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[OLD] Setting pH and Eh of solution reactant


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From: Richard Laffers

Subject: Setting pH and Eh of solution reactant

Dear users, using REACT, I troubled quite a time with this: On the reactants pane, I set a solution as reactant. I wondered, how could I specify the solution's pH, since the amount of any reactant can be constrained only in moles, grams or cm3 respectively.

To find out number of moles of H+ needed for the desired solution's pH, I have done the following:

I made an equilibrium calculation (using REACT) for the solution with the desired pH. In the React_output.txt I checked the bulk concentration of H+ (in the Original basis section). This value I used to constrain H+ on the reactants pane (along with setting the amount of H2O = 1 kg). Is there any more straightforward way to specify pH of solution reactant? Anyone experienced this problem? Similar problem arises when we try to specify the solution's Eh.


From: James Cleverley

Subject: RE: Setting pH and Eh of solution reactant

The best place to get this answer is by looking at example 16.2 (p.237) in Geochemical Reaction Modelling (Craig Bethke, 1996), available through Rockware I believe. This example gives you all the information you need to help you understand how to set a fluid as a reactant in REACT. Also see 'picking up the results of a run' on page 145 of the GWB user's manual.


From: Craig Bethke

Subject: Re: Setting pH and Eh of solution reactant

The easy way to do this is to set up an equilibrium model of the reactant fluid in the Basis pane, setting pH, Eh, etc., as usual. Run the model, then do a Run -> Pickup -> Reactants -> Fluid. Now, the fluid will be converted to a reactant. You can change the total amount of fluid to be reacted by setting a factor in the "reactants times" box on the Reactant pane.

P.S. There are some specific examples of picking up fluids as reactants in the green book as well as the User's Guide. You might find these helpful.


From: Richard Laffers

Subject: Re: Setting pH and Eh of solution reactant

Dear Craig, James, thank you for your advices. Picking up fluid as reactant would certainly works, nevertheless not in my case. I try to model mixing of 2 fluids. The composition of the first fluid is result of former reaction path modeling and it is picked up with "pickup system=fluid" command. That's why I cannot directly use the equilibrium calculation + "pickup reactants=fluid" to specify the other fluid - it would reset the initial system (specifying the first fluid), which cannot be set directly.

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