pH-Concentration Diagram

[MBOTZ]

I'm trying to recreate a pH-concentration diagram for Cu+ / CN- using Act2 with the Minteq database (I've also tried my own personal database but get the same results).  See the attached diagrams.  I can closely recreate this with a different software, but not with GWB.  It seems GWB is not speciating the Cu-CN species vertically, like the other two diagrams.  How can I fix this?  Another question: for a specific log a CN- value in Act2, does this refer to the total activity of all CN- in solution regardless of the species containing CN-, or is the activity only for the single CN- complex and the total amount of CN- is varied with the other species in solution so that the ion CN- has the specified activity?  Thank you!

[Jia Wang]

Hello,

It is difficult to say for sure what might be different here since I don't know how the other program or the diagram in the screenshot was created.

The applications Act2 and Tact are simple diagram generators. The species you specify in the axis variables represents the activity of that species and not a total concentration of the component. In Act2 (and Tact), the diagram is an assemblage of equilibrium equations. You can create a "mosaic diagram" in which species you specify in the "in presence of section" can speciate across the x and y-axes, but the species in the axes themselves do not speciate, unlike an actual multicomponent chemical model in SpecE8 or React. For more information regarding mosaic diagrams, please see section 5.3 in the GWB Essential User Guide.

If you are looking to create phase diagrams of more complex systems, you may find the Phase2 application useful. Unlike Act2 and Tact, Phase2 solves the distribution mass for multicomponent systems of varying conditions along the x and y axes. You can plot predominance diagrams that shows the most abundant species, mineral, or gas in regions.  For more information and examples using Phase2, please see section 7 in the GWB Reaction Modeling User Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC