In accordance with the Clean Air Act, nitrogen dioxide (NO2) is a criteria air pollutant. Thus, it has always been of interest to the gas detection community. But, what about dinitrogen tetroxide (N2O4), a name heard primarily within the aerospace community? In that field, N2O4 is a hypergol (or hypergolic propellant).
Hypergols are used in rocket engines, and spontaneously ignite when they come in contact with each other. Typically, one component is known as the “fuel,” the other as the “oxidizer.” Hypergolic fuels include hydrazine and its derivatives such as monomethyl hydrazine (MMH), unsymmetrical dimethylhydrazine (UDMH), and Aerozine 50 (A-50), which is an equal mixture of hydrazine and UDMH. The oxidizer used with these fuels is usually dinitrogen tetroxide, and various blends of N2O4 with nitric oxide.
As you can see, N2O4 is a dimer of NO2. An equilibrium between the two structures exists as per…
2 NO2 (g) <—> N2O4 (g)
We note Le Chatelier’s Principle: If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change. Temperature can affect this equilibrium, as demonstrated in the video. NO2 is reddish-brown, while N2O4 is clear.
This is why Cameo Chemicals describes nitrogen tetroxide as follows…
Red-brown liquid with a sharp, unpleasant chemical odor. Low-boiling (boiling point 21.15°C / 70.07°F) and held as a liquid by compression. Consists of an equilibrium mixture of brown NO2 (nitrogen dioxide) and colorless N2O4 (dinitrogen tetroxide).