 4 and 5 of the GWB Essentials Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC