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co2(aq) and h2s

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Jowitawl:
Hello everyone,

i was wondering if someone could help me with initial solution data, precisely how to correctly implement them into PHREEQC program. my main problem is CO2 and H2S. I was given thermal fluids compositions samples, where for example CO2 = 26.2mg/kg, while my professor tells me that it is not totalDIC= CO2(aq) + HCO3 + CO3-2, it?s only CO2 pure gas phase in the water. Thereby i am struggling with figuring out how should i input this data. The same is actually with H2S, i have read about partial pressure adjustment, where it is possible to choose CO2(aq) or H2S and change its partial pressure for example in the EQUILIBRIUM_PHASES function, but i am later on equilibrating my model with the atmosphere. In solution input data C(4) element has a choice to set up a phase, so what if i type down c(4)=26.3 mg/kg and use CO2(g) in the phase in GENERAL SOLUTION input, and then the same with H2S will it be correctly applied input solution?

Jowita

dlparkhurst:
How is CO2 measured? If the sample is acidified and then CO2 is measured, then the measurement would be total dissolved inorganic carbon, and it could be entered as C(4). If it is the same for H2S, then you would enter S(-2).

Jowitawl:
then this is correct.

Thank you dr dlparkhurst for clarifying it for me!

Jowitawl:
Dear dlparkhurst,

I ran into another issue with CO2 sample. When i put C(4)=26.2 mg/kgw into PHREEQC, the output data tells me that the molality is C(4)=0,000431, while (26.2mg/kg/1000)/44,001mol= 0,000598 mol/kg of CO2. How PHREEQC calculates this molality differently? I am using phreeqc.dat

Thank you!

dlparkhurst:
The default gram formula weights for converting mass to moles is in the SOLUTION_MASTER_SPECIES data block of the database that you use.

For C(4) in phreeqc.dat, the gfw is calculated from the formula HCO3.

If you want to use CO2, one way to do it is the following:

--- Code: ---SOLUTION
-units mg/kgw
C(4) 26.2 as CO2

--- End code ---