Phosphoric acid

Phosphoric acid
Ball-and-stick model
Space-filling model
IUPAC name
Phosphoric acid
Other names
Orthophosphoric acid
3D model (JSmol)
ECHA InfoCard 100.028.758
EC Number
  • 231-633-2
E number E338 (antioxidants, ...)
RTECS number
  • TB6300000
UN number 1805
  • InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) Y
  • InChI=1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)
  • OP(=O)(O)O
Molar mass 97.994 g·mol−1
Appearance Colorless solid
Odor Odorless
Density 1.6845 g/cm3 (25 °C, 85%),[1] 1.834 g/cm3 (solid)[2]
Melting point 40–42.4 °C (104.0–108.3 °F; 313.1–315.5 K)[3]
Boiling point
  • 212 °C (414 °F)[4](only water evaporates)[5]
  • 392.2 g/(100 g) (−16.3 °C)
  • 369.4 g/(100 mL) (0.5 °C)
  • 446 g/(100 mL) (15 °C)[6]
  • 548 g/(100 mL) (20 °C)[3]
Solubility Soluble in ethanol
log P −2.15[7]
Vapor pressure 0.03 mmHg (20 °C)[8]
Conjugate base Dihydrogen phosphate
−43.8·10−6 cm3/mol[9]
  • 1.3420 (8.8% w/w aq. soln.)[10]
  • 1.4320 (85% aq. soln) 25 °C
Viscosity 2.4–9.4 cP (85% aq. soln.)
147 cP (100%)
145.0 J/(mol⋅K)
150.8 J/(mol⋅K)
−1271.7 kJ/mol
−1123.6 kJ/mol
GHS labelling:
H290, H314[12]
P280, P305+P351+P338, P310[12]
NFPA 704 (fire diamond)
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1530 mg/kg (rat, oral)[13]
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 1 mg/m3[8]
REL (Recommended)
TWA 1 mg/m3 ST 3 mg/m3[8]
IDLH (Immediate danger)
1000 mg/m3[8]
Safety data sheet (SDS) ICSC 1008
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H3PO4. It is commonly encountered as an 85% aqueous solution, 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. Removal of all three H+ ions gives the phosphate ion PO3−4. Removal of one or two protons gives dihydrogen phosphate ion H2PO4, and the hydrogen phosphate ion HPO2−4, respectively. Phosphoric acid forms esters, called organophosphates.[14]

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 one of two routes, wet processes and dry.[15]

Wet process

In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid.[16]

Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O
Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF

Calcium sulfate (gypsum, CaSO4) is a by-product, which is removed as phosphogypsum. The hydrogen fluoride (HF) gas is streamed into a wet (water) scrubber producing hydrofluoric acid. In both cases 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). 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 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.[17]

Acidic properties

In aqueous solution phosphoric acid behaves as a triprotic acid.

H3PO4 ⇌ H2PO4 + H+, pKa1 = 2.14
H2PO4 ⇌ HPO2−4 + H+, pKa2 = 7.20
HPO2−4 ⇌ PO3−4 + H+, pKa3 = 12.37

The difference between successive pKa values is sufficiently large so that salts of either monohydrogen phosphate, HPO2−4 or dihydrogen phosphate, H2PO4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK values.


The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production.[18]

Application Demand (2006) in thousands of tons Main phosphate derivatives
Soaps and detergents1836STPP
Food industry309STPP (Na5P3O10), SHMP, TSP, SAPP, SAlP, MCP, DSP (Na2HPO4), H3PO4
Water treatment164SHMP, STPP, TSPP, MSP (NaH2PO4), DSP
Toothpastes68DCP (CaHPO4), IMP, SMFP
Other applications287STPP (Na3P3O9), TCP, APP, DAP, zinc phosphate (Zn3(PO4)2), aluminium phosphate (AlPO4), H3PO4

Food-grade phosphoric acid (additive E338[19]) 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.[20] 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.[21] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.[22]

Specific applications of phosphoric acid include:


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.[28]

A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men).[29]

See also


  1. Christensen, J. H.; Reed, R. B. (1955). "Design and Analysis Data—Density of Aqueous Solutions of Phosphoric Acid Measurements at 25 °C". Ind. Eng. Chem. 47 (6): 1277–1280. doi:10.1021/ie50546a061.
  2. "CAMEO Chemicals Datasheet – Phosphoric Acid". Archived from the original on 15 August 2019. Retrieved 15 August 2019.
  3. Haynes, p. 4.80
  4. "Phosphoric acid". Archived from the original on 12 March 2020. Retrieved 3 March 2020.
  5. Brown, Earl H.; Whitt, Carlton D. (1952). "Vapor Pressure of Phosphoric Acids". Industrial & Engineering Chemistry. 44 (3): 615–618. doi:10.1021/ie50507a050.
  6. Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Archived from the original on 11 March 2020. Retrieved 2 June 2014.
  7. "phosphoric acid_msds". Archived from the original on 4 July 2017. Retrieved 2 May 2018.
  8. NIOSH Pocket Guide to Chemical Hazards. "#0506". National Institute for Occupational Safety and Health (NIOSH).
  9. Haynes, p. 4.134
  10. Edwards, O. W.; Dunn, R. L.; Hatfield, J. D. (1964). "Refractive Index of Phosphoric Acid Solutions at 25 C.". J. Chem. Eng. Data. 9 (4): 508–509. doi:10.1021/je60023a010.
  11. Haynes, p. 5.13
  12. Sigma-Aldrich Co., Phosphoric acid.
  13. "Phosphoric acid". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  14. Westheimer, F.H. (6 June 1987). "Why nature chose phosphates". Science. 235 (4793): 1173–1178 (see pp. 1175–1176). Bibcode:1987Sci...235.1173W. CiteSeerX doi:10.1126/science.2434996. PMID 2434996.
  15. Becker, Pierre (1988). Phosphates and phosphoric acid. New York: Marcel Dekker. ISBN 978-0824717124.
  16. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 520–522. ISBN 978-0-08-037941-8.
  17. Geeson, Michael B.; Cummins, Christopher C. (2020). "Let's Make White Phosphorus Obsolete". ACS Central Science. 6 (6): 848–860. doi:10.1021/acscentsci.0c00332. PMC 7318074. PMID 32607432.
  18. Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008). "Phosphoric Acid and Phosphates". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a19_465.pub3.
  19. "Current EU approved additives and their E Numbers". Foods Standards Agency. 14 March 2012. Archived from the original on 21 August 2013. Retrieved 22 July 2012.
  20. "Why is phosphoric acid used in some Coca‑Cola drinks?| Frequently Asked Questions | Coca-Cola GB". Archived from the original on 2 August 2021. Retrieved 31 August 2021.
  21. Moynihan, P. J. (23 November 2002). "Dietary advice in dental practice". British Dental Journal. 193 (10): 563–568. doi:10.1038/sj.bdj.4801628. PMID 12481178.
  22. Qaseem, A; Dallas, P; Forciea, MA; Starkey, M; et al. (4 November 2014). "Dietary and pharmacologic management to prevent recurrent nephrolithiasis in adults: A clinical practice guideline from the American College of Physicians". Annals of Internal Medicine. 161 (9): 659–67. doi:10.7326/M13-2908. PMID 25364887.
  23. Toles, C.; Rimmer, S.; Hower, J. C. (1996). "Production of activated carbons from a washington lignite using phosphoric acid activation". Carbon. 34 (11): 1419. doi:10.1016/S0008-6223(96)00093-0.
  24. Wet chemical etching. Archived 25 September 2012 at the Wayback Machine
  25. Wolf, S.; R. N. Tauber (1986). Silicon processing for the VLSI era: Volume 1 – Process technology. p. 534. ISBN 978-0-9616721-6-4.
  26. "Ingredient dictionary: P". Cosmetic ingredient dictionary. Paula's Choice. Archived from the original on 18 January 2008. Retrieved 16 November 2007.
  27. "STAR SAN" (PDF). Five Star Chemicals. Archived (PDF) from the original on 8 February 2016. Retrieved 17 August 2015.
  28. "Phosphoric Acid, 85 wt.% SDS". Sigma-Aldrich. 5 May 2016. Archived from the original on 18 January 2017. Retrieved 16 January 2017.
  29. Tucker KL, Morita K, Qiao N, Hannan MT, Cupples LA, Kiel DP (1 October 2006). "Colas, but not other carbonated beverages, are associated with low bone mineral density in older women: The Framingham Osteoporosis Study". American Journal of Clinical Nutrition. 84 (4): 936–942. doi:10.1093/ajcn/84.4.936. PMID 17023723.

Cited sources

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