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Pourbaix diagram using Phase2

Frank Bok

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Hi Brian et al.

I would like to learn using the new Phase2 program and to do so, I started with something simple: the Eh-pH diagram of Uranium using thermo.tdat. With Act2 this works over the whole range (pH = 0 to 14, Eh = -0.9 to 1.25 V). Trying to do the same with Phase2, I run into several problems:

If I try the whole pH range (starting at pH = 0), the initial system cannot be solved:

Newton-Raphson did not converge after 999 iterations, maximum residual =    5.42e-15, Xi = 0.0000
Largest residual(s):
                       Resid     Resid/Totmol   Cbasis
 Na+                 3.338e-05    5.422e-15    7.396e+05

Reducing the pH range to 4-10 and deactivating charge balancing, the diagram can be calculated. But: The resulting diagrams of Act2 and Phase2 does not match.

If I try to go higher, either GWB runs with full CPU workload and does not respond (Windows then kills the program if I try to close it) or produces an incorrect *.p2p file.

Can you give me a hint how to correctly use Phase2?!

Thank you in advance and best regards,







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  • 1 year later...


I have the same issue as Frank as seen above. I am try to model the Al-Na-Cl-H2O-CO2 system and look at the Pourbaix diagram over the entire eH pH range as mentioned above. I am able to do the calculation from pH = 0 to pH =9, and from Eh = -0.55 to Eh = 0.75, but anything outside of the range crashes and the program must be restarted.

Also as Frank mentioned if I use Act2 to try to replicate the Phase2 calculation, the results are not entirely consistent.

What are some tricks or ways to run the Phase2 at a full Eh/pH range?




Eh pH diagram.ph2

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Hi Frank and Andrew,

Act2 uses a simple analytical method to calculate equilibrium lines and assemble them into Pourbaix diagrams, so there's no difficulty in creating a diagram that spans large Eh and pH ranges. If you wanted, you could set the Eh and pH to span the range -100 to 100, and it would draw the diagram. With Phase2, however, you're setting up numerical reaction path models. In a model of the aqueous phase, working far outside the stability limits of water (i.e., the bottom left and top right corners of an Eh-pH diagram) can be difficult. 

To investigate your calculations, try overlaying some contour plots on your 2D diagram, or better yet, just look at a simple xy plot of the diagram’s left edge. You can select the left-most vertical cross-section through the entire diagram, use the “Go Y” option in Phase2 to calculate only the left edge of the diagram, or set up a React script to reproduce the left edge of the calculation. 

Whatever you choose, start out by plotting the Mass of Solution or Fluid volume vs. Eh. You'll see they're initially enormous. If you make a plot of species concentration, you’ll see it’s due to the extremely high concentration of H2(aq) that would exist under your initial conditions, which are far outside the stability range of water. Next, try plotting  the concentration of your metal component of interest (U++++ or Al+++ in the system) and you should see that your initial conditions are honored. In the U example, U++++ was set to 1e-10 mol/l, but under the initial conditions that’s equivalent to .19 molal! That’s why so many minerals are stable throughout the diagram, compared to the Act2 calculation. As for the Al example, Al+++ was set to 1e-5 mol, and there’s still 1 kg of solvent in the system, so the initial Al+++ molal concentration isn’t skewed the way the U++++ was. You didn’t attach an Act2 script, Andrew, but presumably it’s not as different as the U calculations were.

As for the failures you’re encountering, unfortunately it’s likely to happen when you’re so far outside water’s stability region, as in the corners of an Eh-pH diagram with a traditional range. In a log f O2(g)-pH diagram, by contrast, you don’t have to include such extreme conditions. Since a large portion of the area in an Eh-pH diagram is likely to be masked, log f O2(g)-pH diagrams can be a nice alternative in that they fill more of the plot area with useful information, rather than blank space. If you need to know the Eh, you can always plot contours on the log f O2(g) diagram. 

Whatever diagram you choose, it’s common to use a narrower range in these types of diagrams. I think when you do that you’ll find that you can reproduce an Act2 diagram much more closely. There will still be differences, of course. Boundary lines can be somewhat curved, rather than straight, reflecting the fact that Phase2 is solving a complete multicomponent calculation at each point in the grid instead of drawing equilibrium lines. It includes mass balance and activity calculations, unlike Act2. And whereas an Act2 diagram shows the stability of minerals and predominance of aqueous species (in terms of highest activity), Phase2/P2plot’s predominance map strictly shows the species accounting for the most mass at each point in the diagram. In other words, a mineral’s stability field (which you can see with an assemblage map) can be slightly larger than the area in which that mineral predominates all other species.

Sorry for the delay in responding. It looks like we missed the original post. 

Hope this helps,

Brian Farrell
Aqueous Solutions

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