Plotting reaction traces on Act2 solubility diagrams

[John C. Ayers]

I want to plot three reaction traces on an Act2 solubility diagram. When the fluid is saturated in plotted phases the reaction traces should plot on the solubility boundaries. However, the reaction traces plot at much lower activities because the React reaction path is plotting the activity of the aqueous species (Zr(OH)2++), while Act2 is plotting the fluid elemental composition (sum of the aqueous Zr species concentrations = molality of Zr). I want Act2 to plot the fluid elemental composition for the reaction trace. Is there a way to do that? I attach the Act2 script and one of the React scripts that generates a reaction trace. I also attach the dataset I used in both (an edited version of the thermo.com.V8.R6+.tdat dataset).

[Jia Wang]

Hello John,

Your input file and dataset was not attached. Could you please try attaching again so we can take a closer?

Also please note that the axes in Act2 are reflecting the activity or log activity of species, not concentrations. Activity is equal to the concentration when the activity coefficient is 1. There's no way to plot concentration on activity diagram, the program can only plot species activities from the reaction trace.

Best regards,
Jia

[John C. Ayers]

Here are the files that were supposed to be attached. The zircon_dissolution React output shows that when baddeleyite is saturated log activity of Zr(OH)2++ is -21.6 but the Act2 output shows log activity Zr(OH)2++ at baddeleyite saturation is -12.1. Maybe it's coincidence, but in the React output the basis shows that log10 molalities of Zr(OH)2++ and Zr(total) = -12.1. The activity coefficients of all but the most highly charged species are ~1.

zircon_solubility_SiO2_v3.ac2 EQ3_6_Zr_JCA.tdat Zircon_dissolution.rea

[John C. Ayers]

I just tried switching to the original thermos dataset thermo.com.V8.R6+.tdat in the Act2 script and I get the expected output:

 

[Jia Wang]

Hello John,

Thank you for attaching the scripts and the thermo dataset.

Act2 creates a diagram to display the predominant aqueous species and the stability of minerals in a chemical system. The parameters used to create the diagram include all the reactions in the thermodynamic database and the constraints set in Act2. While you have set the y-axis to vary with the activity of Zr(OH)2++, the predominant aqueous species is Zr(OH)5- given the thermodynamic dataset and the temperature of 200C. The boundaries that you see in the plot represent the equilibrium reaction equation between the stable mineral and the predominant aqueous species. You can view this equation by clicking and hovering over the boundary. You can swap in Zr(OH)5- for Zr(OH)2++ in the basis pane as the diagramming species and the marker will fall on the boundary where you expect it.

If you plot your React results in Gtplot and look under the variable type "Species Activity", you will find that the predominant Zr aqueous species in your system is Zr(OH)5- which is in agreement with Act2.

In Act2, you can suppress a species such that it is removed from the program's calculations. In your case, I can sequentially suppress the predominant Zr species until Zr(OH)2++ appears on the plot. This would be the same as loading thermo.V8.R6+.tdat that came installed with the software since Log K information at higher temperature for some Zr species are not included. Unless set to extrapolate Log Ks, reactions that do not have thermodynamic information at the temperature of your calculation will not be considered.

Hope this helps,
Jia

 

[John C. Ayers]

Thank you Jia, swapping in Zr(OH)5- for Zr(OH)2++ in the basis pane of Act2 as you suggested worked.

[Jia Wang]

You're welcome. Glad to hear that helped.