HNO3 Contamination with nitrogen dioxide

Fuming nitric acid contaminated with yellow nitrogen dioxide

Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:

4 HNO3 → 2 H2O + 4 NO2 + O2

This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.

The nitrogen dioxide (NO2) and/or dinitrogen tetroxide (N2O4) remains dissolved in the nitric acid coloring it yellow or even red at higher temperatures. While the pure acid tends to give off white fumes when exposed to air, acid with dissolved nitrogen dioxide gives off reddish-brown vapors, leading to the common names “red fuming nitric acid” and “white fuming nitric acid”. Nitrogen oxides (NOx) are soluble in nitric acid.