Partial pressure GAS_PHASE input

[Franco Galarce]

Hello,

I have concentration data of a list of gas species, and I need to enter that data into phreeqc. I decided to calculate the partial pressures dividing each concentration by the sum, but in that case the values would be dimensionless, and the software specifies that it is in atmospheres.

I have another question. I don't know very well the function of the options "fixed pressure" and "fixed volume", and which one would be useful to me.

GAS_PHASE 1
    -fixed_volume ?
    -equilibrium with solution 1
    -pressure 1
    -volume 1
    -temperature 112
    Ar(g)     1.35e-06
    CH4(g)    1e-12
    CO(g)     4.46e-09
    CO2(g)    0.002183911
    H2(g)     1e-12
    H2O(g)    0.998
    H2S(g)    6.02e-06
    He(g)     2.9e-08
    N2(g)     2.17e-05

Regards

[dlparkhurst]

You should multiply those values by the total pressure to get atm.

The next consideration is fixed_pressure or fixed_volume. For fixed_pressure, when the gas reacts with a solution, the volume of the gas increases or decreases to maintain a specified pressure after the reaction, like a balloon or a gas bubble in lake sediments. For fixed_volume, the volume is constant, but the pressure adjusts to reactions between gas and solution, like a fixed-head space in a syringe.

If you want to know the water composition in equilibrium with that gas phase, you can make the gas volume large relative to the water volume; if the volume is large enough, the mole transfers to the solution will cause little change in the gas phase.

Finally, when you react the gas phase with a solution, redox reactions will occur for CH4, CO, H2, H2S, and N2. You can ignore the redox reactions if you define unreactive versions of these gases. Phreeqc.dat has definitions for Hdg (H2), Oxg (O2), Mtg (CH4), Sg (H2S), and Ntg (N2). If you use these definitions, the gases cannot react with each other.