Different Eh-reaction

[Helge]

Hallo,

 

a logK for Eh-reaction is given in each parameter file for Geochemist's Workbench. Further below O2(aq) is entered as basis species. All redox species to follow are defined using O2(aq). My first question is, how the Eh-reaction as entered in the parameter files relates to equation 3.40 in Craigs book: e- + 1/4 O2(aq) + H+ <-> 1/2 H2O (logK = 25.5 at 25°C).

 

Second question is: is it possible to choose a different Eh-reaction and use H2(g) instead of O2(aq)? I suppose I would have to enter different logKs in place of those given in the parameter files. But could GWB handle that or is a specific Eh-reaction invariably coded within GWB?

 

Regards,

 

Helge

[Tom Meuzelaar]

Hallo,

 

a logK for Eh-reaction is given in each parameter file for Geochemist's Workbench. Further below O2(aq) is entered as basis species. All redox species to follow are defined using O2(aq). My first question is, how the Eh-reaction as entered in the parameter files relates to equation 3.40 in Craigs book: e- + 1/4 O2(aq) + H+ <-> 1/2 H2O (logK = 25.5 at 25°C).

 

Second question is: is it possible to choose a different Eh-reaction and use H2(g) instead of O2(aq)? I suppose I would have to enter different logKs in place of those given in the parameter files. But could GWB handle that or is a specific Eh-reaction invariably coded within GWB?

 

Regards,

 

Helge

 

Hi Helge:

 

The log K in the header section of the thermo dataset corresponds to the half-cell reaction listed in section A.2.2 of the Thermo Datasets Appendix of the v7 GWB Reference Manual. This reaction represents equation 3.4 from Craig's Geochemical Reaction Modeling text (or 3.46 in the new edition) multiplied by a factor of 4 (see attached RXN file).

 

There is no half cell reaction involving H2(aq) for the hydrolysis of water, so you'll want to stay with the default reaction.

 

Best regards,

 

Tom Meuzelaar

RockWare, Inc.

[Helge]

Hi Tom,

 

this is my output from the attached .rxn file:

 

O2(aq) + 4 e- + 4 H+ = 2 H2O

 

Log K's:

0 °C: 93.7064 150 °C: 60.9559

25 °C: 86.0018 200 °C: 54.5990

60 °C: 77.1101 250 °C: 49.3866

100 °C: 68.9882 300 °C: 45.0942

 

Polynomial fit:

log K = 93.7 - .3318 × T + .001056 × T^2 - 2.34e-6 × T^3 + 2.363e-9 × T^4

 

Equilibrium equation:

log K = - log a[O2(aq)] - 4 × log a[e-] + 2 × log a[H2O]

- 4 × log a[H+]

 

At 25°C it says that logK = 86.0018

In thermo.dat it is logK = 83.1028

 

* log k for eh reaction

-91.0454 -83.1028 -74.0521 -65.8632

-57.8929 -51.6850 -46.7266 -42.6842

 

In Craigs Book, refering to eq. (3.40), logK (25°C) is given as 25.5; multiplied with 4 this gives 102.

 

Second question is: when I add to your half-cell reaction the half-cell reaction of the standard hydrogen electrode

H<+> + e<-> --> 1/2 H2(g)

 

I end up with a standard Eh-reaction which involves H2(g):

H2O <--> H2(g) + O2(g)

 

The background of my question is: when we produce own parameterfiles for GWB, are we fixed on the Eh-reaction given in Craigs book or may we decide to use H2(g) as basis species for redox reactions? If you say, that the Eh-reaction is fixed, we know how to proceed.

 

Is it possible, to have O2(g) as basis species and recalculate the Eh-reaction accordingly?

 

Regards,

 

Helge

[Tom Meuzelaar]

Hi Helge,

 

At 25°C it says that logK = 86.0018

In thermo.dat it is logK = 83.1028

 

In Craigs Book, refering to eq. (3.40), logK (25°C) is given as 25.5; multiplied with 4 this gives 102.

 

Indeed, there's an apparent discrepancy here.

 

1. The first logK, 86.0018, as per my original attached script represents the logK for the half cell reaction written in terms of dissolved O2: O2(aq) + 4e- + 4H+ = 2 H2O
   
2. The second logK, 83.1028, as given in the thermo.dat header section represents the logK for the half cell reaction written in terms of O2 gas: O2(g) + 4e- + 4H+ = 2H2O
   
3. The third logK, 102 (or 25.5), as given in Craig's book, represents the half cell reaction again written in terms of dissolved O2. This value is a typo, and should be 21.5 (or 86), as per thermo.dat
   

 

The background of my question is: when we produce own parameterfiles for GWB, are we fixed on the Eh-reaction given in Craigs book or may we decide to use H2(g) as basis species for redox reactions? If you say, that the Eh-reaction is fixed, we know how to proceed.

 

The reaction H2O <--> H2(g) + O2(g) is not a half-cell reaction, since it does not liberate or consume electrons.

 

By default, the software keeps track of the oxidation state state via the half-cell reaction for oxygen (assumes redox equilibrium), although redox reactions can also be constrained independently:

 

1. Redox equilibrium - use O2 as the controlling or "Master" redox species- all redox couples are speciated according to equilibrium with O2, as per the equation given for each couple in the redox couples section of the database.
   
2. Redox disequilibrium - decouple any redox couple in the database (ie. Fe++/Fe+++) and constrain the couple independently. For the ferrous/ferric iron couple, as an example, decoupling them adds Fe+++ into the Basis- the system now tracks a separate half cell reaction and Eh for this couple.
   

 

So, if your question is "can the software use H2 instead of O2 as the controlling redox species", then the answer is "no".

 

Is it possible, to have O2(g) as basis species and recalculate the Eh-reaction accordingly?

 

You can represent this half cell reaction by swapping O2(g) into the basis for O2(aq) - it is the same reaction as given in the header section of thermo.dat (with logK 83.1 - see attached). GWB will report Eh according the the half-cell reaction for dissolved oxygen, and for any redox couples in disequilibrium. You could write a Nernst equation for the half cell reaction involving O2(g), but it should not yield a different Eh, since O2(aq) and O2(g) are in equilibrium.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, inc.

[Helge]

Hi Tom,

 

having thougth this over and over I still think this is a severe constraint for the use of GWB as soon as one is interested in producing own parameter files (as we are currently doing within the THEREDA-project in Germany). No problem with EQ3/6, PHREEQC or CHEMAPP, only with GWB!

 

I therefore dare to ask: do you see any possibility to make GWB flexible in this point, to have other redox-controlling species instead?

 

Cheers,

 

Helge

[Tom Meuzelaar]

Hi Tom,

 

having thougth this over and over I still think this is a severe constraint for the use of GWB as soon as one is interested in producing own parameter files (as we are currently doing within the THEREDA-project in Germany). No problem with EQ3/6, PHREEQC or CHEMAPP, only with GWB!

 

I therefore dare to ask: do you see any possibility to make GWB flexible in this point, to have other redox-controlling species instead?

 

Cheers,

 

Helge

 

Hi Helge:

 

I think we're still missing each other on a couple of points. First of all, everything that can be done pertaining to redox equilibrium and disequilibrium in the programs you mention can also be done with GWB. In fact, GWB has the advantage that it is not necessary to re-write the thermodynamic database to configure redox disequilibrium - you simply use the Config- Redox Couples option to Decouple any redox reaction.

 

Second, in my answer to the question "can the software use H2 instead of O2 as the controlling redox species", I did not mean to imply that you cannot model a redox system where H2 predominates. You can accomplish this by swapping H2(aq) or H2(g) in for O2(aq) as your Basis species.

 

I hope that helps,

 

Tom

[Helge]

Hi Tom,

 

I am well aware that GWB can swap H2(g) for O2(aq). The point is: GWB cannot process a datafile that originally builds on H2(g) as primary redox species. The datafile itself builds on O2(aq), which is quite uncommon and not in comliance of standards set up by the IUPAC where redox reactions are written in terms of H2(g).

 

In the project a hot discussion is going on on the question whether redox reactions should be written in terms of O2(g) or H2(g). When we produce a GWB-datafile with redox reactions based on H2(g), GWB - we fear - will not be able to process that file.

 

Cheers,

 

Helge

[Tom Meuzelaar]

Hi Tom,

 

I am well aware that GWB can swap H2(g) for O2(aq). The point is: GWB cannot process a datafile that originally builds on H2(g) as primary redox species. The datafile itself builds on O2(aq), which is quite uncommon and not in comliance of standards set up by the IUPAC where redox reactions are written in terms of H2(g).

 

In the project a hot discussion is going on on the question whether redox reactions should be written in terms of O2(g) or H2(g). When we produce a GWB-datafile with redox reactions based on H2(g), GWB - we fear - will not be able to process that file.

 

Cheers,

 

Helge

 

Hi Helge:

 

Thank you for the additional clarification - that helps. Could you make a copy of an example datafile and send it to me? (via personal email is fine). We'll take a look at it and see if we can either modify the database, or the code, as necessary.

 

Best regards,

 

Tom

[Brian Farrell]

For anyone who comes across this old post, recent versions of the GWB programs allow reactions to be written in an arbitrary format. A dataset can be set up with either O2(aq) or H2(aq) as a basis species. Redox reactions can be written in terms of aqueous or gaseous forms of oxygen or dihydrogen, as desired, as well as the e-. And in general, reactions can be written in terms of any species in the dataset. For more information, please see the Thermo Datasets chapter in the GWB Reference Manual.

Regards,

Brian Farrell
Aqueous Solutions