Production
Phosphoric acid is produced industrially by one of two routes, wet processes and dry.Wet process
In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid. : : Calcium sulfate (gypsum, ) is a by-product, which is removed as phosphogypsum. The hydrogen fluoride (HF) gas is streamed into a Wet scrubber, wet (water) scrubber producing hydrofluoric acid. In both cases the phosphoric acid solution usually contains 23–33% P2O5 (32–46% ). It may be concentrated to produce ''commercial-'' or ''merchant-grade'' phosphoric acid, which contains about 54–62% Phosphorus pentoxide, (75–85% ). Further removal of water yields ''superphosphoric acid'' with a concentration above 70% (corresponding to nearly 100% ). The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.Dry process
To produce food-grade phosphoric acid, phosphate ore is first reduced with Coke (fuel), coke in an electric arc furnace, to give elemental phosphorus. Silica is also added, resulting in the production of calcium silicate slag. Elemental phosphorus is distilled out of the furnace and burned with air to produce high-purity phosphorus pentoxide, which is dissolved in water to make phosphoric acid.Acidic properties
In aqueous solution phosphoric acid behaves as a triprotic acid. :, p''K''a1 = 2.14 :, p''K''a2 = 7.20 :, p''K''a3 = 12.37 The difference between successive p''K''a values is sufficiently large so that salts of either monohydrogen phosphate, or dihydrogen phosphate, , can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective p''K'' values.Uses
The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production. Food-grade phosphoric acid (additive E number, E338) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. The phosphoric acid also serves as a preservative. Soft drinks containing phosphoric acid, which would include Coca-Cola, are sometimes called phosphate sodas or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion. Phosphoric acid also has the potential to contribute to the formation of Kidney stone disease, kidney stones, especially in those who have had kidney stones previously. Specific applications of phosphoric acid include: * in anti-rust treatment by phosphate conversion coating or Passivation (chemistry), passivation * to prevent iron oxidation by means of the Parkerization (metallurgy), Parkerization process * as an external standard for phosphorus-31 nuclear magnetic resonance * in phosphoric acid fuel cells * in activated carbon production * in compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide * in microfabrication to etch silicon nitride selectively with respect to silicon dioxide * as a pH adjuster in cosmetics and skin-care products * as a sanitizing agent in the dairy, food, and brewing industriesSafety
Phosphoric acid is not a strong acid. However, at moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage. A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men).See also
* Phosphate fertilizers, such as ammonium phosphate fertilizersReferences
Cited sources
*External links