Sodium nitrite is the 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. It is a useful precursor to a variety of organic compounds, such as pharmaceuticals, dyes, and pesticides, but it is probably best known as a food additive to prevent botulism.
It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.
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.
Other applications include uses in photography. It may also be used as an electrolyte in electrochemical grinding manufacturing processes, typically diluted to about 10% concentration in water. It is used in a variety of metallurgical applications, for phosphatizing and detinning.
Sodium nitrite is an effective corrosion inhibitor and is used as an additive in industrial greases, as an aqueous solution in closed loop cooling systems, and in a molten state as a heat transfer medium.
It is in the section "antidotes and other substances used in poisonings" of the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.
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. Through this research, sodium nitrite has been found to inhibit growth of disease-causing microorganisms; give taste and color to the meat; and inhibit lipid oxidation that leads to rancidity. The ability of sodium nitrite to address the above-mentioned issues has led to production of meat with improved food safety, extended storage life and improving desirable color/taste. It has the E number E250. Potassium nitrite (E249) is used in the same way. It is approved for usage in the EU, USA and Australia and New Zealand.
Sodium nitrite is well known for its role in inhibiting the growth of Clostridium botulinum spores in refrigerated meats. The mechanism for this activity results from the inhibition of iron-sulfur clusters essential to energy metabolism of Clostridium botulinum. However, sodium nitrite has had varying degrees of effectiveness for controlling growth of other spoilage or disease causing microorganisms. 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. Furthermore, the type of bacteria also affects sodium nitrites effectiveness. It is generally agreed upon that sodium nitrite is not considered effective for controlling gram-negative enteric pathogens such as Salmonella and Escherichia coli.
The appearance and taste of meat is an important component of consumer acceptance. Sodium nitrite is responsible for the desirable red color (or shaded pink) of meat. Very little nitrite is needed to induce this change. It has been reported that as little as 2 to 14 parts per million (ppm) is needed to induce this desirable color change. However, to extend the lifespan of this color change, significantly higher levels are needed. 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. The unique taste associated with cured meat is also affected by the addition of sodium nitrite. However, the mechanism underlying this change in taste is still not fully understood.
Sodium nitrite is also able to effectively delay the development of oxidative rancidity. Lipid oxidation is considered to be a major reason for the deterioration of quality of meat products (rancidity and unappetizing flavors). Sodium nitrite acts as an antioxidant in a mechanism similar to the one responsible for the coloring affect. Nitrite reacts with heme proteins and metal ions, neutralizing free radicals by nitric oxide (one of its byproducts). Neutralization of these free radicals terminates the cycle of lipid oxidation that leads to rancidity.
While this chemical will prevent the growth of bacteria, it can be toxic in high amounts for animals and humans. Sodium nitrite's 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 death. 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. Nitrites are not naturally occurring in vegetables in significant quantities. 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). Boiling vegetables lowers nitrate but not nitrite. 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). The presence of nitrite in animal tissue is a consequence of metabolism of nitric oxide, an important neurotransmitter. Nitric oxide can be created de novo from nitric oxide synthase utilizing arginine or from ingested nitrate or nitrite.
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. 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. 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.
A principal concern about sodium nitrite is the formation of carcinogenic nitrosamines in meats containing sodium nitrite when meat is charred or overcooked. 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. Around 1970, it was found that ascorbic acid (vitamin C), an antioxidant, inhibits nitrosamine formation. 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. The nitrosonium ion formed in acidic nitrite solutions is commonly mislabeled nitrous anhydride, an unstable nitrogen oxide that cannot exist in vitro.
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).
Sodium nitrite consumption has also been linked to the triggering of migraines in individuals who already suffer from them.
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.
Carcinogenic nitrosamines are formed when amines that occur naturally in food react with sodium nitrite found in cured meat products.
In the presence of acid (such as in the stomach) or heat (such as via cooking), nitrosamines are converted to diazonium ions.
Industrial production of sodium nitrite follows one of two processes, the reduction of nitrate salts, or the oxidation of lower nitrogen oxides.
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.
Sodium nitrite has also been produced by reduction of nitrate salts by exposure to heat, light, ionizing radiation, metals, hydrogen, and electrolytic reduction.
The nitrous acid then, under normal conditions, decomposes: