Phosphoric acid

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Phosphoric acid
Structural formula of phosphoric acid, showing dimensions
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 E337 (antioxidants, ...)
RTECS number TB6300000
UN number 1805
Appearance White, deliquescent, solid
viscous liquid
Odor odorless
Density 2.030 g cm-3 (25 °C)
Melting point 42.3°C(anhydrous)
29.3°C (hemihydrate)
Boiling point 158°C decomposes
392.2 g/100 g (−16.3 °C)
369.4 g/100 mL (0.5 °C)
446 g/100 mL (14.95 °C) [1]
Solubility soluble in ethanol
log P -2.15[2]
Vapor pressure 0.03 mmHg (20°C)[3]
Acidity (pKa) pKa1 = 2.15
pKa2 = 7.20
pKa3 = 12.32
Conjugate base Dihydrogen phosphate
Hydrogen phosphate
−43.8·10−6 cm3/mol
1.3420 (8.8% w/w aq. soln)[4]
1.4320 (85% aq. soln) 25 °C
Viscosity 2.4–9.4 cP (85% aq. soln.)
147 cP (100%)
158 J/(mol·K)[5]
−1288 kJ/mol[5]
Safety data sheet ICSC 1008
GHS pictograms The corrosion pictogram in the Globally Harmonized System of Classification and Labelling of Chemicals (GHS)[6]
GHS signal word Danger
H290, H314[6]
P280, P305+351+338, P310[6]
NFPA 704
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 codeNFPA 704 four-colored diamond
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
1530 mg/kg (rat, oral)[7]
US health exposure limits (NIOSH):
PEL (Permissible)
TWA 1 mg/m3[3]
REL (Recommended)
TWA 1 mg/m3 ST 3 mg/m3[3]
IDLH (Immediate danger)
1000 mg/m3[3]
Related compounds
Hypophosphorous acid
Phosphorous acid
Pyrophosphoric acid
Triphosphoric acid
Peroxomonophosphoric acid
Peroxodiphosphoric acid
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 H3PO4. 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.[8]

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 is acidic enough to be corrosive. Because of the high percentage of phosphoric acid in this reagent, at least some of the orthophosphoric acid is condensed into polyphosphoric acids; for the sake of labeling and simplicity, the 85% represents H3PO4 as if it were all in the ortho form. Dilute aqueous solutions of phosphoric acid exist in the ortho form.


Phosphoric acid is produced industrially by two general routes.[9] In the wet process a phosphate-containing mineral such as calcium hydroxyapatite is treated with sulfuric acid.[10]

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[edit]

Phosphoric acid speciation

Equilibrium pKa value[11]
+ H+
pKa1 = 2.14
+ H+
pKa2 = 7.20
+ H+
pKa3 = 12.37

values are for 25 °C and zero 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]2-, 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]-, 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.[12]

Application Demand (2006) in thousands of tons Main phosphate derivatives
Soaps and detergents 1836 STPP
Food industry 309 STPP (Na5P3O10), SHMP, TSP, SAPP, SAlP, MCP, DSP (Na2HPO4), H3PO4
Water treatment 164 SHMP, STPP, TSPP, MSP (NaH2PO4), DSP
Toothpastes 68 DCP (CaHPO4), IMP, SMFP
Other applications 287 STPP (Na3P3O9), TCP, APP, DAP, zinc phosphate (Zn3(PO4)2), aluminium phosphate (AlPO4, H3PO4)

Food-grade phosphoric acid (additive E338[13]) is used to acidify foods and beverages such as various colas and jams. It provides a tangy or sour taste. Phosphoric acid in soft drinks has the potential to cause dental erosion.[14] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.[15]

Specific applications of phosphoric acid include:


Products such as soft drinks that contain phosphoric acid pose no threat to human health in general. At moderate concentrations phosphoric acid solutions are irritating to the skin. Contact with concentrated solutions can cause severe skin burns and permanent eye damage.[21]

See also[edit]


  1. ^ Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Retrieved 2014-06-02. 
  2. ^ "phosphoric acid_msds". 
  3. ^ a b c d "NIOSH Pocket Guide to Chemical Hazards #0506". National Institute for Occupational Safety and Health (NIOSH). 
  4. ^ O. W. Edwards, R. L. Dunn, J. D. Hatfield (1964). "Refractive Index of Phosphoric Acid Solutions at 25 C". J. Chem. Eng. Data. doi:10.1021/je60023a010. 
  5. ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 0-618-94690-X. 
  6. ^ a b c Sigma-Aldrich Co., Phosphoric acid. Retrieved on 2014-05-09.
  7. ^ "Phosphoric acid". Immediately Dangerous to Life and Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH). 
  8. ^ Westheimer, F.H. (6 June 1987). "Why nature chose phosphates". Science. 235 (4793): 1173–1178 (see pp. 1175–1176). Bibcode:1987Sci...235.1173W. doi:10.1126/science.2434996. 
  9. ^ Becker, Pierre. (1988). Phosphates and phosphoric acid. New York: Marcel Dekker. ISBN 0824717120. 
  10. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. pp. 520–522. ISBN 0-08-037941-9. 
  11. ^ 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.
  12. ^ Klaus Schrödter, Gerhard Bettermann, Thomas Staffel, Friedrich Wahl, Thomas Klein, Thomas Hofmann "Phosphoric Acid and Phosphates" in Ullmann's Encyclopedia of Industrial Chemistry 2008, Wiley-VCH, Weinheim. doi:10.1002/14356007.a19_465.pub3
  13. ^ "Current EU approved additives and their E Numbers". Foods Standards Agency. 14 March 2012. Retrieved 22 July 2012. 
  14. ^ "Dietary advice in dental practice". British Dental Journal. 23 November 2002. Retrieved 29 June 2017. 
  15. ^ 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. 
  16. ^ 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. 
  17. ^ Wet chemical etching.
  18. ^ Wolf, S.; R. N. Tauber (1986). Silicon processing for the VLSI era: Volume 1 – Process technology. p. 534. ISBN 0-9616721-6-1. 
  19. ^ "Ingredient dictionary: P". Cosmetic ingredient dictionary. Paula's Choice. Archived from the original on 18 January 2008. Retrieved 16 November 2007. 
  20. ^ "STAR SAN" (PDF). Five Star Chemicals. Retrieved 17 August 2015. 
  21. ^ "Phosphoric Acid, 85 wt.% SDS". Sigma-Aldrich. 5 May 2016. 

External links[edit]