Nitrous acid

Nitrous acid
Nitrous acid
CAS number 7782-77-6 YesY
PubChem 24529 YesY
ChemSpider 22936 YesY
EC number 231-963-7
KEGG C00088 YesY
MeSH Nitrous+acid
ChEBI CHEBI:25567 YesY
Gmelin Reference 983
3DMet B00022
Jmol-3D images Image 1
Molecular formula HNO2
Molar mass 47.013 g/mol
Appearance Pale blue solution
Density Approx. 1 g/ml
Melting point

Only known in solution

Acidity (pKa) 3.398
EU Index Not listed
Flash point Non-flammable
Related compounds
Other anions Nitric acid
Other cations Sodium nitrite
Potassium nitrite
Ammonium nitrite
Related compounds Dinitrogen trioxide
 N (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Nitrous acid (molecular formula HNO2) is a weak and monobasic acid known only in solution and in the form of nitrite salts.

Nitrous acid is used to make diazides from amines; this occurs by nucleophilic attack of the amine onto the nitrite, reprotonation by the surrounding solvent, and double-elimination of water. The diazide can then be liberated to give a carbene or carbenoid.



In the gas phase, the planar nitrous acid molecule can adopt both a cis and a trans form. The trans form predominates at room temperature, and IR measurements indicate it is more stable by around 2.3 kJ mol−1.[1]

dimensions of the trans form
(from the microwave spectrum)
model of the trans form
cis form


When cold, dilute solutions of nitrite ion, NO2 are carefully acidified, a light blue solution of nitrous acid is produced. Free nitrous acid is unstable and decomposes rapidly.


In anything other than very dilute, cold solutions, nitrous acid rapidly decomposes into nitrogen dioxide, nitric oxide, and water:

2 HNO2 → NO2 + NO + H2O

Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous solution:[2]

2 NO2 + H2O → HNO3 + HNO2

In warm or concentrated solutions, the overall reaction amounts to production of nitric acid, water, and nitric oxide:

3 HNO2 → HNO3 + 2 NO + H2O


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.[3] 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:[4]

NaNO2 + HCl → HNO2 + NaCl
2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH

Reaction with two α-hydrogen atoms in ketones creates oximes, which may be further oxidized to a carboxylic acid, or reduced to form amines. This process is used in the commercial production of adipic acid.

Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl nitrites, which are potent vasodilators:

(CH3)2CH-CH2-CH2-OH + HNO2 → (CH3)2CH-CH2-CH2-ONO + H2O

Atmosphere of the earth

Nitrous acid is involved in the ozone budget of the lower atmosphere: the troposphere. The heterogeneous reaction of nitrogen monoxide (NO) and water produces nitrous acid. When this reaction takes place on the surface of atmospheric aerosols, product readily photolyses to hydroxyl radicals.

See also


  1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419.  p. 462
  2. ^ Kameoka, Yohji; Pigford, Robert (February 1977). "Absorption of Nitrogen Dioxide into Water, Sulfuric Acid, Sodium Hydroxide, and Alkaline Sodium Sulfite Aqueous". Ind. Eng. Chem. Fundamen. 16 (1): 163–169. doi:10.1021/i160061a031. 
  3. ^ Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941).
  4. ^ Prudent practices in the laboratory: handling and disposal of chemicals. Washington, D.C.: National Academy Press. 1995. ISBN 0309052297. 

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Look at other dictionaries:

  • Nitrous acid — Nitrous Ni trous, a. [L. nitrosus full of natron: cf. F. nitreux. See {Niter}.] 1. Of, pertaining to, or containing, niter; of the quality of niter, or resembling it. [1913 Webster] 2. (Chem.) Of, pertaining to, or designating, any one of those… …   The Collaborative International Dictionary of English

  • nitrous acid — ► NOUN ▪ an unstable, weak acid made by the action of acids on nitrites …   English terms dictionary

  • nitrous acid — n. an acid, HNO2, known only in solution or in the form of its salts ( nitrites) …   English World dictionary

  • nitrous acid — nitrous acid. См. азотистая кислота. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • nitrous acid — Chem. an acid, HNO2, known only in solution. [1670 80] * * * ▪ chemical compound       (HNO2), an unstable, weakly acidic compound that has been prepared only in the form of cold, dilute solutions. It is useful in chemistry in converting amines… …   Universalium

  • nitrous acid — HNO2; a standard biologic and clinical laboratory reagent. * * * nitrous acid n an unstable acid HNO2 known only in solution or in the form of its salts * * * ni·trous ac·id (niґtrəs) a weak acid, HNO2, existing only in aqueous solution …   Medical dictionary

  • nitrous acid — nitrito rūgštis statusas T sritis chemija formulė HNO₂ atitikmenys: angl. nitrous acid rus. азотистая кислота ryšiai: sinonimas – vandenilio dioksonitratas (1–) …   Chemijos terminų aiškinamasis žodynas

  • nitrous acid — noun an unstable inorganic acid known only in solution and as nitrite salts • Derivationally related forms: ↑nitrify • Hypernyms: ↑acid …   Useful english dictionary

  • nitrous acid — noun Date: 1788 an unstable acid HNO2 known only in solution or in the form of its salts or esters …   New Collegiate Dictionary

  • nitrous acid — noun A weak acid, HNO, known only in solution, or as its salts and esters, the nitrites; it has a linear structure, H O N=O …   Wiktionary

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