Nitrous acid

• Hydrogen nitrite
• Nitrogen ( III) acid

Stable only in cold aqueous solution

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Nitrous acid (named after the nomenclature of the IUPAC hydrogen nitrite) is only in cold and dilute aqueous solution resistant, medium strong acid.

Synthesis

Nitrogen dioxide NO2 disproportionates for the introduction into water, eg in the framework of the Ostwald process, in nitric acid and nitrous acid:

Therefore, NO2 can be formally regarded as a mixed anhydride of HNO 2 and HNO 3. In a similar way, the salts of nitrous acid, NO2 produce nitrites with the anion by disproportionation of nitric oxide NO in concentrated alkaline solutions at high temperatures:

As an intermediate product in this case is formed hyponitrite N2O22, which is unstable under the reaction conditions and decomposes with the release of nitrous oxide. On a laboratory scale nitrous acid from the reaction of dilute sulfuric acid can be synthesized with Bariumnitritlösung.

Properties

You can not be isolated as a pure substance because it decomposes when heated in a disproportionation reaction to form nitric acid and nitric oxide:

Puts you nitrous acid with alkaline solutions, the more stable form nitrites. For example, formed by the reaction of nitrous acid with dilute sodium hydroxide solution, sodium nitrite:

Since the nitrogen atom has an oxidation state of III, in nitrous acid, nitrous acid, can be oxidized to nitric acid by a strong oxidizing agent (oxidation state V). This allows for example oxidation of nitrous acid with sulphate of potassium permanganate:

In an acidic medium, the acid is a strong oxidizing agent.

Gaseous nitrous acid consists of planar molecules of cis -and trans- HNO2, wherein the trans isomer of 2 kJ mol -1 is stable.

Proof

A simple, but non-specific detection method is the oxidation of iodide to iodine by means of nitrous acid in acid solution. They are using here the iodine -starch reaction, the very low concentrations (a few ppm) indicates nitrous acid by the resulting Polyiodide -starch complex by blue staining. For this purpose, the so-called potassium iodide used.

Specifically, there can be detected with nitrous acid or nitrites Lunges reagent, forming a red azo dye.

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 impact on organisms. The acid can convert cytosine to uracil. But uracil pairs with adenine in contrast to cytosine, which is why the base pair CG converts after two replications to TA ( a point mutation ). Since uracil but does not occur in the DNA, such errors are relatively easily detected and corrected.