Sodium nitrite

Sodium nitrite is an inorganic compound with the chemical formula NaNO2. It is a white to slightly yellowish crystalline powder that is very soluble in water and is hygroscopic. From an industrial perspective, it is the most important nitrite salt. It is a precursor to a variety of organic compounds, such as pharmaceuticals, dyes, and pesticides, but it is probably best known as a food additive used in processed meats and (in some countries) in fish products.[3]

This article is about the chemical compound. For the use of sodium nitrite as a medication, see Sodium nitrite (medical use).
Not to be confused with sodium nitrate, sodium nitride, or nitratine.
"E250" redirects here. For the mobile phone, see Samsung SGH-E250.

Sodium nitrite
The nitrite anion (space-filling model)
The sodium cation

Unit cell of sodium nitrite under standard conditions.
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.028.687
EC Number
  • 231-555-9
E number E250 (preservatives)
RTECS number
  • RA1225000
UNII
UN number 1500
Properties
NaNO2
Molar mass 68.9953 g/mol
Appearance white or slightly yellowish solid
Density 2.168 g/cm3
Melting point 271 °C (520 °F; 544 K) (decomposes at 320 °C)
71.4 g/100 mL (0 °C)
84.8 g/100 mL (25 °C)
160 g/100 mL (100 °C)
Solubility soluble in methanol (4.4 g/100 mL)
ethanol
slightly soluble in diethyl ether (0.3 g/100 mL)
very soluble in ammonia
Acidity (pKa) ~9
−14.5·10−6 cm3/mol
1.65
Structure[1]
orthorhombic
Im2m
a = 3.5653(8) Å, b = 5.5728(7) Å, c = 5.3846(13) Å
2
Thermochemistry
106 J/mol K
−359 kJ/mol[2]
−295 kJ/mol
Pharmacology
V03AB08 (WHO)
Hazards
Safety data sheet External MSDS
R-phrases (outdated) R8, R25, R50
S-phrases (outdated) (S1/2), S45, S61
NFPA 704 (fire diamond)
489 °C (912 °F; 762 K)
Lethal dose or concentration (LD, LC):
180 mg/kg (rats, oral)
Related compounds
Other anions
 Sodium nitrate
Other cations
 Potassium nitrite
Ammonium nitrite
Lithium nitrite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Uses

Industrial chemistry

The main use of sodium nitrite is for the industrial production of organonitrogen compounds. It is a reagent for conversion of amines into diazo compounds, which are key precursors to many dyes, such as diazo dyes. Nitroso compounds are produced from nitrites. These are used in the rubber industry.[3]

It is used in a variety of metallurgical applications, for phosphatizing and detinning.[3]

Sodium nitrite is an effective corrosion inhibitor and is used as an additive in industrial greases,[4] as an aqueous solution in closed loop cooling systems, and in a molten state as a heat transfer medium.[5]

Medication
Main article: Sodium nitrite (medical use)

Sodium nitrite is an efficient drug in case of cyanide poisoning. It is used together with sodium thiosulfate.[6] It is on the World Health Organization's List of Essential Medicines.[7]

Food additive and preservative

Nitrite is an easy way to give a pink shade to processed meats.[8] Nitrite reacts with the meat myoglobin to cause color changes, first converting to nitrosomyoglobin (bright red), then, on heating, to nitrosohemochrome (a pink pigment).[9]

In the meat-packing industry, nitrite is used to prevent botulism. Several large meat processors also produce processed meats without relying on nitrite or nitrate.[10][11]

Historically, salt has been used for the preservation of meat. The salt-preserved meatproduct was usually brownish-gray in color. When sodium nitrite is added with the salt, the meat develops a red, then pink color, which is associated with cured meats such as ham, bacon, hot dogs, and bologna.[12]

In the early 1900s, irregular curing was commonplace. This led to further research surrounding the use of sodium nitrite as an additive in food, standardizing the amount present in foods to minimize the amount needed while maximizing its food additive role.[13] Through this research, sodium nitrite has been found to give taste and color to the meat; inhibit lipid oxidation that leads to rancidity; and inhibit growth of disease-causing microorganisms.[13] The ability of sodium nitrite to address the above-mentioned issues has led to production of meat with extended storage life and has improved desirable color/taste. According to scientists working for the meat industry,[14] nitrite has improved food safety.[13]

Nitrite has the E number E250. Potassium nitrite (E249) is used in the same way. It is approved for usage in the EU,[15][16] USA[17] and Australia and New Zealand.[18]

Color and taste

The appearance and taste of meat is an important component of consumer acceptance.[13] Sodium nitrite is responsible for the desirable red color (or shaded pink) of meat.[13] Very little nitrite is needed to induce this change.[13] It has been reported that as little as 2 to 14 parts per million (ppm) is needed to induce this desirable color change.[19] However, to extend the lifespan of this color change, significantly higher levels are needed.[19] The mechanism responsible for this color change is the formation of nitrosylating agents by nitrite, which has the ability to transfer nitric oxide that subsequently reacts with myoglobin to produce the cured meat color.[19] The unique taste associated with cured meat is also affected by the addition of sodium nitrite.[13] However, the mechanism underlying this change in taste is still not fully understood.[19]

Inhibition of microbial growth

Sodium nitrite is known for its role in inhibiting the growth of Clostridium botulinum spores in refrigerated meats.[20] The mechanism for this activity results from the inhibition of iron-sulfur clusters essential to energy metabolism of Clostridium botulinum.[20] However, sodium nitrite has had varying degrees of effectiveness for controlling growth of other spoilage or disease causing microorganisms.[13] Even though the inhibitory mechanisms for sodium nitrite are not well known, its effectiveness depends on several factors including residual nitrite level, pH, salt concentration, reductants present and iron content.[19] Furthermore, the type of bacteria also affects sodium nitrites effectiveness.[19] It is generally agreed upon that sodium nitrite is not considered effective for controlling gram-negative enteric pathogens such as Salmonella and Escherichia coli.[19]

Other food additives (such as lactate and sorbate) provide similar protection against bacteria, but do not provide the desired pink color.[21][22]

Inhibition of lipid peroxidation

Sodium nitrite is also able to effectively delay the development of oxidative rancidity.[19] Lipid peroxidation is considered to be a major reason for the deterioration of quality of meat products (rancidity and unappetizing flavors).[19] Sodium nitrite acts as an antioxidant in a mechanism similar to the one responsible for the coloring effect.[19] Nitrite reacts with heme proteins and metal ions, neutralizing free radicals by nitric oxide (one of its byproducts).[19] Neutralization of these free radicals terminates the cycle of lipid oxidation that leads to rancidity.[19]

Toxicity

Sodium nitrite is weakly toxic. The LD50 in rats is 180 mg/kg and its human LDLo is 71 mg/kg, meaning a 65 kg person would likely have to consume at least 4.6 g to result in a 50% chance of death.[23] To prevent toxicity, sodium nitrite (blended with salt) sold as a food additive is dyed bright pink to avoid mistaking it for plain salt or sugar.

Occurrence in vegetables

Nitrites are not naturally occurring in vegetables in significant quantities.[24] However, nitrates are found in commercially available vegetables and a study in an intensive agricultural area in northern Portugal found residual nitrate levels in 34 vegetable samples, including different varieties of cabbage, lettuce, spinach, parsley and turnips ranged between 54 and 2440 mg/kg, e.g. curly kale (302.0 mg/kg) and green cauliflower (64 mg/kg).[25][26] Boiling vegetables lowers nitrate but not nitrite.[25] Fresh meat contains 0.4–0.5 mg/kg nitrite and 4–7 mg/kg of nitrate (10–30 mg/kg nitrate in cured meats).[24]

The presence of nitrite in animal tissue is a consequence of metabolism of nitric oxide, an important neurotransmitter.[27] Nitric oxide can be created de novo from nitric oxide synthase utilizing arginine or from ingested nitrate or nitrite.[28]

Pigs

Because of sodium nitrite's high level of toxicity to swine (Sus scrofa) it is now being developed in Australia to control feral pigs and wild boar.[29][30] The sodium nitrite induces methemoglobinemia in swine, i.e. it reduces the amount of oxygen that is released from hemoglobin, so the animal will feel faint and pass out, and then die in a humane manner after first being rendered unconscious.[31] The Texas Parks and Wildlife Department operates a research facility at Kerr Wildlife Management Area, where they examine feral pig feeding preferences and bait tactics to administer sodium nitrite.[32]

Cancer

Carcinogenicity is the ability or tendency of a chemical to induce tumors, increase their incidence or malignancy, or shorten the time of tumor occurrence.[33]

Nitrosamines can potentially form when sodium nitrite-treated meat is cooked. Such carcinogenic nitrosamines can also be formed from the reaction of nitrite with secondary amines under acidic conditions (such as occurs in the human stomach) as well as during the curing process used to preserve meats. Dietary sources of nitrosamines include US cured meats preserved with sodium nitrite as well as the dried salted fish eaten in Japan. In the 1920s, a significant change in US meat curing practices resulted in a 69% decrease in average nitrite content. This event preceded the beginning of a dramatic decline in gastric cancer mortality.[34] Around 1970, it was found that ascorbic acid (vitamin C), an antioxidant, inhibits nitrosamine formation.[35] Consequently, the addition of at least 550 ppm of ascorbic acid is required in meats manufactured in the United States. Manufacturers sometimes instead use erythorbic acid, a cheaper but equally effective isomer of ascorbic acid. Additionally, manufacturers may include α-tocopherol (vitamin E) to further inhibit nitrosamine production. α-Tocopherol, ascorbic acid, and erythorbic acid all inhibit nitrosamine production by their oxidation-reduction properties. Ascorbic acid, for example, forms dehydroascorbic acid when oxidized, which when in the presence of nitrosonium, a potent nitrosating agent formed from sodium nitrite, reduces the nitrosonium into nitric oxide.[36] The nitrosonium ion formed in acidic nitrite solutions is commonly[37][38] mislabeled nitrous anhydride, an unstable nitrogen oxide that cannot exist in vitro.[39]

Nitrate or nitrite (ingested) under conditions that result in endogenous nitrosation has been classified as "probably carcinogenic to humans" by International Agency for Research on Cancer (IARC).[40][41]

Sodium nitrite consumption has also been linked to the triggering of migraines in individuals who already suffer from them.[42]

One study has found a correlation between highly frequent ingestion of meats cured with pink salt and the COPD form of lung disease. The study's researchers suggest that the high amount of nitrites in the meats was responsible; however, the team did not prove the nitrite theory. Additionally, the study does not prove that nitrites or cured meat caused higher rates of COPD, merely a link. The researchers did adjust for many of COPD's risk factors, but they commented they cannot rule out all possible unmeasurable causes or risks for COPD.[43][44]

Production

Industrial production of sodium nitrite follows one of two processes, the reduction of nitrate salts, or the oxidation of lower nitrogen oxides.

One method uses molten sodium nitrate as the salt, and lead which is oxidized, while a more modern method uses scrap iron filings to reduce the nitrate.[3][45]

A more commonly used method involves the general reaction of nitrogen oxides in alkaline aqueous solution, with the addition of a catalyst. The exact conditions depend on which nitrogen oxides are used, and what the oxidant is, as the conditions need to be carefully controlled to avoid over oxidation of the nitrogen atom.[3]

Sodium nitrite has also been produced by reduction of nitrate salts by exposure to heat, light, ionizing radiation, metals, hydrogen, and electrolytic reduction.[46]

Chemical reactions
Main articles: nitrite and nitrous acid

In the laboratory, sodium nitrite can be used to destroy excess sodium azide.[47][48]

2 NaN3 + 2 NaNO2 + 4 H+ → 3 N2 + 2 NO + 4 Na+ + 2 H2O

Above 330 °C sodium nitrite decomposes (in air) to sodium oxide, nitric oxide and nitrogen dioxide.[49]

2 NaNO2 → Na2O + NO + NO2

Sodium nitrite can also be used in the production of nitrous acid:

2 NaNO
2 + H
2SO
4 → 2 HNO
2 + Na
2SO
4

The nitrous acid then, under normal conditions, decomposes:

2 HNO
2 → NO
2 + NO + H
2O

The resulting nitrogen dioxide hydrolyzes to a mixture of nitric and nitrous acids:

2 NO
2 + H
2O → HNO
3 + HNO
2

Isotope labelling 15N
15N isotope enriched NaNO2

In organic synthesis isotope enriched sodium nitrite-15N can be used instead of normal sodium nitrite as their reactivity is nearly identical in most reactions.

The obtained products carry isotope 15N and hence Nitrogen NMR can be efficiently carried out.[50]

References
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