3D model (JSmol)
|E number||E338 (antioxidants, ...)|
|Molar mass||97.99 g·mol−1|
|Appearance||White, deliquescent, solid|
|Density||2.030 g cm-3 (25 °C)|
|Boiling point||158 °C (316 °F; 431 K) decomposes|
|392.2 g/100 g (−16.3 °C)|
369.4 g/100 mL (0.5 °C)
446 g/100 mL (14.95 °C) 
|Solubility||soluble in ethanol|
|Vapor pressure||0.03 mmHg (20°C)|
|Acidity (pKa)||pKa1 = 2.15|
pKa2 = 7.20
pKa3 = 12.32
|Conjugate base||Dihydrogen phosphate|
Refractive index (nD)
|1.3420 (8.8% w/w aq. soln)|
1.4320 (85% aq. soln) 25 °C
|Viscosity||2.4–9.4 cP (85% aq. soln.)|
147 cP (100%)
Std enthalpy of
|Safety data sheet||ICSC 1008|
|GHS signal word||Danger|
|P280, P305+351+338, P310|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|1530 mg/kg (rat, oral)|
|US health exposure limits (NIOSH):|
|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).
|‹See TfM› (what is ‹See TfM› ?)|
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
4, which in turn has a conjugate base of hydrogen phosphate, HPO2−
4, which has a conjugate base of phosphate, PO3−
4. Phosphates are essential for life.
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.
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.
|H3PO4 ⇌ H
4 + H+
|pKa1 = 2.14†|
4 ⇌ HPO2−
4 + H+
|pKa2 = 7.20|
4 ⇌ PO3−
4 + 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]-, 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]−
|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. It provides a tangy or sour taste. 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
- 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.
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.
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