NaOH Solution in Contact with CO2

Given is a 20 mM NaOH solution. How does the pH change when the solution is in contact with air for a long time? How much CO2 from air is captured in the solution?

pH before and after CO2 Contact

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

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

Closed CO2 System.  Start with pure water (button H2O), then click on button Reac, select the reactant “NaOH” and enter 20 mmol/L. Run the calculation with button Start. The result is displayed immediately:

  pH = 12.24 (without CO2 exchange)

input NaOH with air contact

Open CO2 System.  Repeat the calculation, but now click on Setup, and activate “Open CO2 System” as shown in the screenshot on the right. The default value (pCO2 = 3.408) corresponds to normal conditions in the atmosphere. We adopt it. Then click on Start.

Output NaOH with air contact

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

  pH = 9.33 (with CO2 exchange)

Answer: Due to the air contact and the capture of CO2 (as carbonic acid), the pH value of the caustic soda solution decreases from 12.24 to 9.33.

Amount of Captured CO2 & Speciation

The last calculation delivers the equilibrium composition of the final solution:

pH = 9.33
Na = 20.00 mM
DIC = 17.33 mM

Thus, the amount of 17.33 mM CO2 is absorbed from the atmosphere into the NaOH solution. The complete speciation is listed in aqion’s output table Ions (at 25):

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

The mass balance (molar balance) can be easily checked by summing up the species:

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

[Remark: Examples of speciation can be found in the PowerPoint Lecture.]

[last modified: 2023-11-05]