Nitrous acid

Structural formula
General
Surname	Nitrous acid
other names	Hydrogen nitrite Nitrogen (III) acid Acidum nitrosum
Molecular formula	ENT 2
External identifiers / databases
CAS number 7782-77-6 EC number 231-963-7 ECHA InfoCard 100,029,057 PubChem 24529 ChemSpider 22936 DrugBank DB09112 Wikidata Q211891
properties
Molar mass	47.01 g mol −1
Physical state	only stable in cold aqueous solution
pK s value	3.29
safety instructions
GHS hazard labeling no classification available
As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Nitrous acid (according to the nomenclature of the IUPAC Hydrogen nitrite is called) a stable only in cold and dilute aqueous solution, medium strong acid .

synthesis

Nitrogen dioxide NO ₂ disproportionately when introduced into water, e.g. B. in the context of the Ostwald process , in nitric acid and nitric acid:

${\displaystyle \mathrm {2\,NO_{2}+H_{2}O\ \rightarrow \ HNO_{2}+HNO_{3}} }$

Therefore, NO _{2 can} formally be regarded as a mixed anhydride of HNO ₂ and HNO ₃ . In a similar way, the salts of nitrous acid, the nitrites with the anion NO ₂ ^- , can be produced by disproportionation of nitrogen monoxide NO in concentrated alkalis at high temperatures:

${\displaystyle \mathrm {4\,NO+2\,OH^{-}\ \rightarrow \ N_{2}O+2\,NO_{2}^{-}+\ H_{2}O} }$

Hyponitrite N ₂ O ₂ ²⁻ is formed as an intermediate product, which is unstable under the reaction conditions and decomposes with the release of nitrous oxide . On a laboratory scale, nitrous acid can be synthesized from the reaction of dilute sulfuric acid with barium nitrite solution.

properties

Bonding conditions of nitrous acid

Nitric acid is much more unstable than nitric acid and cannot be isolated as a pure substance, because when heated it breaks down in a disproportionation reaction to nitric acid and nitrogen monoxide : With a pKa value of 3.35, nitrous acid is one of the weaker acids and is therefore similarly strong like formic acid.

${\displaystyle \mathrm {3\ HNO_{2}\ \longrightarrow \ HNO_{3}+2\ NO+H_{2}O} }$

If you neutralize nitrous acid with alkaline solutions , the stable salts of the acid, the nitrites, are formed. The reaction of nitrous acid with dilute sodium hydroxide solution produces sodium nitrite :

${\displaystyle \mathrm {HNO_{2}+NaOH\ \longrightarrow \ NaNO_{2}+H_{2}O} }$

The nitrogen atom in nitrous acid has the oxidation state + III. Therefore nitric acid can be oxidized to nitric acid ( oxidation state + V) by oxidizing agents. Oxidation of nitrous acid with potassium permanganate in acidic solution succeeds :

${\displaystyle \mathrm {2\ {MnO_{4}}^{-}+5\ {NO_{2}}^{-}+6\ H^{+}\ \longrightarrow \ 2\ Mn^{2+}+5\ {NO_{3}}^{-}+3\ H_{2}O} }$

In an acidic environment, nitrous acid is a strong oxidizing agent .

Gaseous nitrous acid consists of planar molecules of cis and trans HNO ₂ , with the trans isomer being 2 kJ mol ⁻¹ more stable.

proof

A simple but not specific detection method is the oxidation of iodide to iodine by nitrous acid in acidic solution. The iodine-starch reaction is used , which shows even very low concentrations (a few ppm ) of nitrous acid due to the resulting polyiodide-starch complex by means of a blue color. The so-called potassium iodide starch paper is used for this.

Specifically, nitrous acid or nitrites can be detected with Lunge's reagent , whereby a red azo dye is formed.

Gaseous hydrogen nitrite can also be detected through its absorption in the near UV. This allows the measurement of atmospheric concentrations of nitrous acid, which occurs in urban environments with concentrations of a few ppt to a few ppb.

use

Nitrous acid is mainly used in the chemical industry for the production of diazonium salts .

toxicity

Nitrous acid is a mutagen and has a negative effect on organisms. The acid can convert cytosine into uracil . In contrast to cytosine, uracil pairs with adenine , which is why the base pair CG converts to TA after two replications (a point mutation ). However, since uracil does not appear in DNA, such errors are relatively easy to identify and correct.

various

Esters of nitrous acid can be produced using the Chrétien-Longi reaction .

Individual evidence

1. ↑ Entry on nitrous acid. In: Römpp Online . Georg Thieme Verlag, accessed on July 15, 2014.
2. ^ ^{A b} A. F. Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 101st edition. Walter de Gruyter, Berlin 1995, ISBN 3-11-012641-9 .
3. ↑ This substance has either not yet been classified with regard to its hazardousness or a reliable and citable source has not yet been found.
4. ↑ ^{a b c} M. Binnewies et alii: Allgemeine und Anorganische Chemie . 2nd Edition. Spectrum, 2010, ISBN 3-8274-2533-6 , pp. 491 .
5. ^ ^{A b} E. Schweda: Jander / Blasius: Inorganic Chemistry I - Introduction & Qualitative Analysis . 17th edition. Hirzel, 2012, ISBN 978-3-7776-2134-0 , pp. 235-236 .
6. ↑ Ralf Steudel : Chemistry of the non-metals: with atomic structure, molecular geometry and bond theory . Walter de Gruyter, 1998, ISBN 3-11-012322-3 , p. 416 ( limited preview in Google Book search).
7. ^ J. Stutz, ES Kim, U. Platt, P. Bruno, C. Perrino, A. Febo, "UV-visible absorption cross sections of nitrous acid," J. Geophys. Res. 105, 14585-14592 (2000)
8. ↑ Wong, KW, et al. "Modeling of daytime HONO vertical gradients during SHARP 2009." Atmospheric Chemistry and Physics 7/13 (2013): 3587-3601.