Adding new phase to thermodynamic database and Log K problem

[BlaXy]

Dear GWB users,

I would like to modify thermo.com.V8.R6+.tdat by adding several minerals, primarily nickeliferous phyllosilicates. For example, I would like to add Pimelite:
Ni3Si4O10(OH)2:H2O + 6 H+ = 3 Ni+2 + 4 SiO2 (aq) + 5 H2O (Gali et al., 2012)

According to the data available in the literature, the equilibrium constant (log K) at 25 °C is 11.46. However, the publications lack of ΔH°f data, so using the Van 't Hoff equation may not address my problem. For my research, I need to determine the log K values for pimelite from 0 °C to 300 °C. So, how can I view these values if Act2 uses polynomial expansion to extrapolate log K? When I navigate to Config -> Alter LogKs, the displayed altered values are 500. The only log K value shown at 25 °C is 11.46, which I provided using TEdit when adding the new mineral. 

All the best!

[Jia Wang]

Hello,

I think the best place to see the polynomial fit equation used in the calculation of a log K for a reaction, not at a principal temperature, is in the Rxn application. You can select the species that you wish to balance the reaction for in Rxn, then run the calculation, and view the polynomial in the Results pane.

In your case, you only have 1 equilibrium constant value at 25C. In that case, when you turn on the extrapolate feature, the program will just use the Log K value at 25 degree at all temperatures. In general, I would advise caution when extrapolating log K's for temperature beyond the range of validity prescribed. The further you extrapolate outside the range, the less accurate the values may become.

Please see section 3.2 Temperature expansions in the GWB Reference Manual for more information. If you have not used Rxn before, you can see chapter 4 in the GWB Essentials Guide for detailed explanations and examples.

Hope this helps,
Jia Wang
Aqueous Solutions LLC