Eh pH Diagrams with limits of no species formation

[shensel]

Hello,

Given that boundaries drawn between species in Eh pH diagrams represent a 50/50 composition of the adjacent species, is it possible for GWB to create additional lines that show when a species is not favored to form at all, or some sort of other ratio of adjacent species such as 75/25?

 

Thanks,

Scott

[Jia Wang]

Hello Scott,

Are you referring to the diagram generated in Phase2 or Act2?

In Act2, the boundaries represent the equilibrium reaction between species in the adjacent fields. This cannot be altered in Act2.

In a Phase2 predominance diagram, the boundary denotes nodes in the diagram where the concentration in adjacent fields are equal. This cannot be altered. However, you can add a variable to create a 2D map on top of a Predominance map. For example, you can add a contour map of mineral saturation for a specific mineral or the concentration of a species. You can see an example of contours in section 7.9 Example: Mineral solubility of the GWB Reaction Modeling Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

[shensel]

Hi Jia,

I was referring to Act2, but I'm glad you mentioned Phase2. That is very useful information! I was able to create what I needed.

 

Thanks,
Scott

[Jia Wang]

You're welcome Scott. I want to follow up with some additional information about Act2 and Phase2.

In Act2, boundaries between aqueous species represent equal activity (or if stoichiometry differs, this is accounted for). In other words, they display the predominance of aqueous species. Aqueous species never “do not form at all”. Minerals fields show stability and outside their fields, they do not form. You can play around with equilibrium reaction equations in Rxn to consider species activity ratios besides the standard 50:50. Although the calculation is different, if you want more insight, you can overlay additional data on a P2plot diagram (e.g. using contours above), or make a speciation diagram in React (or cross-section in Phase2/P2plot).

To be precise, P2plot similarly accounts for stoichiometry as necessary when displaying species predominance. In a Predominance map, minerals are treated the same as aqueous species.

Best,
Jia

[shensel]

Hi Jia,

Thank you for the clarification, that is a much better description of the boundaries than what I described at first. However I am confused by your statement quoting my initial question that aqueous species never do not form at all. Could you please provide some further explanation there? I would expect certain aqueous species to not form at all under certain Eh pH conditions (example: the carbonate ion at pH of 1).

Thanks,
Scott

[Jia Wang]

Hi Scott,

You're welcome. It just means that the aqueous species will form in even very small quantity based on the mass action law, which is how the program calculates mass distribution. In your example of carbonate ion at low pH, the program will calculate an extremely small amount but CO3-- never do not form. You can try running a speciation calculation in React with your example and look at the various carbon species concentrations.

Best regards,
Jia

[shensel]

Ah yes that makes sense. Thank you!