Nitrous acid

Nitrous acid ([[HNO 2 ]]) .Also known as nitrite hydrogen , it is an acid monoprotic inorganic found only in solution and their salts, nitrites.
Other names
Dioxonitric (III) acid, hydrogen dioxonitrate (III), scouring acid, spirit of nitrate, Chilean acid, Stickstoff (III) -säure, Spirit of nitro or nitrate acid.


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  • 1 Structure
  • 2 Obtaining
  • 3 Chemical properties
    • 1 Decomposition
    • 2 Reducing properties
    • 3 Oxidizing properties
  • 4 Atmosphere of Earth
  • 5 Applications
  • 6 References
  • 7 Sources
  • 8 External links


Moderately weak and unstable molecule , with a molar mass of 47 g.mol -1 , with a dissociation constant K (HNO 2 ) = 6.0 · 10 -6 E (at 25 ° C), has polar covalent bond, has properties acidic, and it is monobasic.


Nitrous acid can be obtained by:

1.- Reaction of a mineral acid (such as sulfuric) with sodium nitrite.

Ba (NO 2 ) 2 (ac) + H 2 SO 4 (ac) = 2HNO 2 (ac) + BaSO 4 (ac

2.- Reaction of nitrogen oxide with water .

2 O 3 + H 2 O = 2H-O 2 (ac)


This acid is not very stable in aqueous solution and it has not been possible to isolate it as such, decomposing easily. A blue liquid containing nitrous acid is obtained , decomposing quickly giving off brownish vapors.

Chemical properties


In gaseous state it is remarkably dissociated.
Nitrous acid decomposes rapidly into nitrogen dioxide, nitrogen monoxide, and water vapor .

H-NO 2 (g) = NO (g) + NO 2 (g) + H 2 O (g)

If you try to concentrate it decomposes, being stable only in dilute and cold aqueous solutions. Decomposition is accelerated by heating or in contact with large surface substances, and proceeds according to the reversible reaction:

3H-NO 2 (g) = H-NO 3 (ac) + 2NO (g) + H 2 O (g)

Obtaining nitric acid , nitrogen monoxide and water vapor .

Reducing properties

As a consequence of its instability it is a very reactive substance.
It is an oxidant, being able to release iodine from iodides.

2I -1 (ac) + 2NO 2 (g) + 4H +1 (ac) = I 2 (s) + 2H 2 O + 2NO (g)

It is oxidized to nitric acid by strong oxidants such as potassium permanganate .

NO 2 ( 1 (ac) + H 2 O = NO 3 ( 1 (ac) + 2H +1 (ac)

Oxidizing properties

It is reduced to nitrogen monoxide and sometimes to nitrogen dioxide and even to dinitrogen

NO 2 ( 1 (ac) + 2H +1 (ac) = NO (g) + H 2 O

Oxides ferrous salts to ferric against more vigorous reducing agents (alkaline amalgams, electrolysis ) is reduced to N 2 O or NH 3 . On the contrary, it acts as a reducer against strong oxidizing agents such as KMnO 4 (nitrite determination), H 2 O 2 , PbO 2 , halogens, ozone and others, which convert it into nitric acid . Due to its property of diazotizing organic amines, it finds application in the chemistry of dyes.

Atmosphere of earth

Nitrous acid is involved in the ozone budget of the lowest atmosphere : troposphere . When the heterogeneous reaction of nitrous oxide (NO 2 ) and water that produces nitrous acid occurs on the surface of atmospheric aerosols, it is easily obtained as a product photolyses to radical hydroxy.


Nitrous acid is used to convert diamine amines , this occurs by the nucleophilic attack of the amine on nitrite by means of a mutagenic agent that causes the oxidative deamination of adenine and cytosine, causing transitions and the diazide can then be released.
In aqueous solutions, nitrous acid (HNO 2 ) oxidatively deaminates aromatic primary amines, thereby converting cytosine to uracil and adenine into the guanine-like molecule, hypoxanthine (which forms two of the three guanine bonds). with cytosine).
Effect of hypoxanthine as a mutagen. In such a way that the treatment of DNA with HNO 2or compounds like nitrosamines that react to form nitric acid, results in both A • T – G • C and G • C – A • T transitions.
It is used to prepare diazonium salts:
HNO 2 + ArNH 2 + H sup> +1 (ac) → ArN 2 + 2 H 2 O

Such salts are widely used in organic synthesis for the Sandmeyer Reaction and are used, and in the preparation azo dyes, brightly-colored compounds for which it is the basis of a qualitative test for anilines. It is used to destroy toxic and explosive potential sodium azide.
Its property in the diazotization of organic amines is very important in the manufacture of dyes.


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