[Water Activity at High Temperature]

Hi David,

 

Act2 using its default thermo databases can generate diagrams up to 300 C, at pressures of 1 atm up to 100 C, and following the steam saturation curve thereafter. Above 100 C, then, water is still liquid, unless you specify a lower pressure (which you did in your last diagram). The effect of pressure in Act2 influences only the position of water stability limits and the stability of gas phases. Your diagram is entirely composed of minerals, however, so keeping the pressure at 1 bar doesn't really do anything. I also wonder how useful the x-axis you have chosen is - it seems to be more of a mathematical abstraction than a chemical reality. From my understanding water activity should vary from 0 to 1, so it seems Magnesite and Brucite would be the only likely phases. This isn't exactly answering your original question but you might try a different sort of diagram (diagram Mg++ in Tact vs T and f CO2, or in Act2 vs f CO2 and pH or activity Mg++). You can set an activity for water or a pH in any of these diagrams. Hope to have helped.

 

Brian

[can't plot scatter data]

Hi Tam,

 

You mentioned that your diagram was Eh vs log a Mn++, but your text file has values listed for pH, not Eh. The scatter data categories must match what you have on your diagram. If you have Eh and pH data, I might try several diagrams to see which is most meaningful (Eh vs pH, fixed a Mn++; Eh vs. Mn++, pH fixed; pH vs Mn++, Eh fixed). Hope that helps.

 

Brian

[can't plot scatter data]

Hello,

 

I am trying to make a plot in Act2 that has Mn activity on the y-axis and eh on the x-axis. The system is at a constant pH 7 and H2O activity is equal to 1. It appears I was able to do this successfully. Now, I would like to plot field data of eh and Mn activity for a number of samples. I have made the text file but when I tried to "get scatter data", I received the error message:

 

Can't plot scatter data; doesn't contain: H2O

 

Thank you for your help!!!!!

 

 

Hi Tam,

 

You'll want to take a look at the Scatter Data Appendix in the Reference Manual, which you can get to by clicking on the Help tab in Act2. Make sure your two columns are spelled correctly - Mn++ and Eh, and that for Mn++ the data is entered as log activity. Could you post a script for your Act2 diagram and text file so we can try to troubleshoot this? Btw, which version of GWB are you using? 8.0 uses the Geochemist's Spreadsheet which makes adding scatter data really simple, although a text editor like notepad works just fine.

 

Brian Farrell