Using OH- as a master specie

[Kevin Brown]

Apologies, but I am new to PHREEQC.

I have a system where the mass of OH- for a stream is "known," and thus I want to use OH- as a master specie and enter the OH- for the initial SOLUTION. The pH for said stream is an unknown.

I have attempted multiple changes to the SOLUTION_MASTER_SPECIES and SOLUTION_SPECIES blocks to attempt to include OH- as a master specie without success. The typical error I receive is:

     "ERROR: Element or valence name in SOLUTION_MASTER_SPECIES should include only one element, OH."

My approach appears to be a non-standard way of "controlling" OH- (where typically this would be done by controlling pH but I would rather not compute something that is a given for a stream).

Any help would be greatly appreciated, including if there is a better way to parameterize this type of PHREEQC calculation when knowing the OH- in a stream.

Please let me know if you need additional information or clarification.

Thank you and best wishes,

Kevin G. Brown, Ph.D., BCEEM
Research Professor
Department of Civil & Environmental Engineering
Vanderbilt University

[dlparkhurst]

No, you cannot set OH- as a master species.

You can set the activity of OH- by setting the pH. Log activity of OH- will be will be log_k + pH.

Alternatively, you can set the activity of OH with a phase. Here is an example for a SOLUTION definition.

Code: [Select]

PHASES
Fix_OH
OH- = OH-
log_k 0
END
SOLUTION
pH  7 Fix_OH -4
Na  1 charge
END

And here with an EQUILIBRIUM_PHASE

Code: [Select]

PHASES
Fix_OH
OH- = OH-
log_k 0
END
SOLUTION
EQUILIBRIUM_PHASES
Fix_OH -3 NaOH 10
END

These previous methods set the activity, which is the same as what you can do with pH. If you want concentrations of OH, you can set the total alkalinity. In non-carbonate systems, the alkalinity will generally equal the OH- concentration. With carbonates, the alkalinity is the sum of the carbonate alkalinity and the OH- concentration.

Finally, you can add moles of OH- with a REACTION.

Code: [Select]

SOLUTION
REACTION
NaOH 1
0.001 moles
END

You will have to use one of these methods. I suspect the key for you will be to define the initial solution concentrations; for additional reactions, concentration of OH- will be a calculated value. Post a specific example if you have more questions.