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Re: PHREEQC (What else?)

> Tino Maestas (Rick Healy's student) and I are attempting to model some
solutions we hope to use for some experiments.  We are mixing/diluting
standard solutions of dissolved metals.  Unfortunately, the pH values of
the starting solutions are about 0.5.  Are there any versions of PHREEQC
that will allow us to have solutions at that pH?  Is there a  discussion
area where I could go see if anyone has had a similar dilemma?

A pure acid of pH  0.5 has an ionic strength of less than 1, so you're
probably still within a reasonable ionic strength range depending on the
acid. You are in better shape with hydrochloric than sulfuric, but I
wouldn't worry too much. You will probably dilute these anyway and then the
critical factor is whether the number of moles in the acid solution is
correct, rather than the activities.

> Also, as a first pass, we attempted to "make" DI water using water with a
pH = 7.0, Temperature of 25 degrees C, and in equilibrium with CO2.  We
modeled the gas phase two ways--For the first (input 2) we set pressure = 1
atm and adjusted the CO2 partial pressure to 0.000387 to account for the
elevation here (test2.out).  In the second, we set pressure = 0.81 atm, and
used a CO2 partial pressure of 0.000316 (test3.out).  Both yielded
essentially the same results except for pe. In the first simulation
(test2.out) the ending pe was 12.3.  In the second simulation (test3.out)
the final pe was 0.158.  When I use "redox   O(-2)/O(0)" the resulting pe
values match (test4.out). Any clues why they don't if I use "redox   pe"?

A couple of points: First, you probably intended to fix the partial
pressure of carbon dioxide. I guess it is non intuitive, but you don't use
the GAS_PHASE keyword to do this. GAS_PHASE models a variable composition
gas phase, which either has fixed pressure, like a gas bubble in sediments,
or fixed volume, like a steel gas cylinder. The point is, all of the gases
respond to reactions and partial pressures don't remain fixed.

There are two ways to fix the partial pressures that are conceptually
different and will give slightly different answers depending on what you
want to model. First is to fix the pH at 7 while simultaneously adjusting
the amount of carbon in the system such that the partial pressure of CO2(g)
is equal to atmospheric. It makes little practical difference in this case,
but the calculation does not add CO2 (carbon and oxygen)  to the system
only the carbon concentration is adjusted. I look at this a way as simply
another way to define the carbon concentration in the initial solution.
Note the pH of the initial solution remains at 7 and no reaction
calculation is performed, only an "initial solution calculation".

Method two is a batch-reaction calculation. In this case, the initial
solution (pure water + DO) is defined without any carbon. The pH of the
initial solution is fixed at  7. The distribution of species for the
initial solution is calculated ("initial solution calculation"), then a
"batch-reaction calculation is performed that adds CO2 (carbon and oxygen)
until atmospheric partial pressure of carbon dioxide is reached. The result
of this reaction calculation will have a pH of about 5.5.

Which way you go depends on which approach best fits your conceptual model.
The first way has a slight ambiguity because there are no cations to
balance the HCO3- that is introduced (it is possible to add a cation to
achieve charge balance or you can just ignore the imbalance). The second
way should result in charge balance.

As for pe, if you add the oxygen, the pe should be well defined. In the
absence of oxygen, the solutions have no redox buffering and very big
changes in pe have no real effect on the distribution of species. The
program is equally happy with any pH within about a 10 unit range and the
result is somewhat random. You can use method one to estimate the dissolved
oxygen in the solution  [O(0) 8 O2(g) -0.7]. Note that if you simply define
O(0), atmospheric solubility is about 8 mg/kgw, but units are mmol/kgw by


              temp      25
              pH        7
              pe        4
              units     mmol/kgw
              redox     O(-2)/O(0)
              density   1
              O(0)    8.0 mg/kgw
              water    1       # kg
                              C         1      CO2(g)  -3.5

              temp      25
              pH        7
              pe        4
              units     mmol/kgw
              redox     O(-2)/O(0)
              density   1
              O(0)    8.0 mg/kgw
              water    1       # kg
                           CO2(g)    -3.5    10.

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

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