PCO2 Equilibrium from Contact with Ca

[teimah]

Brand new PHREEQC user here. I was wondering how I would set this up.

I have an X amount of Ca concentration at a certain temp and pH in solution and I am looking to calculate equilibrium PCO2 that is in contact with Ca. I know to use SOLUTION and enter in my initial conditions but beyond that I am kinda lost.

Thanks in advance!

[dlparkhurst]

You need a bit more information or an assumption to calculate a PCO2.

(1) Best is to analyze alkalinity and enter it in the SOLUTION definition.

(2) You could assume that HCO3- is the only missing or erroneous analyte and charge balance the solution by adjusting the carbon concentration.

(3) You could assume the solution is in equilibrium with calcite, and adjust the carbon concentration calcite to equilibrium.

(4) You could specify a PCO2 and adjust the carbon concentration to achieve it.

Note that options 3 and 4 will not be charge balance, and a large charge imbalance may indicate that you are missing important ions in your solution.

Here is a script that performs options 2 through 4 assuming 1 mmol/L and pH 7.5.

Code: [Select]

USER_PRINT
10 PRINT "PCO2: ", SI("CO2(g)")

SOLUTION  Charge balance with HCO3-
pH 7.5
-units mmol/kgw
Ca 1
C(4) 5 charge
END

SOLUTION  Adjust carbon to equibrium with calcite
pH 7.5
-units mmol/kgw
Ca 1
C(4) 5 Calcite 0.0
END

SOLUTION  Adjust carbon to specified log10(PCO2)
pH 7.5
-units mmol/kgw
Ca 1
C(4) 5 CO2(g) -3.4
END


[teimah]

Thanks so much for the guidance! I will give it a shot :D