Input of CO2 Partial Pressure


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 blue 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.] The CO2 partial pressure is a very small quantity compared to the other gaseous components of our atmosphere.

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

Equilibrium with Atmospheric CO2

In order to bring an input solution into chemical equilibrium with the atmospheric CO2 one should use a partial pressure of 0.00039 atm – see table above. According to Eq.(1) we obtain

(2) pCO2   =   – log P  =  – log 0.00039   =   3.408

This is exactly the pCO2 used as default value in aqion’s input panel.

[last modified: 2014-12-02]