I am more than a little dubious that this is worth doing, but pe = -logK + log10[O2] - 4pH log10[O2] ~= log10(c(O2)/32/1000), where c(O2) is concentration in mg/L. logK = 38.0229 + 7.99407e-3*T - 2.7655e+4/T -1.4506e1*log(T) + 199838.45/T^2, where T is in K. Eh = pe * log(10) * R * T / F, where Eh is in volts, log(10) ~ 2.303 (natural log), R is .00831470 kJ/deg-mol, F is 96.4935 kJ/volt-eq, and T in K. For c(O2) = 3.2 mg/L, pH = 7.3, T = 5 C: Eh = .812 V; pe = 14.71. For c(O2) = 3.2 mg/L, pH = 7.3, T = 50 C: Eh = 0.7415; pe = 11.565. David David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled "Doug Craft" <dcraft@xxxxxxxxx To: <dlpark@xxxxxxxx> ov> cc: Subject: Re: Estimating Eh from DO 07/15/03 11:12 AM Thanks David- I appreciate the quick response! You ROCK, man! What I need is a formula I could plug into Excel - Running PHREEQC is not an option as I have over 10,000 values to calculate. As to the interpretative value of Eh - I hear you - and am aware of mixed potentials on the Pt surface (as Stumm and Morgan, Williard Lindsay, and Langmuir have discussed) - but I am working with biologists who think redox should be reported. The data are from a tidal zone, too, so there's tons of variability in the data. I tried an empirical approach but the regression has way too much scatter. Sorry to be a bother. Doug >>> "David L Parkhurst" <dlpark@xxxxxxxx> 07/15/03 09:48 AM >>> One way is to run PHREEQC inputting dissolved oxygen as O(0) in PHREEQC notation. A pe and Eh will be calculated and printed under a heading "Redox Couples" in the output. But I have to add that the DO is a more useful quantity than the Eh, which is hard to measure and difficult to interpret. If you have DO, you are automatically close to the upper stability field for water on a Eh-pH diagram. David SOLUTION 1 -temp 15 -units mg/L pH 7.3 O(0) 3.4 END David Parkhurst (dlpark@xxxxxxxx) U.S. Geological Survey Box 25046, MS 413 Denver Federal Center Denver, CO 80225 Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled "Doug Craft" <DCRAFT@xxxxxxxxx To: <dlpark@xxxxxxxx> ov> cc: Subject: Estimating Eh from DO 07/14/03 02:54 PM Hi Dave- Sorry to bother you, I know you are busy... I have been searching for a straightforward calculation to estimate Eh from DO in oxgenated surface waters - I stumbled across your Phreeqe IAQ but did not find the specific question answered. I have field measured T, pH, but had the redox electrode go whack on me. Do you have a simple formula or can you provide me a reference where it can be obtained? Appreciate your help! Doug Doug Craft Fisheries Applications Research Group USDI - Bureau of Reclamation PO Box 25007, D-8290 Denver CO 80225-0007 dcraft@xxxxxxxxxxx office: 303-445-2182 office cell: 303-870-5625 office fax: 303-445-6328 home: bigdoug@xxxxxxxxx 303-458-8946
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