PhreeqcUsers Discussion Forum
Processes => Oxidation and reduction equilibria => Topic started by: Steve on April 10, 2015, 07:55:01 AM

Is PHREEQC really reporting pe @ the actual operating temperature i.e. in the same way it correctly reports pH @ the actual operating temperature?
I am modeling a fairly alkaline hydromet system involving fly ash (pH around 11.0) and at temperatures between 80 and 100 C. If I switch off any redox couples by not defining one both species in the couple and and fix the only redoxrelevant mineral magnetite as insoluble and simply use any arbitrary pe e.g. 4 or 7, I always end up with the same predicted pe regardless of whether I use a starting value of 4 or 7 for example. So far so good. But I am puzzled by the fact that the reported pe values doen't seem to scale correctly in accord with expected number of mV per unit pe. For example, at 298.15 K (25 C) of course pe scales in units of 59.16 mV (from Eh). But at 100 C (398.15 K), pe should surely scale according to ln(10)RT/F i.e. 79.00 mV per unit pe (from Eh). And at say 90 C (388.15 K) pe should surely be scaled according to ln(10)RT/F i.e. 77 mV per unit pe (from Eh). The ratio of these unit pe in mV at these latter two temperatures is 1.0258. However, I seem to find that; even in a redoxinert system, PHREEQC does not do precisely this (all other things being equal), producing larger ratios of the modelreported pe values (@ 90 and 100 C). I have also received hints from some parallel bench scale trials that PHREEQC may not be correctly handling this issue but I'd prefer to trust the underlying theory of course. Somehow, somewhere, I may have thoroughly confused myself on this issue! Help!

I don't see a problem with the calculation below (run at low pH to avoid Fe+3 complexes). It generates a unit change in pe at 25 and then 100 C, with these results
25C
pe Volts
13.51 0.7993
12.51 0.7401
Difference:
1 0.059 and ln(10)RT/F = 0.059 @ 298.15
100 C
pe Volts
12.016 0.8896
11.016 0.8156
Difference:
1 0.074 and ln(10)RT/F = 0.074 @ 373.15
SOLUTION
pH 1
temp 25
Fe(2) 1
Fe(3) 1
END
SOLUTION
pH 1
temp 25
Fe(2) 1
Fe(3) 10
END
SOLUTION
pH 1
temp 100
Fe(2) 1
Fe(3) 1
END
SOLUTION
pH 1
temp 100
Fe(2) 1
Fe(3) 10
END

Thanks for your quick response David. of course I agree those low pH calculation results are obviously correct. I am working at high pH and I think my 'fixing the magnetite as insoluble' by having a line (in EQUILIBRIUM PHASES) which says; 'Magnetite 0.00 0.0100 p' is not quite the same as avoiding Fe(III) complexation effects in solution (even though I don't get any reported mass change). I have also found some accidental typos in my previous post e.g. 100 C is not 398.15 K but 373.15 K and 90 C is 363.15 K so am checking all my calcs to make sure the typos don't represent actual math errors by me. Will get back to you!