IUPAC name: Nitric acid (V)
Other names: Hydrogen nitrate, aqua fortis.
CAS number: 7697-37-2
Strong inorganic acid, colourless liquid (with a density of 1.52 g/cm3 at 25 °C), toxic and highly burning; in contact with the skin it causes severe burns and yellowish stains because it reacts with several proteins, such as tyrosine, producing a yellow xanthoproteic acid; its vapours are extremely dangerous. In nature it is present in acid rain. Nitric acid in aqueous solution is almost completely dissociated into its H+and NO-3 ions. It is a strong oxidant and it dissolves all metals except platinum, iridium, rhodium, tantalum, and titanium, generating the respective nitrates and releasing nitrogen oxides (in some cases, e.g. aluminium, iron and nickel, it induces the formation of a thin oxide film that prevents further oxidation and plays a protective role); it attacks organic substances, often with the formation of nitro-derivatives.
In the past it was called aqua fortis and was produced by the alchemists by reacting nitre or saltpetre with sulphuric acid; at present, all nitric acid is made by the catalytic oxidation of ammonia.
Nitric acid is produced industrially by the Ostwald process: the catalytic oxidation of ammonia to nitrogen monoxide. The nitric oxide is oxidised further to nitrogen dioxide (or dinatrogen tetroxide), which is reacted with water to yield an aqueous solution of nitric acid. Appropriate catalysts are required to obtain satisfactory reaction yields. The catalyst consists of a platinum alloy containing 5-10% of rhodium.
The maximum nitric acid concentration obtained is about 62%.
It is widely used to manufacture nitrogen fertilisers, nitro-derivatives (that is compounds containing the NO or NO2 group), and intermediates for dyes. Other applications are in metallurgy, in the production of phosphoric acid, in photogravure, and in the manufacture of agricultural fertilisers.