Sato question

["Bird, David" <david.bird@xxxxxxxxxxx>]

David

You probably don't remember me, but I attended the Geochem Modeling class in
1992 taught by you and Messrs. Plummer and Glynn, and I have swapped emails
with you occasionally regarding PHREEQE/PHREEQC questions. I also used to
work with John Mahoney at HCI, whom you no doubt remember and perhaps have
nightmares about. I was wondering if you would have time to help me with a
modeling problem. I have been trying to solve this myself but alas I realize
that I must drop back into punt formation. 

My work with the Colorado Geological Survey involves collecting water
samples and taking field measurements. I decided that, to get Redox
information, rather than measure field Eh that I would measure Dissolved
Oxygen in the field, as I remembered that WATEQ has the capability to
calculate pe from DO data using the Sato equation. To save time, I was
hoping to just enter the formula into a spreadsheet. The formula I got from
the PHREEQC manual ( which I notice is commented out) is:

Kw = (0.25O2(aq) + H+ + e-)/(0.5H2O)

where Kw = -11.385

Questions:

1) Is "O2" equal to the molar concentration of O2 calculated from the
Dissolved Oxygen in mg/L?
2) Is "H+ " equal to pH or the activity of hydrogen ion, or something else?
3) Is "H2O" equal to the activity of water (in which case it can be assumed
to be 1.0)?
4) Is "e-" equal to pe? 

I have a field DO of 5.0 mg/L at a pH of 7.42.  Running this through WATEQF
gives a pe of 3.014, but calculating manually I get something different. Can
you shed any light on this mystery?

Thanks !

David A. Bird
Geochemist
Colorado Geological Survey
Department of Natural Resources
1313 Sherman Street
Denver, CO  80203
Tel: 303-894-2169
Fax: 303-894-2174
email: david.bird@xxxxxxxxxxx
http://geosurvey.state.co.us/