How do I calculate pCO2 from alkanity, pH and temp record?

[Komla Kofi]

Please, how do I simulate pCO2 in addition to the CO2(g) and many other saturation indices? I 've attached the dataset below.
Thanks.
Komla Kofi

[dlparkhurst]

The quick answer is the value for CO2(g) under "SI**" in the list of saturation indices is the log10 partial pressure of CO2(g) that would be in equilibrium with the solution, unless you are using GAS_PHASE and phreeqc.dat or pitzer.dat. If you are using GAS_PHASE and either of those databases, then the value is still going to be very close to the partial pressure of CO2(g) unless you are at high temperature or pressure.

[Komla Kofi]

Hello Sir, Please, thank you for the prompt response. If I understand your point, the log10(pCO2) = si_CO2(g), but I want pCO2. If I have say, an si_CO2(g) =-3.42, I assume the natural log of -3.42 should return the pCO2 value but these numbers are undefined. What should I do?

Thank you once again.

[dlparkhurst]

The partial pressure of CO2(g) that would be in equilibrium with the solution is equal to 10^SI(CO2(g)). In your example, it would be 10^-3.42.

[Komla Kofi]

I appreciate this clarification, Sir and thank you.
One more and then I think I will be ok. Since this partial pressure of CO2, in my case is what would be in equilibrium with my solution, does it mean that this solution with SI (CO2) =-3.42, and pCO2 of 380E-6 atm { or f(CO2) =38.52269058, assuming ideal gas phi of 1}), shows the sample can dissolve more CO2(g) from the atmosphere or it is somewhat unable to take in more CO2 (g) from the atmosphere? Or is this the CO2 released when the sample equilibrates with the atmosphere?

[dlparkhurst]

Atmospheric partial pressure of CO2 is approximately 10^-3.4, so the solution would be at equilibrium with the atmosphere within uncertainties If the SI were lower (<-3.4) then CO2 would move from the atmosphere to the solution. If the SI were higher (>-3.4), CO2 would move from the solution to the atmosphere.

[Komla Kofi]

Merci!