Highly toxic and corrosive dinitrogen tetroxide is in equilibrium with NO2 in its liquid and vapor states. Its brownish color comes from NO2; N2O4 is colorless. It is used commercially as an intermediate in the manufacture of HNO3 and as a nitrating and oxidizing agent. N2O4 is the oxidizer of choice for storable rocket propellants because it is hypergolic with hydrazine fuels.
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