Phosphoric acid, also known as orthophosphoric acid or phosphoric(V) acid, is a weak acid with the chemical formula H
4. It is normally encountered as a colorless syrup of 85% concentration in water. The pure compound is a colorless solid.
3D model (JSmol)
|E number||E338 (antioxidants, ...)|
CompTox Dashboard (EPA)
|Molar mass||97.994 g·mol−1|
|Density||1.6845 g⋅cm−3 (25 °C, 85%), 1.834 g⋅cm−3 (solid)|
|Melting point||40–42.4 °C (104.0–108.3 °F; 313.1–315.5 K)|
|Boiling point||212 °C (414 °F; 485 K)|
|Solubility||Soluble in ethanol|
|Vapor pressure||0.03 mmHg (20 °C)|
|Conjugate base||Dihydrogen phosphate|
Refractive index (nD)
|Viscosity||2.4–9.4 cP (85% aq. soln.) |
147 cP (100%)
Heat capacity (C)
Std enthalpy of
Gibbs free energy (ΔfG˚)
|Safety data sheet||ICSC 1008|
|GHS Signal word||Danger|
|P280, P305+351+338, P310|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|1530 mg/kg (rat, oral)|
|NIOSH (US health exposure limits):|
|TWA 1 mg/m3|
|TWA 1 mg/m3 ST 3 mg/m3|
IDLH (Immediate danger)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|what is ?)(|
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
4, and the hydrogen phosphate ion HPO2−
4, respectively. Orthophosphoric acid also forms esters, called organophosphates.
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.
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.
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. The equilibrium equations are:
- H3PO4 + H2O ⇌ H3O+ + H2PO4− Ka1= 7.25×10−3 [pKa1 = 2.14]
- H2PO4−+ H2O ⇌ H3O+ + HPO42− Ka2= 6.31×10−8 [pKa2 = 7.20]
- HPO42−+ H2O ⇌ H3O+ + PO43− Ka3= 3.98×10−13 [pKa3 = 12.37]
|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) 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. 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
- As an external standard for phosphorus-31 nuclear magnetic resonance.
- In phosphoric acid fuel cells.
- In activated carbon production.
- In compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide.
- 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.
A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men). 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.
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