Where does the O2 come from?

[steven]

Hi All,

I'm trying to learn PHREEQC in order to better understand the formation of amorphous precipitates in solutions of both ferrous and ferric chloride that also have NaCl. I want to ensure there is no oxygen gas in the electrolyte (and no oxygen in the headspace above). When I carry out a simple simulation (single-solution)  of 1 M Fe(II) + 2 M NaCl (pe=7.0135), this seems to work fine. But when I simulate a solution containing 1 M Fe(III) + 2 M NaCl (pe=19.01), a lot of oxygen shows up, and I can't figure out why. Does this imply that oxygen is somehow being 'generated' by the Fe(III) reacting with other species solution? Or am I not doing enough to enforce a no-oxygen environment? Any tips would be greatly appreciated! Thanks much

Best,

Steve

[dlparkhurst]

If you add FeCl3 to pure water, thermodynamics says that there should a little a little oxygen. Some of the Fe(3) will be reduced to Fe(2), making a little O2(aq), I get on the order of 1e-7 O2(aq) for 1 mol FeCl3.

If you want to eliminate that O2(aq), you can set the partial pressure of O2(g) to a small number say 1e-10, which will remove O2 from solution, but generate a little more Fe(2).

SOLUTION 1
END
USE solution 1
REACTION
   FeCl3 1
   1 mol
SAVE solution 1
END
USE solution 1
EQUILIBRIUM_PHASES
    O2(g)  -10. 
END

[steven]

Wow, thanks very much! Very helpful