Nitric acid (HNO3), also known as aqua fortis and spirit of nitre, is made by the dissolution of nitrogen dioxide in water, Equation 6. Nitric acid can be concentrated by distillation from concentrated sulfuric acid.
The structure of HNO3 in the gas phase is planar at the sp2 nitrogen (Figure 4a).
Pure 100% nitric acid is a very corrosive liquid that is strongly acidic and protonates and dissolves organic species. In the liquid phase it is slightly dissociated, Equation 7. It is also a powerful oxidizing agent, converting non-metal elements to either the oxide or oxoacid. In contrast with metals it forms either salts or complexes in which the metal is in its highest oxidation state. It is unstable and decomposes upon heating or photolysis.
The pure acid has the highest self ionization of pure liquid acids, Equation 8. However, the loss of water results, Equation 9, such that the overall reaction can be described by Equation 10.
The common concentration of nitric acid is 70%. While the pure acid is colorless, samples often take on a yellow color due to the photochemical decomposition of nitric acid to give brown NO2.
The term the “acid test” is derived from the medieval practice of debasing of gold and silver currencies (often by the Monarchs who issued them) by debasing with copper. With ducats (Milan), livres (France), florins (Florence), maravedies (Spain) and bezants (Constantinople) in widespread use, and each with a nominal gold or silver content, it was important for a merchant to be able verify the worth of any particular coin. If a drop of dilute nitric acid was placed onto a silver coin adulterated with copper, it turned green, due to the formation of copper(II) nitrate. Conversely, if a gold coin reacted in any way with the nitric acid it was not pure. In both cases the coins failed the “acid test”.
Aqua regia (so called because it dissolves gold) is a mixture of 70% nitric acid and hydrochloric acid in a 1:3 ratio. Aqua regia is a very powerful oxidizing agent (it contains Cl2) and stablizes some metals as their chloro complexes (Table 1). If HF is added in place of HCl, tantalum may be dissolved with the formation of [TaF2]-.
Table 1: Metal chloride salts formed by the dissolution of metals in aqua regia.
| Metal |
Chloride salt formed |
| Au |
[AuCl4]- |
| Pt |
[PtCl6]2- |
Fuming nitric acid (100% nitric) is exceedingly corrosive and should not be used. In 100% sulfuric acid, nitric acid acts as a base and gets protonated, Equation 12 and Equation 13, and acts as a powerful nitrating agent.
In water dilute nitric acid is fully ionized, Equation 14.
The nitrate ion is planar (Figure 4b) and forms many salts and complexes. The nitrate anion is most commonly a monodentate ligand, but can also be a bidentate ligand (Figure 5a) or a bridging ligand (Figure 5b and c).
