3D model (JSmol)
|E number||E338 (antioxidants, ...)|
CompTox Dashboard (EPA)
|Molar mass||97.994 g·mol−1|
|Density||1.6845 g/cm3 (25 °C, 85%), 1.834 g/cm3 (solid)|
|Melting point||42.35 °C (108.23 °F; 315.50 K) anhydrous|
29.32 °C (84.78 °F; 302.47 K) hemihydrate
|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⦵)
|P280, P305+P351+P338, 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)
|Safety data sheet (SDS)||ICSC 1008|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
(what is ?)
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 H2PO−4, and the hydrogen phosphate ion HPO2−4, respectively. Phosphoric acid forms esters, called organophosphates.
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.
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.
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.
In aqueous solution phosphoric acid behaves as a triprotic acid.
- H3PO4 ⇌ H2PO−4 + H+, pKa1 = 2.14
- H2PO−4 ⇌ 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, H2PO−4, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective pK values.
The phase diagram of the H3PO4/H2O system is complicated. Solutions up to 62.5% H3PO4 are eutectic, exhibiting freezing-point depression as low as -85°C. Beyond this freezing-point increases, reaching 21°C by 85% H3PO4 (w/w) and a local maximum at 91.6% which corresponds to the hemihydrate 2H3PO4•H2O, freezing at 29.32°C. There is a second smaller eutectic depression at a concentration of 94.75% with a freezing point of 23.5°C. At higher concentrations the freezing point rapidly increases.
Concentrated phosphoric acid tends to supercool before crystallization occurs, and may be relatively resistant to crystallisation even when stored below the freezing point. For many industrial uses 85% represents a practical upper limit, where higher concentrations risk the entire mass freezing solid when transported inside of tankers and having to be melted out, although partial crystallisation can still occur in sub-zero temperatures.
Phosphoric acid is commercially available as aqueous solutions of various concentrations, not usually exceeding 85%. If concentrated further it undergoes slow self-condensation, forming an equilibrium with pyrophosphoric acid:
- 2 H3PO4 ⇌ H2O + H4P2O7
Even at 90% concentration the amount of pyrophosphoric acid present is negligible, but beyond 95% it starts to increase, reaching 15% at what would have otherwise been 100% orthophosphoric acid.
As the concentration is increased higher acids are formed, culminating in the formation of polyphosphoric acids. It is not possible to fully dehydrate phosphoric acid to phosphorus pentoxide, instead the polyphosphoric acid becomes increasingly polymeric and viscous. Due to the self-condensation, pure orthophosphoric acid can only be obtained by a careful fractional freezing/melting process.
|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. The phosphoric acid also serves as a preservative. 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
- to prevent iron oxidation by means of the Parkerization process
- 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
- in microfabrication to etch aluminium
- as a pH adjuster in cosmetics and skin-care products
- as a sanitizing agent in the dairy, food, and brewing industries
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.
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