How to create calcite stability diagram?

[Andrew Wigston]

Hello,

I am trying to create a stability diagram for calcite in pure water as a function of the partial pressure of CO2(g). I think this should be relatively straight-forward, but I don't think I have quite got it. In React I was able to make an X-Y plot of the solubility of calcite as a function of the partial pressure of CO2 (g), where the solubility is represented on the Y-axis by the concentration of Ca++ in fluid. I figure this is a pseudo-stability diagram, as regions above the curve are where calcite is stable (i.e. the fluid would be supersaturated with respect to calcite) and regions below the curve are where it is unstable (i.e. the fluid is undersaturated). 

But, I can't figure out how to do something similar using Act2. I can plot the stability of calcite as a function of CO2(g) fugacity, but I have to set the pH at a fixed value. I was thinking (hoping) that the CO2(g) fugacity could be allowed to determine the pH, as it did in React.

I have attached a Word document with a number of screen shots of what I have done (both the basis inputs and resultant plots). Thanks for any help you can provide.

Sincerely,

Andrew

 

Calcite stability diagrams using React and Act2 .docx

[Jia Wang]

Hello Andrew,

React solves a multicomponent system to determine the system's equilibrium state. In your example, it looks like you swapped in Calcite to set it in equilibrium with the fluid and then looked at the dissolved constituent concentrations as CO2(g) varied. The sliding reaction path feature makes it a very convenient to view the solubility of your reaction of interest against one changing variable. 

In Act2, the set up is much more simple compared to React. Unlike React, Act2 calculates equilibrium equations based on the reactions provided in the thermo database and then assembles them into a diagram. It does not calculate speciation and solve a multicomponent system at each node to account for pH in the same way that a reaction path in React does. Act2 would need a constraint on H+ separately in order to include Calcite in your diagram calculation. 

If you would like a more complex solubility diagram and supply constraints like you would in React, you may be interested in Phase2. Phase2 works by solving the multicomponent system for each node in a 2D grid according to the reaction paths set in the x and y axis, and determine the dominant phase present based on the amount calculated to be present. You can think of each node in a Phase2 diagram as its own React simulation. Another advantage of Phase2 is that you can constrain variables in concentrations like molal and mg/l instead of activities. 

An example that may be similar to what you may be looking for is the AlSolubility.ph2 example that comes installed with your GWB files. For more examples and information on Phase2, please refer to section 7 in the GWB Reaction Modeling user guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC