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Phosphoric acid

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(Redirected from Orthophosphoric acid) Chemical compound (PO(OH)3) This article is about orthophosphoric acid. For other acids commonly called "phosphoric acid", see Phosphoric acids and phosphates. Not to be confused with Phosphorous acid.

Phosphoric acid
Structural formula of phosphoric acid, showing dimensions
Ball-and-stick model
Ball-and-stick model
Space-filling model
Space-filling model
Names
IUPAC name Phosphoric acid
Other names Orthophosphoric acid, hydrogen phosphate
Identifiers
CAS Number
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.028.758 Edit this at Wikidata
EC Number
  • 231-633-2
E number E338 (antioxidants, ...)
KEGG
PubChem CID
RTECS number
  • TB6300000
UNII
UN number 1805
CompTox Dashboard (EPA)
InChI
  • InChI=1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)Key: NBIIXXVUZAFLBC-UHFFFAOYSA-N
  • InChI=1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4)Key: NBIIXXVUZAFLBC-UHFFFAOYAI
SMILES
  • OP(=O)(O)O
Properties
Chemical formula H3PO4
Molar mass 97.994 g·mol
Appearance Colorless solid
Odor Odorless
Density 1.6845 g/cm (25 °C, 85%), 1.834 g/cm (solid)
Melting point 42.35 °C (108.23 °F; 315.50 K) anhydrous
29.32 °C (84.78 °F; 302.47 K) hemihydrate
Boiling point
  • 212 °C (414 °F)(only water evaporates)
Solubility in water
  • 392.2 g/(100 g) (−16.3 °C)
  • 369.4 g/(100 mL) (0.5 °C)
  • 446 g/(100 mL) (15 °C)
  • 548 g/(100 mL) (20 °C)
Solubility Soluble in ethanol
log P −2.15
Vapor pressure 0.03 mmHg (20 °C)
Conjugate base Dihydrogen phosphate
Magnetic susceptibility (χ) −43.8·10 cm/mol
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%)
Structure
Crystal structure Monoclinic
Molecular shape Tetrahedral
Thermochemistry
Heat capacity (C) 145.0 J/(mol⋅K)
Std molar
entropy (S298) 		150.8 J/(mol⋅K)
Std enthalpy of
formation (ΔfH298) 		−1271.7 kJ/mol
Gibbs free energy (ΔfG) −1123.6 kJ/mol
Hazards
GHS labelling:
Pictograms GHS05: Corrosive
Signal word Danger
Hazard statements H290, H314
Precautionary statements P280, P305+P351+P338, P310
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
3 0 0
Flash point Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose) 1530 mg/kg (rat, oral)
NIOSH (US health exposure limits):
PEL (Permissible) TWA 1 mg/m
REL (Recommended) TWA 1 mg/m ST 3 mg/m
IDLH (Immediate danger) 1000 mg/m
Safety data sheet (SDS) ICSC 1008
Related compounds
Related phosphorus oxoacids
Except where otherwise noted, data are given for materials in their standard state (at 25 °C , 100 kPa). ☒N verify (what is  ?) Infobox references
Chemical compound

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.

Production

Phosphoric acid is produced industrially by one of two routes, wet processes and dry.

Wet process

In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid.

Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O
Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF

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.

Dry process

To produce food-grade phosphoric acid, phosphate ore is first reduced with coke in an electric arc furnace, to give elemental phosphorus. This process is also known as the thermal process or the electric furnace process. 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 thermal process produces phosphoric acid with a very high concentration of P2O5 (about 85%) and a low level of impurities.

However, this process is more expensive and energy-intensive than the wet process, which produces phosphoric acid with a lower concentration of P2O5 (about 26-52%) and a higher level of impurities. The wet process is the most common method of producing phosphoric acid for fertilizer use. Even in China, where the thermal process is still used quite widely due to relatively cheap coal as opposed to the sulfuric acid, over 7/8 of phosphoric acid is produced with wet process.

Purification

Phosphoric acids produced from phosphate rock or thermal processes often requires purification. A common purification methods is liquid-liquid extraction, which involves the separation of phosphoric acids from water and other impurities using organic solvents, such as tributyl phosphate (TBP), methyl isobutyl ketone (MIBK), or n-octanol. Nanofiltration involves the use of a premodified nanofiltration membrane, which is functionalized by a deposit of a high molecular weight polycationic polymer of polyethyleneimines. Nanofiltration has been shown to significantly reduce the concentrations of various impurities, including cadmium, aluminum, iron, and rare earth elements. The laboratory and industrial pilot scale results showed that this process allows the production of food-grade phosphoric acid.

Fractional crystallization can achieve highest purities typically used for semiconductor applications. Usually a static crystallizer is used. A static crystallizer uses vertical plates, which are suspended in the molten feed and which are alternatingly cooled and heated by a heat transfer medium. The process begins with the slow cooling of the heat transfer medium below the freezing point of the stagnant melt. This cooling causes a layer of crystals to grow on the plates. Impurities are rejected from the growing crystals and are concentrated in the remaining melt. After the desired fraction has been crystallized, the remaining melt is drained from the crystallizer. The purer crystalline layer remains adhered to the plates. In a subsequent step, the plates are heated again to liquify the crystals and the purified phosphoric acid drained into the product vessel. The crystallizer is filled with feed again and the next cooling cycle is started.

Properties

Acidic properties

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 pKa values.

Aqueous solutions

Aqueous solutions up to 62.5% H3PO4 are eutectic, exhibiting freezing-point depression as low as -85°C. When the concentration of acid rises above 62.5% the freezing-point increases, reaching 21°C by 85% H3PO4 (w/w; the monohydrate). Beyond this the phase diagram becomes complicated, with significant local maxima and minima. For this reason phosphoric acid is rarely sold above 85%, as beyond this adding or removing small amounts moisture risks the entire mass freezing solid, which would be a major problem on a large scale. 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.

Self condensation

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.

Uses

See also: Phosphorus § Food additive

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

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:

Phosphoric acid may also be used for chemical polishing (etching) of metals like aluminium or for passivation of steel products in a process called phosphatization.

Safety

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.

A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men).

See also

References

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Cited sources

External links

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