Nitric acid is a strong acid with the formula HNO3. It is a mineral acid, along with hydrochloric, sulfuric, perchloric, and phosphoric acids. It is a powerful oxidizing agent, especially when mixed with sulfuric acid, which produces the nitronium ion in situ.
Nitric acid is an oxidizing acid at room temperature. It is often used in the nitration of organic compounds. It is capable of dissolving metals such as copper and silver due to its oxidizing nature, and will release toxic nitrogen dioxide as an oxidation by product.
- Cu + 8 HNO3 → 3 Cu(NO3)2 + 2 NO + 4 H2O
Concentrated nitric acid is a clear solution with a density of about 1.2 g/ml. When the concentration of the acid surpasses 70%, it becomes classified as fuming nitric acid, this can be identified by visible fuming when air is blown into it. It forms an azeotrope with water at 68% concentration, which makes it difficult to produce the pure substance.
Nitric acid is available from lab suppliers like Elemental Scientific and Duda Diesel. While the acid itself is not expensive, it requires a mandatory HAZMAT shipping fee of $37.50, making this acid rather expensive for the amateur chemist.
Nitric acid may be prepared by distilling a mixture of a nitrate salt and one equivalent of sulfuric acid at 83ºC. This will produce 70% azeotropic concentrated acid, but the acid will be brown due to dissolved nitrogen dioxide. Dissolved nitrogen dioxide can be oxidized to nitric acid with oxygen or ozone. To produce higher concentrations with dissolved nitrogen oxides, also known as fuming nitric acid, the concentrated acid may be vacuum distilled with additional concentrated sulfuric acid. Hydrochloric acid should never be used, as it reacts with nitric acid to form nitrosyl chloride.
A somewhat less efficient method of producing nitric acid is by the reaction of a mixture containing sulfuric acid and a nitrate salt with copper metal, generating large amounts of nitrogen dioxide gas. The gas may then be bubbled into hydrogen peroxide or water, with hydrogen peroxide producing a higher yield. Sodium bisulfate can also be use to replace sulfuric acid.
Nitric acid can be used for many projects including make nitrate salts. When mixed with concentrated sulfuric acid or hydrofluoric acid, nitric acid acts like a base and releases a nitronium ion:
- 2H2SO4 + HNO3 → NO2+ + 2HSO4- + H2O
This mixture, known as mixed acid can be used for nitrating many organic compounds.
Nitric acid solutions are highly corrosive and will stain the skin yellow as the proteins are nitrated. Care should be taken to not let nitric acid contact the skin. Most glove types (with the notable exceptions butyl rubber or neoprene) are incompatible due to the strong oxidizing effects of nitric acid and may burn upon contact with the acid. Nitrates should not be used with hydrochloric acid, because this generates nitrosyl chloride.
Nitric acid is incompatible with most plastics due to its oxidizing nature, though bottle lids made of polypropylene (PP) are acceptable. High concentrations of nitric acid are light sensitive and should be kept in amber glass bottles with ample headroom to prevent pressure build up from nitrogen oxides.
Nitric acid can be neutralized with neutralizing compounds, such as carbonates, bicarbonates, oxides, hydroxides. Calcium carbonate is a good neutralizing agent, and, as long as the acid isn't contaminated with heavy metals, the resulting calcium nitrate can be discarded in ground or poured down the drain.