I know $\ce{NaOH}$ can absorb $\ce{NO2}$, but I could not find any data of solubility of $\ce{NO2}$ in $\ce{NaOH}$ at room temperature and atmospheric pressure.

I am trying to figure out how much NaOH solution I will need to absorb the $\ce{NO2}$ gas being produced from a result of another reaction. My setup is having $\ce{NaOH}$ solution in a beaker (how much is what I want to calculate) and feed in the $\ce{NO2}$ gas into the solution. Is there any way to theoretically calculate the quantity of $\ce{NaOH}$ I will need to absorb the $\ce{NO2}$ gas without solubility data?

  • 4
    $\begingroup$ NO2 reacts with NaOH, forming NaNO2 + NaNO3, so solubility does not make sense. $\endgroup$
    – Poutnik
    Nov 28, 2021 at 7:05
  • $\begingroup$ You may find useful these links for text formatting ( not to be applied to titles ): notation , formulas/expressions/equations and upright vs italic $\endgroup$
    – Poutnik
    Nov 28, 2021 at 7:46
  • $\begingroup$ @Poutnik Relevant line of argument: chemistry.stackexchange.com/questions/42696/… $\endgroup$ Nov 28, 2021 at 9:39
  • $\begingroup$ @NilayGhosh Well, bromine water is in very majority a true solution, with the equilibrium strongly shifted to the left. For NO2/NaOH, it is the opposite. $\endgroup$
    – Poutnik
    Nov 28, 2021 at 9:50
  • $\begingroup$ By other words, practically, it is possible to achieve saturated solution Br2(l)<=>Br2(g)<=>Br2(aq), but not NO2(g)<=>[NaOH]NO2(aq). NO2 or NaOH will be spent. $\endgroup$
    – Poutnik
    Nov 28, 2021 at 10:31

1 Answer 1


A numerous studies have shown that the absorbtion of $\ce{NO2}$ in alkaline solutions is due to the reaction (For example, Ref.1):

$$\ce{2NO2 + 2OH- -> NO2- + NO3- + H2O} \tag1$$

Thus, as Poutnik pointed out, you may use dilute solution of $\ce{NaOH}$ solution, if you preferred. For insterd, Ref.2 has used sodalime, mainly $\ce{Ca(OH)2}$, in its solid form (following is the method in the paper for your benefit):

The test gas containing $\ce{NO2}$ $\pu{57 ppm}$ in high purity nitrogen and a trace of oxygen was passed at a flow rate of $\pu{1 litre min-1}$ through the column filled with soda lime $\pu{120 g}$ (Soda sorb) and the $\ce{NO2}$ concentration was measured continuously at the outlet of the column by an electrochemical fuel cell technique (TM-100, Taiyo-Sanso, Co., Ltd, Osaka). Break point was determined as the time when $5\%$ of the initial concentration of $\ce{NO2}$ was detected.

The authors have used two different varieties of soda limes, Soda sorb (which has the chemical composition of $73\% \ \ce{Ca(OH)2}, 5\% \ \ce{KOH}, 3\% \ \ce{NaOH}$, and less than $19\% \ \ce{H2O}$ with ethyl violet colour indicator) and Wako lime-A (which has the chemical composition of $81\% \ \ce{Ca(OH)2}, 4\% \ \ce{NaOH}$, and $15\% \ \ce{H2O}$ with ethyl violet colour indicator). The difference between these two types is the absence of $\ce{KOH}$ in Wako lime-A. The authors have claimed that they have shown that soda lime completely absorbed $\ce{NO2}$ regardless of the type of soda lime used. Thus, it is safe to suggest that the only necessity is the presence of $\ce{OH-}$ in the system.

Further, according to Ref.2, following equilibrium would establishe in aqeous medium:

$$\ce{2NO2(g) + H2O (l) <=> HNO2 (aq) + NO3- (aq) + H+ (aq)} \tag2$$

The following is the best result of particular experiment measuring the absorption of $\ce{NO2}$ (for example, flue gas used: $\pu{800 ppm}$ $\ce{NO2}$ in $\ce{N2}$ with flowrate of $\pu{1 L/min}$) in caustic aqueous scrubbing solution, which, for example, is $\pu{1 L}$ of $\pu{0.5 M}$ $\ce{NaOH}$ for the best result (all experiments have been conducted at room temperature and pressure). The average percent absorption $(n = 3)$ is reported as $98.1\%$ in $\pu{0.5 M}$ $\ce{NaOH}$ solution. For compartion, it was reported as $89.9\%$ in $\pu{0.1 M}$ $\ce{NaOH}$ solution and $81.8\%$ in $\pu{0.01 M}$ $\ce{NaOH}$ solution. Thus, even though change in absorption varies from different concentration of $\ce{NaOH}$ used, the difference is close enough to say concentration would not be effective if you have increased the time of exposure of gas to the scrubbing solution.


  1. T. Ishibe, T. Sato, T. Hayashi, N. Kato, and T. Hata, "Absorption of nitrogen dioxide and nitric oxide by soda lime," British Journal of Anaesthesia 1995, 75(3), 330-333 (DOI: https://doi.org/10.1093/bja/75.3.330)(PDF).
  2. Chen-Lu Yang, "Aqueous Absorption of $\ce{NO_x}$ Induced by Sodium Chlorite Oxidation," Master's Thesis 1989, New Jersey Institute of Technology, NJ.

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