Switching off Redox

[GeeqC]

Dear David Parkhurst,

We discussed earlier that it is difficult to switch off entirely all redox equilibration in PhreeqcRM. I have now tried to solve the issue with a minimal database in which all redox half reactions have been removed. Redox states have been decoupled by replacing the oxidation numbers of the master species by roman numbers.

Only for H and O PhreeqcRM still wants the redox states defined as master species (otherwise it gives an error message). Then also the half reactions for H2 and O2 are still required:

2 H+ + 2 e- = H2
- 4 H+ - 4 e- + 2 H2O = O2

Both H2 and O2 can be prevented from forming by setting log_k to a very small value (log_k = -100). 

Individual redox-reactions can now proceed in a kinetically controlled way as part of the reaction-transport model. There, all reactions and transport are charge balanced. For charge balance, chloride is set as variable.

Best wishes
Patrick

[dlparkhurst]

I'm pretty sure I have pointed you to this database that separated all redox states. Good luck keeping thermodynamic plausibility.

Code: [Select]

https://phreeqcusers.org/index.php/topic,2023.msg7203.html#msg7203


[GeeqC]

Yes, indeed, you recommended this procedure earlier.
Furthermore, the pitzer.dat database has already all redox couples separated. For each master species only one redox state is defined.

Here comes now my question: while Phreeqc gives an error message if I do not at least define the two half reactions for H2 and O2 in my minimal database, the pitzer.dat database does not contain any half reaction. How is this possible?

[dlparkhurst]

You are right. The original (and current) pitzer.dat did not have any redox definitions, that is, no e- definition. So, there is a special case that allows pitzer.dat to omit e-, H2(aq), and O2(aq) and to avoid the database checks that require these species. Later, people wanted the possibility of redox calculations with Pitzer, and the -redox option in the PITZER keyword was added. By default, the option is false, but when true, the special case is not used.

I don't think this option makes much difference to you. It only applies to databases that use the PITZER keyword. You have made definitions for O2(aq) and H2(aq) that will work in your modified database, and you still have to defined every redox state as a separate element.

[GeeqC]

Thanks, this mostly answers my question.

The keyword PITZER occurs in the pitzer.dat database as the heading of the datablock that contains the pitzer parameters. So, it is this keyword "PITZER" that triggers the special case?

Then this is clear.