Phosphoric acid

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Phosphoric acid
Ball-and-stick model Phosphoric-acid-3D-balls.png
Ball-and-stick model
Space-filling model Phosphoric-acid-3D-vdW.png
Space-filling model
IUPAC name
OrthoPhosphoric acid
Other names
Phosphoric acid
3D model (JSmol)
ECHA InfoCard 100.028.758
EC Number
  • 231-633-2
E number E338 (antioxidants, ...)
PubChem CID
RTECS number
  • TB6300000
UN number 1805
Molar mass 97.994 g·mol−1
Odor Odorless
Density 1.6845  gcm−3 (25 °C, 85%), [1] 1.834  gcm−3 (solid) [2]
Melting point 42.4 °C (108.3 °F; 315.5 K) [3]
Boiling point 407 °C (765 °F; 680 K) [3]
  • 392.2 g/100g (−16.3 °C)
  • 369.4 g/100mL (0.5 °C)
  • 446 g/100mL (15 °C) [4]
  • 548 g/100mL (20 °C) [3]
Solubility Soluble in ethanol
log P −2.15 [5]
Vapor pressure 0.03 mmHg (20 °C) [6]
Conjugate base Dihydrogen phosphate
Hydrogen phosphate
−43.8·10−6 cm3/mol [7]
  • 1.3420 (8.8% w/w aq. soln.) [8]
  • 1.4320 (85% aq. soln) 25 °C
Viscosity 2.4–9.4  cP (85% aq. soln.)
147 cP (100%)
Thermochemistry [9]
145.0 J/molK
150.8 J/molK
−1271.7 kJ/mol
-1123.6 kJ/mol
Safety data sheet ICSC 1008
GHS pictograms GHS-pictogram-acid.svg [10]
GHS Signal word Danger
H290, H314 [10]
P280, P305+351+338, P310 [10]
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no codePhosphoric acid
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1530 mg/kg (rat, oral) [11]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3 [6]
REL (Recommended)
TWA 1 mg/m3 ST 3 mg/m3 [6]
IDLH (Immediate danger)
1000 mg/m3 [6]
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid with the chemical formula H
. It is a non-toxic compound which, when pure, is a solid at room temperature and pressure.


All three hydrogens are acidic to varying degrees and can be lost from the molecule as H+ ions (protons). When all three H+ ions are removed, the result is an orthophosphate ion PO43−, commonly called "phosphate". Removal of one or two protons gives dihydrogen phosphate ion H
, and the hydrogen phosphate ion HPO2−
, respectively. Orthophosphoric acid also forms esters, called organophosphates. [12]

Phosphoric acid is commonly encountered in chemical laboratories as an 85% aqueous solution, which is a colourless, odourless, and non-volatile syrupy liquid. Although phosphoric acid does not meet the strict definition of a strong acid, the 85% solution can still severely irritate the skin and damage the eyes.

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.


Phosphoric acid is produced industrially by two general routes. [13] In the wet process a phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid. [14]

Fluoroapatite is an alternative feedstock, in which case fluoride is removed as the insoluble compound Na2SiF6. The phosphoric acid solution usually contains 23–33% P2O5 (32–46% H3PO4). It may be concentrated to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P2O5 (75–85% H3PO4). Further removal of water yields superphosphoric acid with a P2O5 concentration above 70% (corresponding to nearly 100% H3PO4). Calcium sulfate (gypsum) is produced as a by-product and is removed as phosphogypsum.

To produce food-grade phosphoric acid, phosphate ore is first reduced with coke in an electric arc furnace, to make 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.

The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.

Acidic properties

All three hydrogens are acidic, with dissociation constants pKa1 = 2.14, pKa2 = 7.20, and pKa3 = 12.37. It follows that, in water solutions, phosphoric acid is mostly dissociated into some combination of its three anions, except at very low pH.

When phosphoric acid is dissolved in a superacid, poorly characterized products are formed. It is likely that a reaction such as

H3PO4 + HSbF6 [P(OH)4]+ + [SbF6]

occurs. The ion [P(OH)4]+ is isoelectronic with orthosilicic acid, Si(OH)4.


The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production. [15]

ApplicationDemand (2006) in thousands of tonsMain phosphate derivatives
Soaps and detergents1836 STPP
Food industry309 STPP (Na5P3O10), SHMP, TSP, SAPP, SAlP, MCP, DSP (Na2HPO4), H3PO4
Water treatment 164SHMP, STPP, TSPP, MSP (NaH2PO4), DSP
Toothpastes 68 DCP (CaHPO4), IMP, SMFP
Other applications287 STPP (Na3P3O9), TCP, APP, DAP, zinc phosphate (Zn3(PO4)2), aluminium phosphate (AlPO4, H3PO4)

Food-grade phosphoric acid (additive E338 [16] ) is used to acidify foods and beverages such as various colas and jams, providing a tangy or sour taste. 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. [17] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously. [18]

Specific applications of phosphoric acid include:


A link has been shown between long-term regular cola intake and osteoporosis in older women (but not men). [24] This was thought to be due to the presence of phosphoric acid, and the risk for women was found to be greater for sugared and caffeinated colas than diet and decaffeinated variants, with a higher intake of cola correlating with lower bone density.

At moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage. [25]

See also

Related Research Articles

Phosphorus Chemical element with atomic number 15

Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Earth. It has a concentration in the Earth's crust of about one gram per kilogram. In minerals, phosphorus generally occurs as phosphate.

Phosphate salt or ester of phosphoric acid

In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid H

Polyphosphates are salts or esters of polymeric oxyanions formed from tetrahedral PO4 (phosphate) structural units linked together by sharing oxygen atoms. Polyphosphates can adopt linear or a cyclic ring structures. In biology, the polyphosphate esters ADP and ATP are involved in energy storage. A variety of polyphosphates find application in mineral sequestration in municipal waters, generally being present at 1 to 5 ppm. GTP, CTP, and UTP are also nucleotides important in the protein synthesis, lipid synthesis, and carbohydrate metabolism, respectively.

Phosphine Chemical compound hydrogen phosphide

Phosphine (IUPAC name: phosphane) is the compound with the chemical formula PH3. It is a colorless, flammable, toxic gas and is classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like garlic or rotting fish, due to the presence of substituted phosphine and diphosphane (P2H4). With traces of P2H4 present, PH3 is spontaneously flammable in air (pyrophoric), burning with a luminous flame. Phosphines are also a group of organophosphorus compounds with the formula R3P (R = organic derivative). Organophosphines are important in catalysts where they complex to various metal ions; complexes derived from a chiral phosphine can catalyze reactions to give chiral, enantioenriched products.

Calcium phosphate is a family of materials and minerals containing calcium ions (Ca2+) together with inorganic phosphate anions. Some so-called calcium phosphates contain oxide and hydroxide as well. They are white solids of nutritious value. Calcium phosphates are found in many living organisms, e.g., bone mineral and tooth enamel. In milk, it exists in a colloidal form in micelles bound to casein protein with magnesium, zinc, and citrate - collectively referred to as colloidal calcium phosphate (CCP). Various calcium phosphate minerals are used in the production of phosphoric acid and fertilizers. Overuse of certain forms of calcium phosphate can lead to nutrient-containing surface runoff and subsequent adverse effects upon receiving waters such as algal blooms and eutrophication.

Tetrasodium pyrophosphate chemical compound

Tetrasodium pyrophosphate, also called sodium pyrophosphate, tetrasodium phosphate or TSPP, is an inorganic compound with the formula Na4P2O7. As a salt, it is a white, water-soluble solid. It is composed of pyrophosphate anion and sodium ions. Toxicity is approximately twice that of table salt when ingested orally. Also known is the decahydrate Na4P2O7 · 10(H2O).

Diammonium phosphate chemical compound

Diammonium phosphate (DAP) (chemical formula (NH4)2HPO4, IUPAC name diammonium hydrogen phosphate) is one of a series of water-soluble ammonium phosphate salts that can be produced when ammonia reacts with phosphoric acid. Solid diammonium phosphate shows a dissociation pressure of ammonia as given by the following expression and equation:

Phosphorus pentachloride chemical compound

Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moisture-sensitive solid, although commercial samples can be yellowish and contaminated with hydrogen chloride.

Phosphorus trichloride chemical compound

Phosphorus trichloride is a chemical compound of phosphorus and chlorine, having the chemical formula PCl3. It is a toxic and volatile liquid which reacts violently with water to release HCl gas. It has a trigonal pyramidal shape, owing to the lone pairs on the phosphorus. It is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds for a wide variety of applications. It has a 31P NMR signal at around +220 ppm with reference to a phosphoric acid standard.

Phosphoric acids and phosphates

A phosphoric acid, in the general sense, is a phosphorus oxoacid in which each phosphorus atom is in the oxidation state +5, and is bonded to four oxygen atoms, one of them through a double bond, arranged as the corners of a tetrahedron. Two or more of these PO
tetrahedra may be connected by shared single-bonded oxygens, forming linear or branched chains, cycles, or more complex structures. The single-bonded oxygen atoms that are not shared are completed with acidic hydrogen atoms. The general formula of a phosphoric acid is Hn+2−2xPnO3n+1−x, where n is the number of phosphorus atoms and x is the number of fundamental cycles in the molecule's structure, betwen 0 and (n+2)/2.

Ammonium phosphate chemical compound

Ammonium phosphate is an ammonium salt of orthophosphoric acid. It is a highly unstable compound with the formula (NH4)3PO4. Because of its instability, it is elusive and of no commercial value. A related "double salt", (NH4)3PO4.(NH4)2HPO4 is also recognized but is too unstable for practical use. Both triammonium salts evolve ammonia. In contrast to the unstable nature of the triammonium salts, the diammonium phosphate (NH4)2HPO4 monoammonium salt (NH4)H2PO4, are stable materials that are commonly used as fertilizers to provide plants with fixed nitrogen and phosphorus.

Phosphorus oxoacid is a generic name for any acid whose molecule consists of atoms of phosphorus, oxygen, and hydrogen. There is a potentially infinite number of such compounds. Some of them are unstable and have not been isolated, but the derived anions and organic groups are present in stable salts and esters. The most important ones — in biology, geology, industry, and chemical research — are the phosphoric acids, whose esters and salts are the phosphates.

Tricalcium phosphate chemical compound Ca₃O₈P₂

Tricalcium phosphate (sometimes abbreviated TCP) is a calcium salt of phosphoric acid with the chemical formula Ca3(PO4)2. It is also known as tribasic calcium phosphate and bone phosphate of lime (BPL). It is a white solid of low solubility. Most commercial samples of "tricalcium phosphate" are in fact hydroxyapatite.

Monocalcium phosphate chemical compound CaH₄O₈P₂

Monocalcium phosphate is an inorganic compound with the chemical formula Ca(H2PO4)2 ("AMCP" or "CMP-A" for anhydrous monocalcium phosphate). It is commonly found as the monohydrate ("MCP" or "MCP-M"), Ca(H2PO4)2·H2O. Both salts are colourless solids. They are used mainly as superphosphate fertilizers and are also popular leavening agents..

Phosphoryl chloride chemical compound

Phosphoryl chloride (commonly called phosphorus oxychloride) is a colourless liquid with the formula POCl3. It hydrolyses in moist air releasing phosphoric acid and fumes of hydrogen chloride. It is manufactured industrially on a large scale from phosphorus trichloride and oxygen or phosphorus pentoxide. It is mainly used to make phosphate esters such as tricresyl phosphate.

Dicalcium phosphate chemical compound CaHPO₄

Dicalcium phosphate is the calcium phosphate with the formula CaHPO4 and its dihydrate. The "di" prefix in the common name arises because the formation of the HPO42– anion involves the removal of two protons from phosphoric acid, H3PO4. It is also known as dibasic calcium phosphate or calcium monohydrogen phosphate. Dicalcium phosphate is used as a food additive, it is found in some toothpastes as a polishing agent and is a biomaterial.

Disodium phosphate chemical compound Na2HPO4

Disodium phosphate (DSP), or sodium hydrogen phosphate, or sodium phosphate dibasic, is the inorganic compound with the formula Na2HPO4. It is one of several sodium phosphates. The salt is known in anhydrous form as well as forms with 2, 7, 8, and 12 hydrates. All are water-soluble white powders; the anhydrous salt being hygroscopic.

Monosodium phosphate chemical compound

Monosodium phosphate (MSP), also known as monobasic sodium phosphate and sodium dihydrogen phosphate, is an inorganic compound of sodium with a dihydrogen phosphate (H2PO4) anion. One of many sodium phosphates, it is a common industrial chemical. The salt exists in an anhydrous form, as well as mono- and dihydrates..

Ammonium dihydrogen phosphate chemical compound

Ammonium dihydrogen phosphate (ADP), also known as monoammonium phosphate (MAP) is a chemical compound with the chemical formula (NH4)(H2PO4). ADP is a major ingredient of agricultural fertilizers and some fire extinguishers. It also has significant uses in optics and electronics.

Di-(2-ethylhexyl)phosphoric acid chemical compound

Di-(2-ethylhexyl)phosphoric acid (DEHPA or HDEHP) is an organophosphorus compound with the formula (C8H17O)2PO2H. The yellow liquid is a diester of phosphoric acid and 2-ethylhexanol. It is used in the solvent extraction of uranium, as well as the rare-earth metals.


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