This example demonstrates the capabilities of PHREEQC to model
the evolution of gas compositions in equilibrium with a solution with a
fixed (total) pressure or a fixed volume of the gas phase. In the case of a
fixed-pressure gas phase, a gas bubble forms as soon as the sum of the
partial pressures of the component gases exceeds the specified pressure of
the gas phase. Once the bubble forms, its volume and composition will vary
with the extent of reactions. This case applies to gas bubbles forming in
surface water or groundwater at a given depth, where the total pressure is
constant. With a fixed-volume gas phase, the aqueous solution is in contact
with a head space of a fixed volume, which is typical for a laboratory
experiment with a closed bottle. The gas phase always exists in this head
space, but its pressure and composition will vary with the reactions.
Another way to model gas-liquid reactions in PHREEQC is to maintain a fixed
partial pressure by using the EQUILIBRIUM_PHASES data block. This
fixed-partial-pressure approach is illustrated in this example by fixing
the CO2 pressure for a SOLUTION. The example can be run using the
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