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Keywords

GAS_PHASE

This keyword is used to define the composition of a fixed-total-pressure multicomponent gas phase. A GAS_PHASE data block is needed if a gas bubble (with a volume that is not infinite) at a fixed pressure equilibrates with an aqueous phase. A GAS_PHASE data block is not needed if fixed partial pressures of gas components are desired, which corresponds to an infinite-volume gas phase; use EQUILIBRIUM_PHASES instead. The gas phase defined with this keyword data block subsequently may be equilibrated with an aqueous phase in combination with pure-phase, surface, and exchange assemblages. As a consequence of reactions, the gas phase may exist or not, depending on the fixed pressure for the gas phase and the sum of the partial pressures of the dissolved gases in solution. The thermodynamic properties of the gas components are defined with PHASES input.

Example 

Line 0:  GAS_PHASE 1-5  Air
Line 1:       -pressure       1.0
Line 2:       -volume         1.0
Line 3:       -temperature    25.0
Line 4a:      CH4(g)          0.0
Line 4b:      CO2(g)          0.000316
Line 4c:      O2(g)           0.2
Line 4d:      N2(g)           0.78

Explanation

Line 0: GAS_PHASE [number] [description]

GAS_PHASE is the keyword for the data block.

number--positive number to designate this gas phase and its composition. Default is 1. A range of numbers may also be given in the form m-n, where m and n are positive integers, m is less than n, and the two numbers are separated by a hyphen without intervening spaces.

description--is an optional character field that describes the gas phase.

Line 1: -pressure pressure

-pressure--identifier defining the fixed pressure of the gas phase that obtains during all reactions. Optionally pressure, or -p[ressure].

pressure--the pressure of the gas phase, in atmospheres. Default is 1.0 atm.

Line 2: -volume volume

-volume--identifier defining the initial volume of the gas phase. Optionally, volume, or -v[olume].

volume--the initial volume of the gas phase, in liters. Default is 1.0 liter. The volume and temperature are used to compute the initial number of moles present in the gas phase.

Line 3: -temperature temp

-temperature--identifier defining the initial temperature of the gas phase. Optionally, temperature, or -t[emperature].

temp--the initial temperature of the gas phase, in Celsius. Default is 25.0. The volume and temperature are used to compute the initial number of moles present in the gas phase.

Line 4: phase name, partial pressure

phase name--name of a gas. A phase with this name must be defined by PHASES input.

partial pressure--initial partial pressure of this gas in the gas phase, in atmospheres. The partial pressure along with volume and temperature are used to compute the initial number of moles of this gas present in the gas phase.

Notes

Line 4 may be repeated as necessary to define all of the components initially present in the gas phase as well as any components which may subsequently enter the gas phase. The initial number of moles, of any gases that are defined to have positive partial pressures in GAS_PHASE input, will be computed using the ideal gas law, n = PV/RT, where n is the number of moles of the gas, P is the defined partial pressure (line 4), V is given by -volume, and T is given by -temperature. It is probable that the sum of the partial pressures of the defined gases will not be equal to the pressure given by -pressure. However, when the initial moles of gas components are brought in contact with a solution during a reaction simulation, the moles of gases and volume of the gas phase will adjust so that each component is in equilibrium with the solution and the total pressure is that specified by -pressure. It is possible that the gas phase will not exist if the sum of the partial pressures of dissolved gases does not exceed the pressure given by -pressure.

Some gas components may be defined to have initial partial pressures of zero. In this case, no moles of that component will be present initially, but the component will enter the gas phase when in contact with a solution. If no gas phase exists initially, the initial partial pressures of all components should be set to 0.0.

Example problems

The keyword GAS_PHASE is used in example problem 7.

Related keywords

EQUILIBRIUM_PHASES, PHASES, SAVE gas_phase, and USE gas_phase.

Example
Explanation
Notes
Example problems
Related keywords
User's Guide to PHREEQC - 07 MAY 96
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