Sodium nitrite

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.

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.

Medicine

Sodium nitrite is used together with sodium thiosulfate to treat cyanide poisoning.

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.

Food additive

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.

Inhibition of microbial growth

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.

Taste and color

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.

Inhibition of lipid oxidation

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.

Toxicity

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 LD_Lo 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.

Humane toxin for feral hog/wild boar control

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.

Nitrosamines

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. About 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.

Mechanism of action

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.

Certain nitrosamines such as N-nitrosodimethylamine and N-nitrosopyrrolidine form carbocations that react with biological nucleophiles (such as DNA or an enzyme) in the cell.

If this nucleophilic substitution reaction occurs at a crucial site in a biomolecule, it can disrupt normal cell functions, leading to cancer or cell death.

Production

The salt is prepared by treating sodium hydroxide with mixtures of nitrogen dioxide and nitric oxide:

The conversion is sensitive to the presence of oxygen, which can lead to varying amounts of sodium nitrate.

In former times, sodium nitrite was prepared by reduction of sodium nitrate with various metals.

Chemical reactions

In the laboratory, sodium nitrite can be used to destroy excess sodium azide.

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

Sodium nitrite can also be used in the production of nitrous acid via sulfuric acid. This reaction first yields nitrous acid and sodium sulfate:

The nitrous acid then, under normal conditions, decomposes:

The nitrogen dioxide of the prior decomposition is then routed through a condensor or fractional distillation apparatus to react with water and yield nitric acid: