Appropriate gas volume assumption for water exposed to atmosphere

[AnotherSLGuy]

Hello,

I tried to simulate the HCO3- in water exposed to atmosphere using gas phases and noticed that it is sensitive to the volume of gas I use.

So, I would be very thankful if I can get some information on what the appropriate assumptions/inputs in regard to air volume to simulate the dissolved HCO3- amount in a water container exposed to atmosphere are.

Following is the code I used.

Code: [Select]

SOLUTION 1
    temp      10
    pH        7
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    C(4)      0
    -water    1 # kg

GAS_PHASE 1
    -fixed_pressure
    -pressure 1
    -volume 1
    -temperature 10
    CO2,g     0.00043


[dlparkhurst]

It depends on your assumptions. If there is an infinite amount of gas, then use EQUILIBRIUM_PHASES to fix the pCO2 of the solution. Otherwise, it depends on the physical conditions that you are modeling.

[AnotherSLGuy]

Thank you for the input.

There is an infinite amount of gas. So I suppose this is the solution;

Code: [Select]

SOLUTION 1
    temp      10
    pH        7
    pe        4
    redox     pe
    units     mol/kgw
    density   1
    C(4)      0
    -water    1 # kg

EQUILIBRIUM_PHASES 1
    CO2,g     -3.3665


[dlparkhurst]

That is one way. The pH will change with the addition of CO2, and you probably want to SAVE solution 2 or similar.

Alternatively, you can adjust the C(4) to equilibrium within the SOLUTION definition.

Code: [Select]

C(4) 1 CO2(g) -3.4

With this approach you can optionally adjust the pH to charge balance, which makes sense for your solution, or you can leave the pH as specified, which makes sense for field measurements.

Your choice.