# Solubility of nitrogen dioxide in sodium hydroxide

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?

• NO2 reacts with NaOH, forming NaNO2 + NaNO3, so solubility does not make sense. Nov 28, 2021 at 7:05
• You may find useful these links for text formatting ( not to be applied to titles ): notation , formulas/expressions/equations and upright vs italic Nov 28, 2021 at 7:46
• @Poutnik Relevant line of argument: chemistry.stackexchange.com/questions/42696/… Nov 28, 2021 at 9:39
• @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. Nov 28, 2021 at 9:50
• 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. Nov 28, 2021 at 10:31

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.

References:

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.