Important Q and A about fixing redox potential

[Charlie]

The following was taken from the PHREEQC FAQ, and answered by D. Parkhurst
(http://wwwbrr.cr.usgs.gov/projects/GWC_coupled/phreeqc/faq.html)

Q. In the general frame of nuclear wastes storage, we have performed leaching experiments of uranium dioxide. I would like to calculate solubility limit of uranium secondary phases under my experimental conditions. I have determined experimentally pH, Eh, temperature and water composition (Na, Cl, HCO3-, SO42-,K, Mg, Ca and U). After calculations, I obtain solubility limits for phases but the pe varies (-3.08 to -5.9) during calculations. Can you fix the pe for reaction calculations?

A. I generally discourage people from trying to fix pe during reaction calculations (it's fine for speciation calculation). The reactions should determine the pe just as the reactions determine the pH. Bad things can happen, for example it is possible to slip out of the stability field for water if pe is fixed, say at low pH initially, and then the pH rises.

If the modeled pe is not what you expect, I think there are two possible reasons (1) redox disequilibrium in your system, or (2) unexpected reactions.

Disequilibrium can be modeled with some effort. It is necessary to rewrite the database to split out each redox state as an "element". If all redox elements are split, then all redox reactions would have to be performed with REACTIONs that transfer elements from one redox state to another. Partial disequilibrium may be more reasonable.

You can fix pe the same way as pH. However, you must choose a reactant that enters or leaves the solution to cause the pe to be fixed. It is best if this is a plausible reactant. The example below assumes atmospheric oxygen entered your system? Oxygen could also leave the system, which would be implausible.

 PHASES
 Fix_pe
 e- = e-
 log_k 0.0
 EQUILIBRIUM_PHASES
 Fix_pe  4      O2(g)   # fixes pe to be -4 by adding or removing O2

[kishu5694]

Hey,

I have some further question.

How can you define Pe/ Redox Potential from Oxygen concentration in Open system? To Measure theoretically, There is any formula for it?

[dlparkhurst]

The following reaction for O2(aq) is from phreeqc.dat. It allows you to calculate a pe:

Code: [Select]

2 H2O = O2 + 4 H+ + 4 e-
-log_k -86.08

pe = (-2*lg[H2O] + lg[O2] - 4pH)/4

where [] indicates activity. lg[H2O] is approximately zero, and activity of O2 is approximately equal to molality. PHREEQC does this calculation for each SOLUTION that contains dissolved oxygen [redox state O(0)].

The concentration of O2(aq) in equilibrium with the atmosphere can be calculated from the Henry's law constant. Again from phreeqc.dat, the equation is

Code: [Select]

O2(g) = O2(aq)
-log_k   -2.8983
or

K = [O2(aq)]/P(O2(g))
where P (actually fugacity) is approximately the partial pressure of O2(g) in atm.

[kishu5694]

Thank you so much for your Reply,
But the Pe equation should be like derived below. Right?

K = [O2]*[H+]^4*[e-]^4/[H2O]^2
Take log both side,
logK = log[O2]+4log[H+]+4log[e-]-2log[H2O]
-4 log[e-] = log[O2]+4log[H+]-2log[H2O]-logK
4Pe = log[O2]-4pH-2log[H2O]-logK

Pe = (log[O2]-4pH-logK)/4
As log[H2O] = 0

I dont understand, how you derived that equation.

Also what is the reason for considering lg[H2O] approximately zero because we don't know about the purity and other ions eg. Fe, Mn, Na available in solution?

[dlparkhurst]

Yes, my algebra was not right.

Activity of water is approximately the mole fraction of water in the solution. Until you get to concentrated brines, it is very close to 1. At seawater concentrations activity of water is still about 0.98.

[kishu5694]

Thank you so much for your help