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Re: Seawater




> I'm calculating equilibriums in seawater and, as you
say in the manual, this is a bit beyond the possibilities
of PHREEQC. On the other side, there are examples provided
with PHRREQC which include seawater. So, what do you think
is the approximate error I have to expect ?

The best way to answer that is to compare PHREEQC results to results with a
Pitzer model (EQ3/6, TEQUIL, Geochemists Workbench, PHRQPITZ) or, for
seawater, the conditional constant approach like you have done. Pitzer
models generally lack trace metals, redox, aluminum, and silica. PHREEQC
will work best with the major ions. Minor elements are worse, probably much
worse.

> For example, if I compare calculated ion speciations to
Millero's "Chemical oceanography", my results agree with
Millero's for most ions (like Na, Mg, K, Ca). But for Cu++,
I calculate 93.2 % Cu(OH)_2 and 5.5 % CuCO_3, while Millero
says 73.8 % CuCO_3, 14.2 % Cu(CO_3)_2-- and 2.2 % Cu(OH)_2.

> Do you have an explanation for that result ?

First, systematic analysis of thermodynamic data is nearly absent. Second,
whichever approach is chosen, the selection of ionic complexes is likely to
be arbitrary. Ideally, there is spectroscopic data that defines the aqueous
complexes, but usually, the researcher picks species because they fit the
data best with the model that he or she is using. For ion association
models, different researchers use different models and different complexes,
perhaps to describe the same phenomenon. Then a collator comes along and
puts all the constants together into a "selected" database, but there is
usually no attempt to put the "selected" constants into a model and
reproduce the original experimental data, solubility data, titration data,
etc. So you have a terrible mess.  The Pitzer approach systematically fit a
collection of experimental data, so it is a more consistent and believable
model where it is applicable.

> By the way, is there a database which includes iodine ?

wateq4f.dat, minteq.dat, and llnl.dat have I-. llnl.dat also has other
oxidation states of iodine.

David

David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

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