Nitric acid is an inorganic compound with the formula HNO3. It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into oxides of nitrogen. Most commercially available nitric acid has a concentration of 68% in water. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as red fuming nitric acid at concentrations above 86%, or white fuming nitric acid at concentrations above 95%. Nitric acid is the primary reagent used for nitration – the addition of a nitro group, typically to an organic molecule. While some resulting nitro compounds are shock- and thermally-sensitive explosives, a few are stable enough to be used in munitions and demolition, while others are still more stable and used as synthetic dyes and medicines (e.g. metronidazole). Nitric acid is also commonly used as a strong oxidizing agent.
History
Medieval alchemy The discovery of mineral acids such as nitric acid is generally presumed to go back to 13th-century European alchemy. The conventional view is that nitric acid was first described in pseudo-Geber's De inventione veritatis ("On the Discovery of Truth", after c. 1300). However, according to Eric John Holmyard and Ahmad Y. al-Hassan, nitric acid was also referenced in various earlier Arabic works such as the Ṣundūq al-ḥikma ("Chest of Wisdom") attributed to Jabir ibn Hayyan (8th century) or the Taʿwīdh al-Ḥākim attributed to the Fatimid caliph al-Hakim bi-Amr Allah (985–1021). The recipe in the Ṣundūq al-ḥikma attributed to Jabir has been translated as follows:
Take five parts of pure flowers of nitre, three parts of Cyprus vitriol and two parts of Yemen alum. Powder them well, separately, until they are like dust and then place them in a flask. Plug the latter with a palm fibre and attach a glass receiver to it. Then invert the apparatus and heat the upper portion (i.e. the flask containing the mixture) with a gentle fire. There will flow down by reason of the heat an oil like cow's butter. Nitric acid is also found in post-1300 works falsely attributed to Albert the Great and Ramon Llull (both 13th century). These works describe the distillation of a mixture containing niter and green vitriol, which they call eau forte (aqua fortis).
Modern era In the 17th century, Johann Rudolf Glauber devised a process to obtain nitric acid by distilling potassium nitrate with sulfuric acid. In 1776 Antoine Lavoisier cited Joseph Priestley's work to point out that it can be converted from nitric oxide (which he calls "nitrous air"), "combined with an approximately equal volume of the purest part of common air, and with a considerable quantity of water." In 1785 Henry Cavendish determined its precise composition and showed that it could be synthesized by passing a stream of electric sparks through moist air. In 1806, Humphry Davy reported the results of extensive distilled water electrolysis experiments concluding that nitric acid was produced at the anode from dissolved atmospheric nitrogen gas. He used a high voltage battery and non-reactive electrodes and vessels such as gold electrode cones that doubled as vessels bridged by damp asbestos. The industrial production of nitric acid from atmospheric air began in 1905 with the Birkeland–Eyde process, also known as the arc process. This process is based upon the oxidation of atmospheric nitrogen by atmospheric oxygen to nitric oxide with a very high temperature electric arc. Yields of up to approximately 4–5% nitric oxide were obtained at 3,000 °C, and less at lower temperatures. The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to nitrogen dioxide, and this was subsequently absorbed in water in a series of packed column or plate column absorption towers to produce dilute nitric acid. The first towers bubbled the nitrogen dioxide through water and non-reactive quartz fragments. About 20% of the produced oxides of nitrogen remained unreacted so the final towers contained an alkali solution to neutralize the rest. The process was very energy intensive and was rapidly displaced by the Ostwald process once cheap ammonia became available. Another early production method was invented by French engineer Albert Nodon around 1913. His method produced nitric acid from electrolysis of calcium nitrate converted by bacteria from nitrogenous matter in peat bogs. A pit was dug into the peat, then lined with tarred timber stakes around the sides. Into this pit was placed a porous earthenware vessel, surrounded by crushed limestone. The interior was filled with coke around a graphite anode. Nitric acid was pumped out via a glass tube that was sunk down nearly to the bottom of the pot, while fresh water was pumped into the top through another glass pipe to replace the fluid removed. Cast iron cathodes were sunk into the peat surrounding it. Resistance was about 3 ohms per cubic meter and the power supplied was around 10 volts. Production from a one hectare deposit, 6.5 feet deep, was estimated to be in excess of 600 tons per year. Once the Haber process for the efficient production of ammonia was introduced in 1913, nitric acid production from ammonia using the Ostwald process overtook production from the Birkeland–Eyde process. This method of production is still in use today.
Physical and chemical properties Commercially available nitric acid is an azeotrope with water at a concentration of 68% HNO3. This solution has a boiling temperature of 120.5 °C (248.9 °F) at 1 atm (100 kPa; 15 psi). It is known as "concentrated nitric acid". The azeotrope of nitric acid and water is a colourless liquid at room temperature. Two solid hydrates are known: the monohydrate HNO3·H2O or oxonium nitrate [H3O]+[NO3]− and the trihydrate HNO3·3H2O. An older density scale is occasionally seen, with concentrated nitric acid specified as 42 Baumé.
Contamination with nitrogen dioxide
Nitric acid is subject to thermal or light decomposition and for this reason it was often stored in brown glass bottles:
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