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nitric acid (HNO3)





nitric acid
A clear, colorless liquid that is one of the strongest mineral acids. In air, nitric acid (HNO3) fumes and turns yellow due to its decomposition into nitrogen dioxide (NO2). Nitric acid is highly corrosive and a powerful oxidizing agent. It attacks most metals, resulting in the formation of nitrates. With many organic compounds, nitration occurs. Melting point -42°C, boiling point 83°C.

Nitric acid is used in the production of fertilizers, explosives, plastics, dyes, and rocket propellants. It was called aqua fortis ("strong water") by the alchemists, a name by which it continued to be known commercially.





How nitric acid is made

Nitric acid can be prepared in the lab by the distillation of a mixture of an alkali-metal nitrate and concentrated sulfuric acid. Industrially, it is produced by the Ostwald process (named after the Latvian-born German physical chemist Friedrich Wilhelm Ostwald who developed it in 1902). In this process, ammonia is oxidized by air to nitric oxide, at 900°C and 1–8 atm using a platinum or platinum-rhodium catalyst in the form of a thin wire gauze. The nitric oxide is further oxidized to nitrogen dioxide, which is dissolved in water to give 60% nitric acid.


Use of nitric acid in rocket engines

Nitric acid was commonly used as an oxidizer in liquid-propellant rocket engines between 1940 and 1965. It most often took the form of RFNA (red fuming nitric acid), containing 5–20% dissolved nitrogen dioxide. Compared to concentrated nitric acid (also known as white fuming nitric acid), RFNA is more energetic and more stable to store but produces poisonous red-brown fumes. Because nitric acid is normally highly corrosive it can only be stored and piped by a few materials such as stainless steel. However, the addition of a small concentration of fluoride ions inhibits the corrosive action and gives a form known as IRFNA (inhibited red fuming nitric acid). Like nitrogen tetroxide, it is hypergolic (reacts upon contact with) hydrazine, MMH (monomethyl hydrazine), and UDMH (unsymmetrical dimethyl hydrazine).


Medical effects of nitric acid

Concentrated nitric acid is capable of producing severe burns of the skin. Swallowing the acid leads to intense burning pain and ulceration of the mouth and throat. Treatment is by immediate administration of alkaline solutions, followed by milk or olive oil.


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