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Phase2_ionic strength is out of range

Jueun Kim

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I have one question about speciation diagram using Phase2 

I want to check the Ni Pourbaix diagram at this solution 



So, I input some parameters as below file (KURT_Ni)


However, Phase2 does not working due to ionic strength is out of range.


In this modeling, I used SIT thermodata.


What's the problem with Phase2?


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Hello Jueun,

Thank you for attaching your scripts. Here are a few suggestions to help you get started with troubleshooting, I first ran a "go initial" run with your Phase2 input file. Doing so shows the error you mentioned above, so right away, this tells you there's something not quite right with the way your basis is set up. So before adding any reaction paths, I suggest taking the composition in your basis pane to React and start troubleshooting there.

Based on the concentrations of your components, I don't think the fluid is really out of ionic strength range. If your starting pH is very low, then swapping in a more abundant species for the corresponding component is going to help the program solve the equilibrium state. In particular, SO4-- is not going to be the most dominant form at pH of 1, swapping in HS- helped with the initial calculation. Similarly with the carbonates, CO3-- is presently at very small quantity at low pH and swapping in CH4 seems to help. I initially tried CO2(aq), but given the low Eh, CH4 is likely to be abundant.

Charge balancing ion is typically defaulted to chloride because of its high abundance in many natural waters. In this case, your most abundant anion is Fluorite, so I would use that as the charge balancing ion.

Once you have gotten React to solve the speciation of initial composition successfully, you can return to Phase2 and adjust your X and Y axis. I noticed that you have set the Eh path to slide to 50 volts, which seems really high. To get an idea of the range for pH and Eh, I built a simple Act2 diagram for Ni++ (diagram Ni++ as the main species and assign pH and Eh as axes) and see that the water stability limits range between -.5 and 1.25 Volts across the range of pH. If you are going too far outside of this range in Phase2 or React, the program will fail because it can't calculate equilibrium when water is not stable. Unlike Act2, Phase2 and React solves a full solution to a multi-component chemical system.  On the other hand Act2, simply solves the equilibrium equations used in assembling an activity diagram. These equilibrium equations can be solved for at any range of pH and Eh. Because Phase2 is essential solving a series of reaction models that include mass balance and activity calculations, it is oftentimes necessary to choose a narrower range for the variable changing in the x and y axis. For more information on Act2, please refer to chapter 5 of the GWB Essentials User Guide.

One tip for using Phase2 is to always start with a relatively coarse grid. By default, the grid is set to 301 on the X and Y axes but you can adjust this by going to Config -> Output... I suggest starting with a smaller grid (e.g. 20 by 20) so that you can save time until you are ready to make a more detailed diagram. For more information regarding Phase2 configurations and examples, please refer to Chapter 7 in the GWB Reaction Modeling User Guide.

Hope this helps,
Jia Wang
Aqueous Solutions LLC

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I am not sure what your diagram would show if you were not able to complete your simulation due to the ionic strength error. Were you able to launch P2plot after running the attached input file?

Your file attached has the sliding path set to 10 Volts, not 10 millivolts as you stated above. When I changed the unit in the Y Axis pane from V to mV (by clicking on the unit "V" and selecting "mV"), and ran the simulation, it went to completion and I was able to plot various Predominance diagrams. In this new file, I noticed that you swapped in Ni(OH)2(s) for your Ni component. Was this intentional? Doing so would set the initial concentration of Ni in the fluid in equilibrium with Ni(OH)2(s).

I am not really sure what you mean by not much of a difference. If the reactions and species that you expect are not present in your simulation, you would need to check if they are present in your thermo database.  Also, remember that the predominance diagrams only show the highest abundance species or mineral at each node. You can investigate further the concentrations and minerals present if you plot a slice of the grid, across or along scanning paths. You also have the option to plot a Mineral Assemblage diagram under the option Format menu. For more information on Phase2 plot configurations, please see section 8 in the GWB Reaction Model User Guide.

Hope this helps,

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