Sodium nitrite--NaNO2--is a salt commonly used in the food industry for curing meat. In pure form, it has a grainy crystalline texture and an off-white colour. It is valued for its ability to prevent botulism by inhibiting the growth mechanism of certain bacteria. Understanding how it reacts with other substances is important for anyone planning to work with the substance.
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When left in open air, sodium nitrate gradually oxidises. Oxygen molecules from the air bond with the nitrite ions, turning it into sodium nitrate, another white crystal with formula NaNO3. You can distinguish between them by the fact sodium nitrate has a sweet smell, as opposed to being odourless. Nitrite is itself a strong oxidiser, but not quite as strong as molecular oxygen, so the process is very slow.
As a strong oxidiser, sodium nitrite will react very rapidly with reducing agents. Reducing agents are chemicals that readily give up electrons into covalent or ionic bonds, such as potassium, lithium and ammonia. Reactions with such substances remove oxygen from the nitrite ion, turning it into neutral nitric oxide (NO). Because of its high reactivity, it is important to keep sodium nitrite away from strong reducers or open flame.
When in a strong acid, nitrite will reduce into water and nitrosyl, a cation (positively charged ion) with the formula NO+. When this comes into contact with a secondary amine, a common substance in the body, it can react to form a nitrosoamine, a known carcinogen (cancer-causing molecule). This is of high concern because it can happen very easily when sodium nitrite is digested in the body by the stomach's hydrochloric acid. So far, it is not considered a serious enough risk to stop the use of the chemical in curing meat.
If the nitrosyl cation reacts with a primary amine rather than a secondary amine, something very different happens. Primary amines differ from secondary amines in that they have two hydrogen atoms bonded to the key nitrogen atom, rather than one. Because of this, the nitrosyl ion ends up volunteering its oxygen atom to form water with the two hydrogen atoms, leaving behind what is known as a diazonium cation. Diazonuim ions are used in the synthesis of a number of important photographic chemicals and dyes. The chemical dye industry owes its success in part to this reaction sequence of sodium nitrite.
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