Nitrous acid (7782-77-6)

Nitrous acid is used to prepare diazonium salts:
HNO₂ + ArNH₂ + H+ → ArN₂+ + 2 H₂O
where Ar is an aryl group.
Such salts are widely used in organic synthesis, e.g., for the Sandmeyer reaction and in the preparation azo dyes, brightly-colored compounds that are the basis of a qualitative test for anilines. Nitrous acid is used to destroy toxic and potentially-explosive sodium azide. For most purposes, nitrous acid is usually formed in situ by the action of mineral acid on sodium nitrite:
NaNO₂ + HCl → HNO₂ + NaCl
2 NaN₃ + 2 HNO₂ → 3 N₂ + 2 NO + 2 NaOH

EPA Genetic Toxicology Program. Reported in EPA TSCA Inventory.