Phosphoric acid

Phosphoric acid (also known as orthophosphoric acid or phosphoric(V) acid) is a weak acid with the chemical formula H
. Orthophosphoric acid refers to phosphoric acid, which is the IUPAC name for this compound. The prefix ortho- is used to distinguish the acid from related phosphoric acids, called polyphosphoric acids. Orthophosphoric acid is a non-toxic acid, which, when pure, is a solid at room temperature and pressure. The conjugate base of phosphoric acid is the dihydrogen phosphate ion, H
, which in turn has a conjugate base of hydrogen phosphate, HPO2−
, which has a conjugate base of phosphate, PO3−
. Phosphates are essential for life, being building blocks for both DNA and RNA.[12]

Phosphoric acid
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, ...)
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%)
145.0 J/molK
150.8 J/molK
−1271.7 kJ/mol
-1123.6 kJ/mol
Safety data sheet ICSC 1008
GHS pictograms [10]
GHS Signal word Danger
H290, H314[10]
P280, P305+351+338, P310[10]
NFPA 704 (fire diamond)
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).
N verify (what is YN ?)
Infobox references

The most common source of phosphoric acid is an 85% aqueous solution; such solutions are colourless, odourless, and non-volatile. The 85% solution is a syrupy liquid, but still pourable. 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.


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

Equilibrium Disassociation constant, pKa[15]
+ H+
pKa1 = 2.14[lower-alpha 1]
+ H+
pKa2 = 7.20
+ H+
pKa3 = 12.37
  1. Values are at 25 °C and 0 ionic strength.

Phosphoric acid, H3PO4, is a tribasic acid. The speciation diagram shows that, in aqueous solution, there are five main pH regions.

  • There are 3 regions, centred where the pH is equal to a pK value, which are buffer regions.
  • In the region centred around pH 4.7 (mid-way between the first two pK values) the dihydrogen phosphate ion, [H2PO4], is the only species present.
  • In the region centred around pH 9.8 (mid-way between the second and third pK values) the monohydrogen phosphate ion, [HPO4]2−, is the only species present.

This means that salts of the mono- and di-phosphate ions can be selectively crystallised from aqueous solution by setting the pH value to either 4.7 or 9.8.

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 silicic acid, Si(OH)4.


The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production.[16]

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[17]) 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.[18] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.[19]

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).[25] 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.[26]

See also


  1. Christensen, J. H. and 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.CS1 maint: multiple names: authors list (link)
  2. "CAMEO Chemicals Datasheet - Phosphoric Acid".
  3. Haynes, p. 4.80
  4. Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Retrieved 2 June 2014.
  5. "phosphoric acid_msds".
  6. NIOSH Pocket Guide to Chemical Hazards. "#0506". National Institute for Occupational Safety and Health (NIOSH).
  7. Haynes, p. 4.134
  8. Edwards, O. W.; Dunn, R. L. and 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.CS1 maint: multiple names: authors list (link)
  9. Haynes, p. 5.13
  10. Sigma-Aldrich Co., Phosphoric acid. Retrieved on 2014-05-09.
  11. "Phosphoric acid". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
  12. 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.
  13. Becker, Pierre. (1988). Phosphates and phosphoric acid. New York: Marcel Dekker. ISBN 978-0824717124.
  14. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 520–522. ISBN 978-0-08-037941-8.
  15. Powell, Kipton J.; Brown, Paul L.; Byrne, Robert H.; Gajda, Tamás; Hefter, Glenn; Sjöberg, Staffan; Wanner, Hans (2005). "Chemical speciation of environmentally significant heavy metals with inorganic ligands. Part 1: The Hg2+, Cl, OH, CO2−
    , SO2−
    , and PO3−
    aqueous systems". Pure Appl. Chem. 77 (4): 739–800. doi:10.1351/pac200577040739.
  16. Schrödter, Klaus; Bettermann, Gerhard; Staffel, Thomas; Wahl, Friedrich; Klein, Thomas; Hofmann, Thomas (2008) "Phosphoric Acid and Phosphates" in Ullmann's Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_465.pub3
  17. "Current EU approved additives and their E Numbers". Foods Standards Agency. 14 March 2012. Retrieved 22 July 2012.
  18. 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.
  19. 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.
  20. 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.
  21. Wet chemical etching.
  22. Wolf, S.; R. N. Tauber (1986). Silicon processing for the VLSI era: Volume 1 – Process technology. p. 534. ISBN 978-0-9616721-6-4.
  23. "Ingredient dictionary: P". Cosmetic ingredient dictionary. Paula's Choice. Archived from the original on 18 January 2008. Retrieved 16 November 2007.
  24. "STAR SAN" (PDF). Five Star Chemicals. Retrieved 17 August 2015.
  25. 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.
  26. "Phosphoric Acid, 85 wt.% SDS". Sigma-Aldrich. 5 May 2016.

Cited sources

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