NaOH Solution in Contact with CO2

Given is a 20 mM NaOH solution. How the pH changes when the solution is in (long-term) contact with atmospheric air? How much CO2 will be absorbed?

pH Values Before and After CO2 Adsorption

Due to the long-term contact with atmospheric air the solution will be in equilibrium with CO2. We solve this task in two steps:

  •  pH calculation of 20 mM NaOH without air contact (closed CO2 system)
  •  pH calculation of 20 mM NaOH with air contact (open CO2 system)

Closed CO2 System. We begin with pure water (click on button New), then click on Reac and enter for “NaOH” the value 20 mmol/L. Run the calculation with Start. The result will be instantly displayed:

pH = 12.24       (without CO2 absorbtion)

Open CO2 System. Repeat the above calculation with the only change: Activate the selection box Open CO2 System in the input panel before you click on Reac. The default value of pCO2 = 3.41 corresponds to the partial pressure of CO2 under normal conditions.

The result of the calculation is displayed in the right screenshot:

pH = 9.33       (with CO2 absorption)

Answer: Due to the contact with air the pH of the NaOH solution decreases from 12.24 to 9.33.

Amount of Adsorbed CO2 & Speciation

The last calculation delivers the composition of the final solution with pH 9.33:

Na = 20.00 mM
DIC = 17.33 mM

Thus, the amount of 17.33 mM CO2 will be absorbed from the atmosphere into the NaCl solution. The complete speciation you will find in the table Ions (at 25°C):

CO2(aq) 0.01 mM
HCO3- 14.53 mM
CO3-2 2.20 mM
Na+ 19.42 mM
NaHCO3 0.12 mM
NaCO3- 0.46 mM

It is easy to check that

Na+ + NaHCO3 + NaCO3-  =  20.00 mM
CO2(aq) + HCO3- + CO3-2 + NaHCO3 + NaCO3-  =  17.33 mM

[last modified: 2013-08-06]