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The chemical compound nitric acid (HNO3) is a colorless, corrosive liquid, a toxic acid which can cause severe burns. At room temperature it gives off red or yellow fumes. Commonly used as a laboratory reagent, it is used in the manufacture of explosives such as nitroglycerin and trinitrotoluene (TNT), and as well as of fertilizers such as ammonium nitrate. It has additional uses in metallurgy and refining as it reacts with most metals, and in organic syntheses. Nitric acid is also a component of acid rain.
Nitric acid is a strong acid: in aqueous solution, it completely dissociates into the nitrate ion NO3− and a hydrated proton. The salts of nitric acid (which contain the nitrate ion) are also known as nitrates. The overwhelming majority of them are very soluble in water.
Nitric acid is made by mixing nitrogen dioxide (NO2), with water. Creating a very pure nitric acid usually involves distillation with sulfuric acid, as nitric acid forms an azeotrope with water with a composition of 68% nitric acid and 32% water. Commercial grade nitric acid solutions are usually between 52% and 68% nitric acid. If the nitric acid solution contains more than 86% nitric acid, it is referred to as fuming nitric acid, and can be separated into two kinds of fuming acids, white fuming nitric acid, and red fuming nitric acid.
White fuming nitric acid, or WFNA, is very close to the anhydrous nitric acid product. One specification for white fuming nitric acid is that it has a maximum of 2 percent water and a maximum of 0.5 percent dissolved NO2. Red fuming nitric acid, or RFNA, contains substantial quantities of dissolved nitrogen dioxide (NO2) leaving the solution with a reddish-brown color. One formulation of RFNA specifies a minimum of 17% NO2, another specifies 13% NO2. In either event, an inhibited fuming nitric acid (either IWFNA, or IRFNA) can be made by the addition of 0.6-0.7% hydrogen fluoride, HF. This fluoride is added for corrosion resistance in metal tanks (the fluoride creates a metal fluoride layer that protects the metal). The obvious use for such a corrosion inhibited product is as an oxidizer in liquid fuel rockets.
Nitric acid is a powerful oxidizing agent and the reactions of nitric acid with compounds such as cyanides, carbides, and metallic powders can be explosive. Reactions of nitric acid with many organic compounds, such as turpentine, are violent and hypergolic (i.e. self igniting).
Concentrated nitric acid dyes human skin yellow on contact. Funnily, these yellow stains turn orange when alkalised.
Nitric acid and its salts, the nitrates, should not be confused with nitrous acid and its salts, the nitrites.
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