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 In-Reply-To: <OFC81E35F3.4E4CAEB8-ON85256AA0.005DD52E@xxxxxxxx> AM Subject: Question about PHREEQCI ----- 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> In-Reply-To: <OFC81E35F3.4E4CAEB8-ON85256AA0.005DD52E@xxxxxxxx> Subject: Question about PHREEQCI 08/05/01 11:42 p.m. Dear Sir, I am a user of PHREEQC interactive. And I have some questions about the calculation. 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. Do I have some misunderstanding about this ? Please give me some comments about it. 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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