Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the chemical formula . It is commonly encountered as an 85%

aqueous solution An aqueous solution is a solution in which the solvent is water. It is mostly shown in chemical equations by appending (aq) to the relevant chemical formula. For example, a solution of table salt, or sodium chloride (NaCl), in water would be re ...

, which is a colourless, odourless, and non- volatile syrupy liquid. It is a major industrial chemical, being a component of many fertilizers. The compound is an

acid In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...

. Removal of all three ions gives the phosphate ion . Removal of one or two protons gives dihydrogen phosphate ion , and the hydrogen phosphate ion , respectively. Phosphoric acid forms esters, called

organophosphate In organic chemistry, organophosphates (also known as phosphate esters, or OPEs) are a class of organophosphorus compounds with the general structure , a central phosphate molecule with alkyl or aromatic substituents. They can be considered a ...

s. The name "orthophosphoric acid" can be used to distinguish this specific acid from other " phosphoric acids", such as pyrophosphoric acid. Nevertheless, the term "phosphoric acid" often means this specific compound; and that is the current IUPAC nomenclature.

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 Hydroxyapatite, also called hydroxylapatite (HA), is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but it is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. ...

and

fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, bl ...

are treated with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

. : : Calcium sulfate (gypsum, ) is a by-product, which is removed as phosphogypsum. The

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

(HF) gas is streamed into a wet (water) scrubber producing

hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...

. 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% (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 in an electric arc furnace, to give elemental phosphorus. Silica is also added, resulting in the production of

calcium silicate Calcium silicate is the chemical compound Ca2SiO4, also known as calcium orthosilicate and is sometimes formulated as 2CaO·SiO2. It is also referred to by the shortened trade name Cal-Sil or Calsil. It occurs naturally as the mineral larnite. ...

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 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 soda Phosphate soda is a type of beverage that has a tangy or sour taste. These beverages became popular among children in the 1870s in the United States. Phosphate beverages were made with fruit flavorings, egg, malt, or wine. In the 1900s, the bever ...

s 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 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 * to prevent iron oxidation by means of the Parkerization process * as an external standard for phosphorus-31 nuclear magnetic resonance * in phosphoric acid fuel cells * in

activated carbon Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that increase the surface area avail ...

production * in compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to

indium phosphide Indium phosphide (InP) is a binary semiconductor composed of indium and phosphorus. It has a face-centered cubic ("zincblende") crystal structure, identical to that of GaAs and most of the III-V semiconductors. Manufacturing Indium phosphide ca ...

* 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 industries

Safety

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 Osteoporosis is a systemic skeletal disorder characterized by low bone mass, micro-architectural deterioration of bone tissue leading to bone fragility, and consequent increase in fracture risk. It is the most common reason for a broken bone ...

in later middle age in women (but not men).

References

Cited sources

External links

National pollutant inventory – Phosphoric acid fact sheet

{{DEFAULTSORT:Phosphoric Acid Mineral acids Flavors Food acidity regulators Glassforming liquids and melts Phosphates Phosphorus oxoacids Hydrogen compounds Acid catalysts E-number additives Phosphorus(V) compounds