Phosphoric acid


Phosphoric acid is used in the manufacture of superphosphate fertilisers, livestock feeds, phosphate salts, polyphosphates, soaps, waxes, polishes and detergents. Phosphoric acid is used as a soil stabiliser, in the manufacture of fire control agents, opal glasses, electric lights, in cotton dyeing, tile cleaning, ceramic binding, dental cement, water treatment, electro-polishing, operating lithography, photoengraving operations, process engraving, as a petrol additive and in coagulating rubber latex. It is used in metal rust proofing before painting, in the polishing of metals, in pickling and in hot stripping for aluminium and zinc substrates. Phosphoric acid is used as an acid catalyst in making ethylene and purifying hydrogen peroxide, in the manufacture of chemicals (ethylbenzene, propylene, cumene), as a bonding agent for refractory bricks, in extracting penicillin and as an analytical agent. It is used as an anti-oxidant in food, as a flavour additive for sharp taste in food (jellies, preserves) and soft drinks (e.g. Coca-Cola), as a tang (Food Additive 338) and for the manufacture of yeasts and gelatine. It is used to manufacture the phosphoric acid electrolyte fuel cell system and it has been used to treat lead poisoning.

Substance details

Substance name: Phosphoric acid

CASR number: 7664-38-2

Molecular formula: H3O4P (or PO(OH)3)

Synonyms: Orthophosphoric acid, white phosphoric acid, Sonac

Physical properties

Pure phosphoric acid is a non-combustible, colourless, odourless and hygroscopic crystal. Commercial phosphoric acid comes as a viscous solution in water which contains 75-85% phosphoric acid. The liquid can solidify at lower temperatures.

Melting Point: 42°C (pure)
21°C (85%)
18°C (75%)

Boiling Point: 260°C (pure)
154°C (85%)
135°C (75%)

Specific Gravity: 1.88 (pure)
1.69 (85%)
1.58 (75%)

Vapour Density: 3.4 (pure)

Chemical properties

Phosphoric acid can be made using either of two different methods. One method is by direct reaction of ground phosphate rock with sulfuric acid which also produces a lot of gypsum (calcium sulfate) as waste by-product. The second method is by burning elemental phosphorous and subsequent hydration of the phosphorous oxide. Phosphoric acid is a corrosive acid that can form three different classes of salts, namely primary phosphates, dibasic phosphates and tribasic phosphates. Phosphoric acid is soluble in water. It is incompatible with strong caustics and it is corrosive to ferrous metals and alloys. It readily reacts with metals to form flammable hydrogen gas. Phosphoric acid decomposes under formation of toxic fumes on contact with alcohols, aldehydes, cyanides, ketones, phenols, esters, sulfides, mercaptans and halogenated organic compounds. Phosphoric acid forms toxic phosphorous oxide fumes on combustion.

Further information

The National Pollutant Inventory (NPI) holds data for all sources of phosphoric acid in Australia.


Phosphoric acid can severely affect human health through the inhalation of mist, ingestion and contact with skin and eyes. Eye contact can cause redness, pain, tearing, eyelid spasms, blurred vision and permanent damage. Phosphoric acid can attack the mucous membranes of the mouth, throat and oesophagus, leading to immediate pain and difficulty in swallowing (dysphagia). Other symptoms of phosphoric acid poisoning include a sour acrid taste, coughing, difficult breathing, conjunctivitis, severe gastrointestinal irritation, nausea, vomiting, bloody diarrhoea, severe abdominal pains, extreme thirst, convulsion, shock and even death through choking (asphyxia). Higher phosphoric acid levels can cause a build-up of fluid in the lungs (pulmonary oedema) which may result in death. It can cause circulatory collapse with clammy skin, weak and rapid pulse, shallow respiration and scanty urine. Repeated exposure to phosphoric acid can cause bronchitis with cough, phlegm and/or shortness of breath. Long-term skin exposure to the liquid may cause dermatitis. People at special risk of exposure to phosphoric acid include those with chronic pulmonary disease or skin disease.

Entering the body

The substance can be absorbed into the body by inhalation of its aerosol and by ingestion.


Occupational exposure to concentrated levels of phosphoric acid may occur in industries manufacturing and using phosphoric acid. The general public may be exposed to small quantities of phosphoric acid in the consumption of food and soft drinks and by using some cleaning agents.

Health guidelines

Worksafe Australia has set the exposure standard for phosphoric acid to 1 milligram/m3 (TWA). The short-term exposure level (STEL) is 3 milligram/m3.


Phosphoric acid has moderate acute and chronic toxicity to aquatic life in waters of low alkalinity. Whilst small quantities of phosphoric acid can be neutralised by the alkalinity in aquatic ecosystems, larger quantities can lower the pH for extended periods of time, posing a potential risk to aquatic organisms. Phosphate (formed when phosphoric acid is dissolved) is unlikely to bioaccumulate in most aquatic species. Insufficient data are available to evaluate or predict the short-term and long-term effects of phosphoric acid on plants, birds or land animals.

Entering the environment

Phosphoric acid can be transported as mist in air and dissolved in water.

Where it ends up

Phosphoric acid entering the environment can acidify soils and waters. Smaller quantities of phosphoric acid will be neutralised forming harmless phosphate salts or it will be diluted to harmless levels.

Environmental guidelines

Australian Water Quality Guidelines for Fresh and Marine Waters (ANZECC, 1992):
Total phosphate: 10 to 100 micrograms/L (ie 0.00001 to 0.0001g/L)

Industry sources

It may enter the environment from industrial discharges, spills or mining operations run-off.

Diffuse sources, and industry sources included in diffuse emissions data

Sub-threshold facilities.

Natural sources

Phosphoric acid is not directly found in nature. It can be obtained from phosphate rock deposits.

Transport sources

There are no mobile sources for phosphoric acid.

Consumer products

Phosphoric acid can be found in soft drinks (e.g. Coca-Cola), food (e.g. jellies, preserves), animal food (e.g. cat food), some cleaning agents.

Sources used in preparing this information

  • Australian and New Zealand Environment and Conservation Council (ANZECC) (1992), Australian Water Quality Guidelines for Fresh and Marine Waters.
  • ChemFinder WebServer Project (1995), Phosphoric acid (accessed, June, 1999)
  • Environmental Health Center (a division of the National Safety Council) Environment Writer - Chemical Backgrounders (July 1, 1997), Phosphoric acid (accessed, June, 1999)
  • Integrated Risk Information System (IRIS, June 1, 1995), Phosphoric acid (accessed, June, 1999)
  • International Programme on Chemical Safety and The Commission of the European Communities (IPCS CEC 1993), International Chemical Safety Cards, Phosphoric acid (accessed, June, 1999)
  • National Environment Protection Council (1998), National Environment Protection Measure for the National Pollutant Inventory. (accessed, March, 1999)
  • New Jersey Department of Health and Senior Services - Hazardous Substance Fact Sheet (May 1997), Phosphoric acid (accessed, June, 1999)
  • Open Data Solutions, EPA factsheets for regulated toxic chemicals (February 1989), Phosphoric acid (accessed, June, 1999)
  • Technical Advisory Panel (1999), Final Report to the National Environment Protection Council
  • Worksafe Australia (1996), Exposure Standards Database, Phosphoric acid (accessed, June, 1999)
  • Worksafe Australia (1996), Hazardous Substances Database, Phosphoric acid (accessed, June, 1999)