Why is the difference between sodium nitrite and nitrate?
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SODIUM NITRITE MOLECULE
Sodium Nitrite has the chemical formula NaNO2
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It is also used in manufacturing diazo dyes, nitroso compounds, and other organic compounds; in dyeing and printing textile fabrics and bleaching fibers; in photography; as a laboratory reagent and a corrosion inhibitor; in metal coatings for phosphatizing and detinning; and in the manufacture of rubber chemicals. Sodium nitrite also has been used in human and veterinary medicine as a vasodilator, a bronchodilator, an intestinal relaxant or a laxative, and an antidote for cyanide poisoning.
Sodium Nitrite as a Food additive
As a food additive, it serves a dual purpose in the food industry since it both alters the color of preserved fish and meats and also prevents growth of Clostridium botulinum, the bacteria which causes botulism. In the European Union it may be used only as a mixture with salt containing at most 0.6% sodium nitrite. It has the E number E250. Potassium nitrite (E249) is used in the same way.
While this chemical will prevent the growth of bacteria, it can be toxic for mammals. (LD50 in rats is 180 mg/kg.) For this reason, sodium nitrite sold as a food additive is dyed bright pink to avoid mistaking it for something else. Cooks and makers of charcuterie often simply refer to sodium nitrite as "pink salt".
Various dangers of using this as a food additive have been suggested and researched by scientists.
A principal concern is the formation of carcinogenic N-nitrosamines by the reaction of sodium nitrite with amino acids in the presence of heat in an acidic environment.
Sodium nitrite has also been linked to triggering migraines. http://www.fda.gov/FDAC/features/1998/398_pain.html
Recent studies have found a link between high processed meat consumption and colon cancer, possibly due to preservatives such as sodium nitrite. 
Recent studies have also found a link between frequent ingestion of meats cured with nitrites and the COPD form of lung disease.
Disease treatment with Sodium Nitrite
Recently, sodium nitrite has been found to be an effective means to increase blood flow by dilating blood vessels, acting as a vasodilator. Research is ongoing to investigate its applicability towards treatments for sickle cell anemia, cyanide poisoning, heart attacks, brain aneurysms, and pulmonary hypertension in infants.
Sodium nitrate is the chemical compound with the formula NaNO3. and molecular mass of 84.9947 g/mol. This salt, also known as "Chile saltpeter" (to distinguish it from ordinary saltpeter, potassium nitrate), is a white solid which is very soluble in water. Sodium nitrate is used as an ingredient in fertilizers, explosives, and in solid rocket propellants, as well as in glass and pottery enamels; the compound has been mined extensively for those purposes.
Sodium nitrate is also synthesized industrially by neutralizing nitric acid with soda ash.
Applications of Sodium Nitrate
Sodium nitrate was used extensively as a fertilizer and a raw material for the manufacture of gunpowder in the late nineteenth century.
Sodium nitrate has antimicrobial properties when used as a food preservative. It is found naturally in leafy green vegetables.
Sodium nitrate should not be confused with the related compound, sodium nitrite.The presence of sodium nitrite in food is controversial due to the development of nitrosamines when the food, primarily bacon, is cooked at high temperatures. The nitrate compound itself is not harmful, however, and is among the antioxidants found in fresh vegetables. (6) Its usage is carefully regulated in the production of cured products; in the United States, the concentration in finished products is limited to 200 ppm, and is usually lower.
It can be used in the production of nitric acid by combining it with sulfuric acid and subsequent separation through fractional distillation of the nitric acid, leaving behind a residue of sodium bisulfate. Less common applications include its use as a substitute oxidizer used in fireworks as a replacement for potassium nitrate commonly found in black powder and as a component in instant cold packs(7).