Nitrite

The nitrite ion has the chemical formula . Nitrite (mostly sodium nitrite) is widely used throughout chemical and pharmaceutical industries. The nitrite anion is a pervasive intermediate in the nitrogen cycle in nature. The name nitrite also refers to organic compounds having the –ONO group, which are esters of nitrous acid.

Production

Sodium nitrite is made industrially by passing a mixture of nitrogen oxides into aqueous sodium hydroxide or sodium carbonate solution:

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The data can be extended to include products in lower oxidation states. For example:

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Oxidation reactions usually result in the formation of the nitrate ion, with nitrogen in oxidation state +5. For example, oxidation with permanganate ion can be used for quantitative analysis of nitrite (by titration):

The products of reduction reactions with the nitrite ion vary depending on the reducing agent used and its strength. With sulfur dioxide, the products are NO and ; with tin(II) () the product is hyponitrous acid (); reduction all the way to ammonia () occurs with hydrogen sulfide. With the hydrazinium cation () the product of nitrite reduction is hydrazoic acid (), an unstable and explosive compound:

which can also further react with nitrite:

This reaction is unusual in that it involves compounds with nitrogen in four different oxidation states.

Analysis of nitrite

Nitrite is detected and analyzed by the Griess Reaction, involving the formation of a deep red-colored azo dye upon treatment of a -containing sample with sulfanilic acid and naphthyl-1-amine in the presence of acid.

Coordination complexes

Nitrite is an ambidentate ligand and can form a wide variety of coordination complexes by binding to metal ions in several ways. For example, the red nitrito pentaamminecobalt complex is metastable, isomerizing to the yellow nitro complex .

Biochemistry

thumb|upright=1.5|class=skin-invert-image|A schematic representation of the microbial nitrogen cycle. [[Anammox|ANAMMOX is anaerobic ammonium oxidation, DNRA is dissimilatory nitrate reduction to ammonium, and COMMAMOX is complete ammonium oxidation.]]

In nitrification, ammonium is converted to nitrite. Important species include Nitrosomonas. Other bacterial species, such as Nitrobacter, are responsible for oxidizing nitrite to nitrate.

Nitrite can be reduced to nitric oxide or ammonia by many species of bacteria. Under hypoxic conditions, nitrite may release nitric oxide, which causes potent vasodilation. Several mechanisms for nitrite conversion to NO have been described, including enzymatic reduction by xanthine oxidoreductase, nitrite reductase, and NO synthase (NOS), as well as nonenzymatic acidic disproportionation reactions.

Uses

Chemical precursor

Azo dyes and other colorants are prepared by the process called diazotization, which requires nitrite.

The academic and industrial consensus is that nitrites also reduce the growth and toxin production of Clostridium botulinum.

On the other hand, a 2018 study by the British Meat Producers Association determined that legally permitted levels of nitrite do not affect the growth of C. botulinum.

Addition of ascorbic acid, erythorbic acid, or one of their salts enhances the binding of nitrite to the iron atom in myoglobin.

In the U.S., meat cannot be labeled "cured" unless it contains nitrite. In the US, nitrite has been formally used since 1925. According to scientists working for the industry group American Meat Institute, this use of nitrite started in the Middle Ages.

In some countries, cured-meat products are manufactured without nitrate or nitrite, and without nitrite from vegetable sources. Parma ham, produced without nitrite since 1993, was reported in 2018 to have caused no cases of botulism. This is because the muscle's interior is sterile, while its surface is exposed to oxygen.

Historians and epidemiologists argue that the widespread use of nitrite in meat-curing is closely linked to the development of industrial meat-processing. French investigative journalist Guillaume Coudray asserts that the meat industry chooses to cure its meats with nitrite even though it is established that this chemical gives rise to cancer-causing nitroso-compounds. Some traditional and artisanal producers avoid nitrites. As many researchers nowadays try to point out the hazardous generation of nitrosamines as nitrites bind to free peptides in the gastrointestinal system, the EU published a regulation that requires lowering nitrite levels in meat curing from .

In mice, food rich in nitrites together with unsaturated fats can prevent hypertension by forming nitro fatty acids that inhibit soluble epoxide hydrolase, which is one explanation for the apparent health effect of the Mediterranean diet. Adding nitrites to meat has been shown to generate known carcinogens; the World Health Organization (WHO) advises that eating of nitrite processed meat a day would raise the risk of getting bowel cancer by 18% over a lifetime.

95% of the nitrite ingested in modern diets comes from bacterial conversion of nitrates naturally found in vegetables. However, potentially cancer-causing nitroso compounds are not formed in the pH-neutral colon. They are mostly produced in the acidic stomach.

Since the 1980s, sorbic acid and its salts have been used to inhibit Clostridium botulinum in meat products, replacing nitrites to avoid the formation of carcinogenic nitrosamines.

Antidote for cyanide poisoning

Nitrites in the form of sodium nitrite and amyl nitrite are components of many cyanide antidote kits. Both of these compounds bind to hemoglobin and oxidize the ions to ions forming methemoglobin. Methemoglobin, in turn, binds to cyanide (CN), creating cyanmethemoglobin, effectively removing cyanide from the complex IV of the electron transport chain (ETC) in mitochondria, which is the primary site of disruption caused by cyanide. Another mechanism by which nitrites help treat cyanide toxicity is the generation of nitric oxide (NO). NO displaces the CN from the cytochrome c oxidase (ETC complex IV), making it available for methemoglobin to bind.

Organic nitrites

thumb|right|upright=0.5|class=skin-invert-image|A nitrite ester

In organic chemistry, alkyl nitrites are esters of nitrous acid and contain the nitrosoxy functional group. Nitro compounds contain the group. Nitrites have the general formula RONO, where R is an aryl or alkyl group. Amyl nitrite and other alkyl nitrites have a vasodilating action and must be handled in the laboratory with caution. They are sometimes used in medicine to treat heart disease. A classic named reaction for the synthesis of alkyl nitrites is the Meyer synthesis in which alkyl halides react with metallic nitrites to a mixture of nitroalkanes and nitrites.

Safety

Large doses of nitrites cause acute poisoning in the form of methemoglobinemia, which can lead to death.