Nitrous acid

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Nitrous acid
Nitrous acid
Preferred IUPAC name
Nitrous acid
Systematic IUPAC name
3D model (JSmol)
3DMet B00022
ECHA InfoCard 100.029.057
EC Number 231-963-7
MeSH Nitrous+acid
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
Flash point Non-flammable
Related compounds
Other anions
Nitric acid
Other cations
Sodium nitrite
Potassium nitrite
Ammonium nitrite
Related compounds
Dinitrogen trioxide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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. It can be produced by dissolving dinitrogen trioxide in water according to the equation

N2O3 + H2O → 2 HNO2


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

The nitric oxide can subsequently be re-oxidized by air to nitric acid, making the overall reaction:

2 HNO2 + O2 → 2 HNO3


Inorganic chemistry[edit]

Reduction of the acid gives different products, depending on the reducing agent:[3]

With I and Fe2+ ions, NO is formed:

2 KNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4
2 KNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4

With Sn2+ ions, N2O is formed:

2 KNO2 + 6 HCl + 2 SnCl2 → 2 SnCl4 + N2O + 3 H2O + 2 KCl

With SO2 gas, NH2OH is formed:

2 KNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH

With Zn in alkali solution, NH3 is formed:

5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2

With N2H5+, HN3, and subsequently, N2 gas is formed:

HNO2 + [N2H5]+ → HN3 + H2O + H3O+
HNO2 + HN3 → N2O + N2 + H2O

Oxidation by nitrous acid has a kinetic control over thermodynamic control, this is best illustrated that dilute nitrous acid is able to oxidize I to I2, but dilute nitric acid cannot.

I2 + 2 e ⇌ 2 I {Eo = +0.54 V}
NO3 + 3 H+ + 2 e ⇌ HNO2 + H2O {Eo = +0.93 V}
HNO2 + H+ + e ⇌ NO + H2O {Eo = +0.98 V}

It can be seen that the values of Ecello for these reactions are similar, but nitric acid is a more powerful oxidizing agent. Base on the fact that dilute nitrous acid can oxidize iodide into iodine, it can be deduced that nitrous is a faster, rather than a more powerful, oxidizing agent than dilute nitric acid.[3]

Organic chemistry[edit]

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.[4] 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:[5] It is mainly blue in colour

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[edit]

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

See also[edit]


  1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.  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. ^ a b Catherine E. Housecroft; Alan G. Sharpe (2008). "Chapter 15: The group 15 elements". Inorganic Chemistry, 3rd Edition. Pearson. p. 449. ISBN 978-0-13-175553-6. 
  4. ^ Clarke, H. T.; Kirner, W. R. "Methyl Red" Organic Syntheses, Collected Volume 1, p.374 (1941).
  5. ^ Prudent practices in the laboratory: handling and disposal of chemicals. Washington, D.C.: National Academy Press. 1995. ISBN 0-309-05229-7.