Jump to content
Geochemist's Workbench Support Forum

Dissolution of mineral phase at high pH & atmospheric conditions


Recommended Posts

Hello all,

I am using React to model the dissolution of NdPO4 (input as reactant) under atmospheric conditions (fixed fugacities, fixed pH). The pH is adjusted with HNO3, therefore the NO3- concentration.

With the input shown in the attached file "Dissolution pH 5" the program runs without any problems.

In the next step, I would like to model the similar process at pH 10, again in atmospheric conditions. Here, of course, the pH would be adjusted via e.g. NaOH. I tried a few things, e.g. swapping H+ <-> OH- but somehow it will not work (see attachment Dissolution pH 10). Here, I set the OH- activity to 0.01 and this gives a pH of 10, although a a pH of 12 should be expected; especially if one looks at the OH- concentrations, e.g. after step 100 (= 0.016 mol/kg).

Has anyone experience how I can input correctly the fixed pH of 10 while having normal atmospheric conditions?


Thanks in advance for the help.








Kind regards,


Link to comment
Share on other sites



The reaction OH- + H+ = H2O has a log K at 25 C of 13.99, so if you set the OH- activity to 0.01 (log activity of -2) and assume the water activity is 1 (log activity of 0), by mass action the pH would have to be 12, which is what you expect. Since you're working at 100 C, however, the above reaction's equilibrium constant is about 12.25. In that case, if you set the OH- activity to 0.01, the pH should be about 10.25. Does this make sense?


By the way, I moved your topic from the archive of old posts to the main forum's front page. New posts are easier for everyone to find if they're in the main forum.




Brian Farrell

Aqueous Solutions LLC

Link to comment
Share on other sites

  • 1 month later...

Hi together,

now I a have a new problem. I try to model the dissolution of GdPO4:0.5H2O at pH 6 in contact with the atmosphere (fixed fugacities, fixed pH) at 25°C. The thermodynamic data is complete but I need advice regarding the correct data input. Something is wrong with the charge balance, since in the output it is said: "molality of Na+ component forced negative by charge balance". In the attached files you can find my input and output information. I would like to prevent that the Na+ is forced negative, so where is the error in the charge balance? Or is something else not correct?

What I noticed is that it run is working better if the pH is not fixed (although it has in the result the same value as in "fixed mode"). However, still the Na molality is changed but not forced negative.


Thank you in advance for the help.



Link to comment
Share on other sites



Try taking a look a your initial fluid chemistry (before dissolving the GdPO4:0.5 H2O into it) without forcing charge balance. To do so, turn the charge balance off and go to Run -> Go Initial instead of Run -> Go. When you do so, you'll notice that a significant charge imbalance exists in your initial fluid. Next, turn charge balance back on (using Na+ as the balancing ion) and solve for the initial state once again (Go Initial). You'll see that the concentration of the Na+ component needs to be decreased to establish a charge balanced solution.


Now try tracing the reaction path without fixing pH. You should see the pH increasing as the GdPO4:0.5 H2O dissolves into the fluid. When you implement the fixed pH path, then, the program would need to add H+ ions to the solution in order to counteract the pH increase. Adding H+ ions results in a charge imbalance, however, so the program needs to remove Na+ from solution. The program encounters an error when it needs to remove all of the Na+ from solution to balance the large amount of H+ that needs to be added. Try taking a look at section 14.2 (Fixed activity and fugacity paths) in the Geochemical and Biogeochemical Reaction Modeling text for more information on fixed activity reaction paths.


I'm not sure that you should be fixing pH in your model. There may be situations, such as in controlled laboratory experiments, where pH is maintained mechanically by a pH stat (adding small amounts of acid or base). A fixed pH setting would apply well here. Your description of a fluid in contact with the atmosphere would likely require fixed gas fugacities, but I don't think that it requires a fixed pH. Does this make sense?




Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...