IUPAC Name: Nitrous acid
Synonyms: Nitrous acid ; Nitrite anion ; Nitrogen dioxide ; Nitrogen dioxide ion ; Nitrogen peroxide ion
CAS NO:7782-77-6
Molecular Formula of Nitrous acid (CAS NO.7782-77-6) :HNO₂
Molecular Weight of Nitrous acid (CAS NO.7782-77-6) :47.01
Molecular Structure of Nitrous acid (CAS NO.7782-77-6) :
EINECS: 231-963-7
Index of Refraction:1.394
Surface Tension: 61.7 dyne/cm
Density: 1.54 g/cm³

Preparation Products :Sodium nitrite-->2-Chlorobenzoic acid-->NITRIC OXIDE-->ACID BLUE 113-->O-(2-MethoxyEthoxy)BenzeneSulfonamide-->2,5-Dichlorophenol-->BASIC RED 46-->2-[(2-cyanoethyl)[4-[(6-nitrobenzothiazol-2-yl)azo]phenyl]amino]ethyl acetate -->Disperse Yellow 163-->Enoximone 
Nitrous acid can be prepared by adding any mineral acid to sodium nitrite.

EPA Genetic Toxicology Program. Reported in EPA TSCA Inventory.

Mutation data reported. Flammable by chemical reaction; a powerful oxidizer. Explodes on contact with phosphorus trichloride. Reacts violently with PH₃ and PCl₃. Reactions with 1-amino-5-nitrophenol, ammonium decahydroborate(2−), hydrazine (product is hydrogen azide) may give explosive products. Incompatible with anilines (e.g., 4-bromoaniline, 2-chloroaniline, 3-chloroaniline, 2-nitroaniline, 3-nitroaniline, 4-nitroaniline, aniline), semicarbazone, silver nitrate. When heated to decomposition it emits highly toxic fumes of NO_x. See also NITRIC OXIDE.

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