Input of CO2 Partial Pressure
The Input Panel
In an open CO2 System, DIC is determined by the CO2 partial pressure. The corresponding panel (right screenshot) is part of the main input window. Three different types of pressure units can be selected by the light gray drop-down list:
| • pCO2 | (default) |
| • atm | (1 atm = 1.0133 ∙ 105 Pa) |
| • Pa | (SI unit: 1 Pa = 1 kg m-1 s-2 = 1 Nm-2) |
The parameter pCO2 is related to the partial pressure P by:
| (1) | P (atm) = 10-pCO2 |
Note its similarity to the definition of the pH value.
Composition of the Earth’s Atmosphere (Partial Pressures)
The table below presents the composition of the atmosphere at normal conditions (dry atmosphere at sea level):
| partial pressure % | Pa | atm | ||
|---|---|---|---|---|
| nitrogen | N2 | 78.090 | 79 120 | 0.78090 |
| oxygen | O2 | 20.950 | 21 230 | 0.20950 |
| argon | Ar | 0.927 | 939 | 0.00927 |
| carbon dioxide | CO2 | 0.039 | 39 | 0.00039 |
| air | 100 | 101 328 | 1.0 |
In fact, the CO2 partial pressure is a very small quantity compared to the other gaseous components of our atmosphere.
Equilibrium with Atmospheric CO2
To bring an input solution into chemical equilibrium with atmospheric CO2, a partial pressure of 0.00039 atm should be used – see table above. According to 1 we obtain
| (2) | pCO2 = – log P = – log 0.00039 = 3.408 |
This is exactly the pCO2 that is used as the default in aqion’s input panel.