Nitric acid (HNO3), also known as aqua fortis and spirit of niter, is a highly corrosive strong mineral acid. The pure compound is colorless, but older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. Most commercially available nitric acid has a concentration of 68%. 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 or red fuming nitric acid, at concentrations above 95%. Nitric acid is also commonly used as a strong oxidizing agent.
Contamination with nitrogen dioxide
Nitric acid is subject to thermal or light decomposition: 4 HNO3 → 2 H2O + 4 NO2 + O2. This reaction may give rise to some non-negligible variations in the vapor pressure above the liquid because the nitrogen oxides produced dissolve partly or completely in the acid.
The main use of nitric acid is for the production of fertilisers. Nitric acid is neutralised 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 specialty organic compounds.
As an analytical reagent – In elemental analysis by ICP-MS, ICP-AES, GFAA, and Flame AA, dilute nitric acid (0.5 to 5.0%) is used as a matrix compound for determining metal traces in solutions. Ultrapure trace metal grade acid is required for such determination, because small amounts of metal ions could affect the result of the analysis.
It is also typically used in the digestion process of turbid water samples, sludge samples, solid samples as well as other types of unique samples which require elemental analysis.
In electrochemistry, nitric acid is used as a chemical doping agent for organic semiconductors, and in purification processes for raw carbon nanotubes.
Woodworking – In a low concentration (approximately 10%), nitric acid is often used to artificially age pine and maple. The color produced is a grey-gold very much like very old wax or oil finished wood
Etchant and cleaning agent – The corrosive effects of nitric acid are exploited for a number of specialty applications, such as etching of metals to reveal the microstructure.
Concentrated nitric acid is required for many chemical processes. It is produced by feeding the aqueous nitric acid resulting from the oxidation of ammonia (composition about 65% HNO3 by mass), into a concentration unit along with 60 – 67% by mass of concentrated sulphuric acid, H2SO4.
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