Nitric acid is used in a wide variety of chemical processes where cleaning, oxidising or etching is required, including making synthetic fibres, dying, electrical circuit board making, electroplating, explosives, laboratory chemicals, metal cleaning and etching, semiconductors, pharmaceutical manufacture. It is used in the manufacture of fertilisers and other organic chemicals, in the printing industry for photoengraving, in jewellery manufacturing, and for wet chemical etching.
Substance name: Nitric acid
CASR number: 7697-37-2
Molecular formula: HNO3
Synonyms: Hydrogen Nitrate, Azotic acid, aqua fortis, red fuming acid, Nital, Nitryl Hydroxide
Nitric acid is a transparent, colourless or yellowish, corrosive liquid with an acrid or pungent odour.
Melting Point: -41.59°C
Boiling Point: 121°C
Specific Gravity: 1.502
Vapour Density: 2-3.0
Strongly acidic and highly reactive compound. Fumes in moist air, reacts readily with organic and plastic compounds, and many metals.
The National Pollutant Inventory (NPI) holds data for all sources of nitric acid emissions in Australia.
Nitric acid is very corrosive compound. The vapour is very irritating to the eyes, throat, lungs and corrosive to the teeth. If the vapour is inhaled in significant amounts it will result in severe coughing, chest pain and shortness of breath. Contact with the skin will result in a severe corrosive burn.
Entering the body
Trace amounts of nitric acid are present in the air circulating in and around major cities. It may be absorbed in minute amounts by inhaling it as a vapour. or swallowing it in liquid form.
Most people are exposed outdoors to very minute amounts of nitric acid derived from exhaust fumes or the burning of some organic compounds that contain nitrogen. It is also present in small quantities in rain from areas where nitric oxide (a product of combustion) reacts with ozone and water to form nitric acid. People living near industries that produce or use nitric acid may be exposed to minute quantities of the compound as a result of low rates of emission. Nitric acid may form in minute amounts wherever very high temperatures are generated in the presence of air (nitrogen and oxygen) and water.
Workplace exposure standards
Safe Work Australia sets the workplace exposure standard for nitric acid through the workplace exposure standards for airborne contaminants:
- Maximum eight hour time weighted average (TWA): 2 parts per million (5.2 mg/m3)
- Maximum short term exposure limit (STEL): 4 parts per million (10 mg/m3)
These standards are only appropriate for use in workplaces and are not limited to any specific industry or operation. Make sure you understand how to interpret the standards before you use them.
Drinking water guidelines
There is no guideline for nitric acid in the Australian Drinking Water Guidelines.
Nitric acid is a very corrosive compound when in concentrated form such as that commonly used in industrial and chemical processes. It is present in nature in very minute amounts. Severe thunderstorms can produce temperatures high enough to generate nitrogen dioxide which combines with water to form nitric acid. This is either rained or washed out of the atmosphere in very low concentrations. Nitric acid from this source readily reacts with ammonia in the atmosphere to form ammonium nitrate.
Entering the environment
Nitric acid exists in the atmosphere in the gas phase. The atmospheric removal processes for gaseous nitric acid are by wet and dry deposition. The estimated half-life and lifetime for dry deposition of nitric acid is 1.5 to 2 days and 2 to 3 days, respectively, and efficient rain out during episodic precipitation events. Nitric acid reacts with gaseous ammonia to form particulate or aerosol nitrate, which in turn is removed by wet and dry deposition of the particles. The average half-life and lifetime for particles in the atmosphere is about 3.5 to 10 days and 5 to 15 days, respectively.
Where it ends up
In the atmosphere, nitric acid is present in very minute quantities as a gas or vapour. It reacts with ammonia in the atmosphere to form a nitrate compound that is unlikely to persist for more than 10 to 15 days. Nitric acid does not build up in plant or animal tissues largely because of its highly reactive properties. In a concentrated form, nitric acid reacts very quickly with many other commonly occurring natural or manufactured substances and compounds. In some cases this can produce a vigorous reaction in the form of a chemical fire or explosion.
No national guideline for nitric acid.
Released from industries producing, using or handling nitric acid, for example chemical plants, metal, electronic, printing, glass, rubber and plastics plants and industries. Where ever very high temperature combustion takes place in the atmosphere in the presence of nitrogen, oxygen and water. May be present in small amounts in some waste-water from intensive farm factories and other facilities which produce waste-water containing high level of nitrogen.
Diffuse sources, and industry sources included in diffuse emissions data
May be present in exhaust gases from motor vehicles, the exhaust of incinerators or other chemical plants, or where these are in contact with moisture in the air. Unlikely to persist in nature because it readily reacts with a wide variety of naturally occurring substances.
Rare in nature as a gas in the atmosphere, in groundwater around active volcanic regions, or drainage from areas where accumulated organic or animal wastes are present.
Exhaust chambers in motor vehicles.
Could be retained as small amounts in products where it has been used in the manufacturing or treatment process.
Sources used in preparing this information
- ChemFinder WebServer Project (1995), Nitric Acid (accessed, 30 May, 1999)
- Cornell University, Planning Design and Construction, MSDS. (accessed, 30 May, 1999)
- National Environment Protection Council (1998), National Environment Protection Measure for the National Pollutant Inventory. (accessed, March, 1999)
- O'Neil, P(1993), Environmental Chemistry, 2nd edition, Chapman & Hall, UK.
- Technical Advisory Panel (1999), Final Report to the National Environment Protection Council.
- Safe Work Australia, Workplace exposure standards for airborne contaminants, accessed January 2019.
- National Health and Medical Research Council (NHMRC), Australian Drinking Water Guidelines (2011) - Updated October 2017, accessed May 2018.