Nitric acid (HNO3), also known as aqua fortis (Latin for "strong water") and spirit of niter, is a highly corrosive mineral acid.The pure compound is colourless, but older samples tend to acquire a yellow cast due to decomposition into oxides of nitrogen and water. 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 white fuming nitric acid at concentrations above 95%, or red fuming nitric acid at concentrations above 86%.
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 the 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 pigments in inks and dyes. Nitric acid is also commonly used as a strong oxidizing agent.
The main industrial use of nitric acid is for the production of fertilizers. Nitric acid is neutralized with ammonia to give ammonium nitrate. This application consumes 75–80% of the 26M tons produced annually (1987). The other main applications are for the production of explosives, nylon precursors, and speciality organic compounds.
A precursor to organic nitrogen compounds
See also: nitration
In organic synthesis, industrial and otherwise, the nitro group is a versatile functional group. Most derivatives of aniline are prepared via nitration of aromatic compounds followed by reduction. Nitrations entail combining nitric and sulfuric acids to generate the nitronium ion, which electrophilically reacts with aromatic compounds such as benzene. Many explosives, such as TNT, are prepared this way.
Etchant and cleaning agent
The corrosive effects of nitric acid are exploited for a number of speciality applications, such as etching in printmaking, pickling stainless steel or cleaning silicon wafers in electronics.
A solution of nitric acid, water and alcohol, Nital, is used for etching of metals to reveal the microstructure. ISO 14104 is one of the standards detailing this well-known procedure.
Commercially available aqueous blends of 5–30% nitric acid and 15–40% phosphoric acid are commonly used for cleaning food and dairy equipment primarily to remove precipitated calcium and magnesium compounds (either deposited from the process stream or resulting from the use of hard water during production and cleaning). The phosphoric acid content helps to passivate ferrous alloys against corrosion by the dilute nitric acid.
