Determination of pCO2 that produces a given Ca conc. in equilibrium with calcite

[MichaelZ20]

Hello!
I need to find PCO2 that produces the given Ca concentrations in equilibrium with calcite. In a single case, I can do this by trial and error. Does more elegant way exist in a case of necessity of numerous calculations of this sort?

[dlparkhurst]

If you want to fix the pH, you can adjust the total carbon to calcite equilibrium, or, if you want to fix the total carbon, you can adjust the pH as below, or you can fix the alkalinity and adjust the pH.

Code: [Select]

SOLUTION
pH 7
Ca 1
C 1 calcite
END
SOLUTION
pH 7 calcite
Ca 1
C 1
END
SOLUTION
pH 7 calcite
Ca 1
Alkalinity 1
END


[MichaelZ20]

David, thank you!
This is not what I want. I need just pCO2(g) in an open system that in equilibrium with calcite produces the known Ca concentration. I tried using Fix_Ca by CO2(g), but in this case, I get an addition of CO2 and not pCO2. May be, it is possible to receive pCO2 through some script, but I failed in this.

[dlparkhurst]

So, you know the Ca and pH? Alkalinity?

If all you need is the P(CO2), you can obtain it with Basic

USER_PRINT
10 PRINT "log10(P(CO2)): ", SI("CO2(g)")

[MichaelZ20]

Unfortunately, I have no reliable pH. My water is fresh, and I thought that it is possible to estimate pCO2(g) using the described simplified model.

[dlparkhurst]

Do you know the alkalinity? It is independent of pCO2.

[MichaelZ20]

Unfortunately, no. Only HCO3, but also not so reliable.