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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