act2: EH-pH diagrams confusion

[Arne Z.]

Hello,

 

I’ve a question about the act2. I use the database of thermo.com.v8r6+. I tried to make an EH-pH diagram of copper (log a = -3) and sulfate (log a = -3) and this worked well.

But I am worried about the circumstance that

1)Cu++ is the “Diagram species” and SO4-- is “In the presence of” it makes a different EH-pH diagram, compared to the diagram if these two species are swapped to

2)SO4—is the “Diagram species” and Cu++ is “In the presence of”.

 

My question is why are these two diagrams are different from each other and why have the second diagram less phases then the first diagram.

For me it was also unclear from the manual what does it mean if a species is “Diagram species” and what does it mean if a species is “In the presence of”. I thought that in the two cases shown above both diagrams should be the same.

Furthermore, if the species Cu2+ in field of “In the presence of” are “speciate over x-y” in the second case it makes another diagram.

See attached the jpg’s of the input file and the out put file.

 

Greets

Arne

[Brian Farrell]

Hello Arne,

 

With the "diagram species" selection, you choose the main species for which you want information. An Eh-pH diagram of Cu++ show the most stable copper minerals and the predominance of copper species on Eh and pH axes. A similar diagram for SO4-- will consider only sulfur species on the same axes. The "in the presence of field" is used to modify the form of the main species by adding ligands with which the main species can react.

 

If you start with just a diagram of SO4--, the various species formed might include SO4-- and HSO4-, elemental sulfur, H2S(aq), HS-, and S--. By considering Cu++ in the calculation (SO4-- in the presence of Cu++), Brochantite (Cu4(SO4)(OH)6) and Chalcocite (Cu2S) become stable. If you start by diagramming Cu++ with SO4--, on the other hand, many more minerals appear stable on the diagram. Tenorite (CuO), Cuprite (Cu2O), and metallic Copper (Cu) appear in this Cu++ with SO4-- diagram, but not in the SO4-- with Cu++ diagram, since they do not contain any sulfur.

 

The "speciate over x-y" option is used to consider the speciation of the ligand ("in the presence of" species) on the axes chosen, Eh-pH here. For example, if SO4-- is chosen as the ligand, only the SO4-- ion is considered by default. By speciating over x-y, the SO4-- component can be present as SO4-- under oxidizing conditions and as H2S(aq) in reducing conditions, for example. For more information, see the Mosaic Diagrams section (5.3) in the GWB Essentials Guide.

 

Activity diagrams can be quite useful, but keep in mind that they are simplified representations of system chemistry. For a more complete analysis, you might use SpecE8 or React. These will show the predominance of all species at equilibrium, not just the most stable phases of the main species considered. Please let me know if this does not make sense.

 

Regards,

 

Brian Farrell

Aqueous Solutions LLC