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The handling of gases with PHREEQC can be confusing. There are several
different ways to conceive of the physical situation.

If you want to fix the pH of a solution and determine how much carbon(4)
would be in the solution to produce a specified partial pressure of CO2,
then the problem is defined with only a SOLUTION data block. The program
will take the initial estimate of C(4) concentration (.01 mmol/kgw) and
adjust it until the log P(CO2) is -3.5; pH will be 7.0.

SOLUTION 1
temp 25.0
pH 7
pe 4
redox pe
units mg/kgw
C(4)  .01 CO2(g)  -3.5
END

If you want to take pure water and add CO2(g) to produce a specified
partial pressure of CO2, then  SOLUTION and an EQUILIBRIUM_PHASES data
blocks are used. Pure water is speciated and then a reaction calculation
equilibrates the solution with CO2(g). In the reaction calculation, the pH
will adjust as the CO2 is added to the solution. This is the most common
way of handling a gas, and the key is that the partial pressure is desired
to be fixed.

SOLUTION 1
temp 25.0
pH 7
pe 4
redox pe
units mg/kgw
EQUILIBRIUM_PHASES
CO2(g)      -3.5
END

The GAS_PHASE datablock defines an initial gas phase composition. The
program then reacts this gas phase with a solution and the gas components
are distributed between the aqueous and the gas phase. During the reaction,
the partial pressure of the gas components are not fixed, they vary with
reaction. The gas phase maintains either a fixed volume, with the total
pressure varying, or a fixed total pressure with the volume varying.
Neither -fixed_pressure nor -fixed_volume are defined in your example, so
the default of fixed pressure is used. The physical setup for your input
file is that CO2 is present initially in a volume of 1 Liter at a partial
pressure of .00035 atmospheres; you can calculate how many moles of CO2 are
in the gas phase from the ideal gas law PV = nRT. Note that -pressure
defines the pressure that is obtained during the reaction and -volume
defines the initial volume of the gas phase (for a -fixed_pressure gas
phase). This gas phase with a  number of moles of CO2 calculated by the
ideal gas law is allowed to react with a kilogram of pure water. The CO2
dissolves into the water and in this case the gas phase disappears because
even by putting all of the CO2 into the dissolved phase, the partial
pressure of CO2 in the water is less than 1 atmosphere, the pressure
specified to be necessary to maintain a separate gas phase. If you increase
the initial volume of the gas phase to 1000 liters, you will have more CO2,
but it still dissolves completely into the water phase. If you increase the
initial  volume to 5000 Liters, you have enough CO2 that  a liter of gas
remains after equilibration with the water; the partial pressure of CO2 is
1 atmosphere (as specified by -pressure). If you change the pressure to
1000, you get the same result as a pressure of 1 atm because the partial
pressure is less than 1 after the reaction, which is also less than 1000
atm.

GAS_PHASE 1
-pressure      1
-volume        1
-temperature   25
CO2(g) 0.00035
END

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

Michele Y Crouse
To:      David L Parkhurst/WRD/USGS/DOI@xxxx
08/06/01 10:05           cc:      gs-w_h2osoft@xxxxxxxx

----- Forwarded by Michele Y Crouse/WRD/USGS/DOI on 08/06/01 01:04 p.m.
-----

"Kaoru Masuda"
<k-masuda@xxxxxxxxxxx         To:      <h2osoft@xxxxxxxx>
elco.co.jp>                   cc:      <seik@xxxxxxxxxxxxxxxxxx>
08/05/01 11:42 p.m.

Dear Sir,

I am a user of PHREEQC interactive.  And I have some questions
Would you please transfer this e-mail to someone who knows it well?

I want to calculate CO2 gas dissolution into water solution.
For example, If 350ppmV-CO2 gas phase exists with a solution of pH=7,
I think that the *.in file must be as follows;

SOLUTION 1
temp 25.0
pH 7
pe 4
redox pe
units mg/kgw
GAS_PHASE 1
-pressure      1
-volume        1
-temperature   25
CO2(g) 0.00035
END

The result of the calculation with abobve *.in file shows that
total C in the solution is 1.4E-5 mol/L and pH is 5.6.
I think this result is reasonable.
However, by replacing the volume to 1000 instead of 1, the
result shows that total C is  1.4E-2 and pH=4.1.
On the other hand, by replacing pressure value to 1000 instead of 1.
total C is 1.4e-5 and pH=5.6.
I think that the solubility has not be changed with volume
by Henry's Law.

Yours Sincerly,

Mr. KAORU MASUDA
Kobe Steel, Ltd.
1-5-5, Takatsuka-dai Nishi-ku
Kobe 651-2271
JAPAN
Tel  Japan-78-992-5733
Fax Japan-78-992-5547

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Complete Water Resources Division Software