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) |
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.
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.]