What Is Nitrous Acid?
Nitrous acid is a weak and unstable inorganic acid, typically existing only in aqueous solution and vapor form. It is a pale blue compound with a chemical formula of HNO2. Nitrous acid is insoluble in water, acetonitrile, methanol, and ethanol, but it is soluble in chloroform.
Nitrous acid can be produced in an aqueous solution at low temperatures by reacting barium nitrite with dilute sulfuric acid or silver nitrite with hydrochloric acid and then filtering out the precipitate. It can also be generated by dissolving a mixture of nitrogen monoxide and nitrogen dioxide in ice water.
Uses of Nitrous Acid
Nitrous acid is primarily used in the synthesis of diazonium salts, which are intermediates in the production of various aromatic compounds. These salts are formed when nitrous acid reacts with organic amines. Nitrous acid and its salts are also used in the food industry as colorants for processed meats and as substances to inhibit botulism bacteria. They contribute to the bright red color of processed meat products, such as sausages, by interacting with heme iron.
Properties of Nitrous Acid
Nitrous acid is a weak acid with a pKa of about 3.3, making it significantly less acidic than nitric acid. In its pure form, it is unstable and decomposes readily into nitric oxide and nitric acid in water. Nitrous acid exhibits both oxidative and reductive properties, being oxidized to nitric acid by strong oxidants, and reduced to various compounds like nitric oxide, hydroxylamine, and ammonia by different reducing agents. The standard redox potentials as an oxidizing and reducing agent are E° = 0.996 V and E° = 1.093 V, respectively.
Structure of Nitrous Acid
In the gas phase, nitrous acid has a structure of H-O-N-O with bond angles of ∠HON and ∠ONO being 102° and 111°, respectively. It exists in both cis and trans forms, with the trans form being more stable. The molecular mass of nitrous acid is 47.01.
Other Information on Nitrous Acid
1. Diazotization With Nitrous Acid
Nitrous acid reacts with secondary amines to form nitrosamines and with aromatic primary amines to produce aromatic diazonium ions, which are used in the Sandmeyer reaction and in the synthesis of azo compounds for dyes. Diazonium salts are reactive and contain the -N+≡N group.
2. Nitrous Acid-Related Compounds
Nitrous acid ions can coordinate with various metals to form complexes. Nitrogen-coordinated complexes are known as nitro complexes and oxygen-coordinated ones as nitrito complexes. Examples of nitrous acid salts include potassium nitrite (KNO2), calcium nitrite (Ca(NO2)2), silver nitrite (AgNO2), sodium nitrite (NaNO2), and barium nitrite (Ba(NO2)2).