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{{Short description|Chemical compound}} | {{Short description|Chemical compound (PO(OH)3)}} | ||
{{About|orthophosphoric acid|other acids commonly called "phosphoric acid"|Phosphoric acids and phosphates}} | {{About|orthophosphoric acid|other acids commonly called "phosphoric acid" |Phosphoric acids and phosphates}} | ||
{{Distinguish | Phosphorous acid}} | {{Distinguish | Phosphorous acid}} | ||
{{Use dmy dates|date=January 2020}} | {{Use dmy dates|date=January 2020}} | ||
{{chembox|Verifiedfields=changed|Watchedfields=changed|verifiedrevid=470622096|Name=|ImageFile=Phosphoric-acid-2D-dimensions. | {{chembox|Verifiedfields=changed | ||
|Watchedfields=changed | |||
|verifiedrevid=470622096 | |||
|Name= | |||
|ImageFile = Phosphoric-acid-2D-dimensions.svg | |||
|ImageClass = skin-invert-image | |||
|ImageSize = 150px | |||
|ImageName = Structural formula of phosphoric acid, showing dimensions | |||
|ImageFileL1 = Phosphoric-acid-3D-balls.png | |||
|ImageClassL1 = bg-transparent | |||
|ImageSizeL1 = 120px | |||
|ImageNameL1 = Ball-and-stick model | |||
|ImageFileR1 = Phosphoric-acid-3D-vdW.png | |||
|ImageClassR1 = bg-transparent | |||
|ImageSizeR1 = 120px | |||
|ImageNameR1 = Space-filling model | |||
|IUPACName=Phosphoric acid | |||
|OtherNames=Orthophosphoric acid, hydrogen phosphate | |||
|SystematicName= | |||
|Section1={{Chembox Identifiers | |||
| CASNo = 7664-38-2 | |||
| CASNo_Ref = {{cascite|correct|CAS}} | |||
| ChEBI_Ref = {{ebicite|correct|EBI}} | |||
| ChEBI = 26078 | |||
| ChEMBL_Ref = {{ebicite|correct|EBI}} | |||
| ChEMBL = 1187 | |||
| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | |||
| ChemSpiderID = 979 | |||
| DrugBank = DB09394 | |||
| EINECS = 231-633-2 | |||
| Gmelin = 2000 | |||
| KEGG_Ref = {{keggcite|correct|kegg}} | |||
| KEGG = D05467 | |||
| PubChem = 1004 | | PubChem = 1004 | ||
| RTECS = TB6300000 | |||
| UNII_Ref = {{fdacite|correct|FDA}} | | UNII_Ref = {{fdacite|correct|FDA}} | ||
| UNII = E4GA8884NN | | UNII = E4GA8884NN | ||
| | | UNNumber = 1805 | ||
| InChI = 1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) | | InChI = 1/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) | ||
| InChIKey = NBIIXXVUZAFLBC-UHFFFAOYAI | | InChIKey = NBIIXXVUZAFLBC-UHFFFAOYAI | ||
| StdInChI_Ref = {{stdinchicite|correct|chemspider}} | | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | ||
| StdInChI = 1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) | | StdInChI = 1S/H3O4P/c1-5(2,3)4/h(H3,1,2,3,4) | ||
| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | ||
| StdInChIKey = NBIIXXVUZAFLBC-UHFFFAOYSA-N | | StdInChIKey = NBIIXXVUZAFLBC-UHFFFAOYSA-N | ||
| | | SMILES = OP(=O)(O)O | ||
}}|Section2={{Chembox Properties | }}|Section2={{Chembox Properties | ||
| Formula = {{chem2|H3PO4}} | | Formula = {{chem2|H3PO4}} | ||
| Line 32: | Line 53: | ||
| Appearance = Colorless solid | | Appearance = Colorless solid | ||
| Odor = Odorless | | Odor = Odorless | ||
| Density = 1.6845 g/cm<sup>3</sup> (25 °C, 85%),<ref name="Density 85%"> | | Density = 1.6845 g/cm<sup>3</sup> (25 °C, 85%),<ref name="Density 85%">{{Cite journal| author=Christensen, J. H.| author2=Reed, R. B. | year=1955| title=Design and Analysis Data—Density of Aqueous Solutions of Phosphoric Acid Measurements at 25 °C.| journal=Ind. Eng. Chem.| volume=47 | issue=6 | pages=1277–1280| doi=10.1021/ie50546a061}}</ref> 1.834 g/cm<sup>3</sup> (solid)<ref name="Density solid">{{Cite web| url=https://cameochemicals.noaa.gov/chemical/4231| title=CAMEO Chemicals Datasheet – Phosphoric Acid| access-date=15 August 2019| archive-date=15 August 2019| archive-url=https://web.archive.org/web/20190815155917/https://cameochemicals.noaa.gov/chemical/4231| url-status=live}}</ref> | ||
| MeltingPtC = 42.35 | |||
| MeltingPt_notes = anhydrous<ref name="Greenwood">{{cite journal |last1=Greenwood |first1=N. N. |last2=Thompson |first2=A. |title=701. The mechanism of electrical conduction in fused phosphoric and trideuterophosphoric acids |journal=Journal of the Chemical Society (Resumed) |date=1959 |pages=3485 |doi=10.1039/JR9590003485}}</ref><br><!-- | |||
-->{{convert|29.32|C|F K}} [[hemihydrate]]<ref name="Ross"/> | |||
| BoilingPt = {{ubl|212 °C (414 °F)<ref name=chemspider>{{cite web|url=http://www.chemspider.com/Chemical-Structure.979.html|title=Phosphoric acid|website=www.chemspider.com|access-date=3 March 2020|archive-date=12 March 2020|archive-url=https://web.archive.org/web/20200312003851/http://www.chemspider.com/Chemical-Structure.979.html|url-status=live}}</ref> (only water evaporates)<ref>{{Cite journal|url=https://pubs.acs.org/doi/pdf/10.1021/ie50507a050|doi=10.1021/ie50507a050|title=Vapor Pressure of Phosphoric Acids|year=1952|last1=Brown|first1=Earl H.|last2=Whitt|first2=Carlton D.|journal=Industrial & Engineering Chemistry|volume=44|issue=3|pages=615–618|url-access=subscription}}</ref> | |||
| MeltingPtC = | |||
| | |||
| BoilingPt = {{ubl | |||
}} | }} | ||
| Solubility = {{ubl | | Solubility = {{ubl|392.2 g/(100 g) (−16.3 °C)|369.4{{nbsp}}g/(100 mL) (0.5 °C)|446 g/(100 mL) (15 °C)<ref name=sioc>{{cite book|last1 = Seidell|first1 = Atherton|last2 = Linke|first2 = William F.|year = 1952|title = Solubilities of Inorganic and Organic Compounds|publisher = Van Nostrand|url = https://books.google.com/books?id=k2e5AAAAIAAJ|access-date = 2 June 2014|archive-date = 11 March 2020|archive-url = https://web.archive.org/web/20200311230632/https://books.google.com/books?id=k2e5AAAAIAAJ|url-status = live}}</ref>|548{{nbsp}}g/(100 mL) (20 °C)<ref name=crc>Haynes, p. 4.80</ref> | ||
}} | }} | ||
| SolubleOther = Soluble in [[ethanol]] | | SolubleOther = Soluble in [[ethanol]] | ||
| pKa = {{ubl | | pKa = {{ubl|p''K''<sub>a1</sub> = 2.16<ref>Haynes, p. 5.92</ref>|p''K''<sub>a2</sub> = 7.20|p''K''<sub>a3</sub> = 12.32}} | ||
| ConjugateBase = [[Dihydrogen phosphate]] | | ConjugateBase = [[Dihydrogen phosphate]] | ||
| Viscosity = 2.4–9.4{{nbsp}}[[poise (unit)|cP]] (85% {{abbr|aq. soln.|aqueous solution}}) <br />147 | | Viscosity = 2.4–9.4{{nbsp}}[[poise (unit)|cP]] (85% {{abbr|aq. soln.|aqueous solution}}) <br />147 cP (100%) | ||
| RefractIndex = {{ubl | | RefractIndex = {{ubl|1.3420 (8.8% w/w {{abbr|aq. soln.|aqueous solution}})<ref name="Refractive Index">{{Cite journal|author=Edwards, O. W.| author2=Dunn, R. L.|author3=Hatfield, J. D. |year=1964|title=Refractive Index of Phosphoric Acid Solutions at 25 C.|journal=J. Chem. Eng. Data|volume=9|issue=4|pages=508–509|doi=10.1021/je60023a010}}</ref> | ||
| 1.4320 (85% aq. soln) 25 °C | | 1.4320 (85% aq. soln) 25 °C | ||
}} | }} | ||
| VaporPressure = 0.03{{nbsp}}mmHg (20{{nbsp}}°C)<ref name=PGCH/> | | VaporPressure = 0.03{{nbsp}}mmHg (20{{nbsp}}°C)<ref name=PGCH/> | ||
| MagSus = −43.8·10<sup>−6</sup>{{nbsp}}cm<sup>3</sup>/mol<ref>Haynes, p. 4.134</ref> | | MagSus = −43.8·10<sup>−6</sup>{{nbsp}}cm<sup>3</sup>/mol<ref>Haynes, p. 4.134</ref> | ||
| LogP = −2.15<ref name="chemsrc">{{Cite web | | LogP = −2.15<ref name="chemsrc">{{Cite web|url=https://www.chemsrc.com/en/cas/7664-38-2_329226.html|title=phosphoric acid_msds|access-date=2 May 2018| archive-date=4 July 2017|archive-url=https://web.archive.org/web/20170704022858/http://www.chemsrc.com/en/cas/7664-38-2_329226.html| url-status=live}}</ref> | ||
}} | |||
|Section3={{Chembox Structure | |||
}}|Section3={{Chembox Structure | |||
| CrystalStruct = Monoclinic | | CrystalStruct = Monoclinic | ||
| MolShape = Tetrahedral | | MolShape = Tetrahedral | ||
}}|Section4={{Chembox Thermochemistry | }} | ||
|Section4={{Chembox Thermochemistry | |||
| Thermochemistry_ref=<ref>Haynes, p. 5.13</ref> | | Thermochemistry_ref=<ref>Haynes, p. 5.13</ref> | ||
| DeltaHf = −1271.7 kJ/mol | | DeltaHf = −1271.7 kJ/mol | ||
| DeltaGfree = | | DeltaGfree = −1123.6 kJ/mol | ||
| Entropy = 150.8 J/mol⋅K | | Entropy = 150.8 J/(mol⋅K) | ||
| HeatCapacity =145.0 J/mol⋅K | | HeatCapacity =145.0 J/(mol⋅K) | ||
}}|Section5=|Section6=|Section7={{Chembox Hazards | }}|Section5=|Section6=|Section7={{Chembox Hazards | ||
| GHSPictograms = {{GHS05}}<ref name="sigma">{{Sigma-Aldrich|id=466123|name=Phosphoric acid|access-date=9 May 2014}}</ref> | | GHSPictograms = {{GHS05}}<ref name="sigma">{{Sigma-Aldrich|id=466123|name=Phosphoric acid|access-date=9 May 2014}}</ref> | ||
| Line 127: | Line 93: | ||
| NFPA-S = | | NFPA-S = | ||
| FlashPt = Non-flammable | | FlashPt = Non-flammable | ||
| LD50 = 1530 | | LD50 = 1530 mg/kg (rat, oral)<ref>{{IDLH|7664382|Phosphoric acid}}</ref> | ||
| IDLH = 1000 | | IDLH = 1000 mg/m<sup>3</sup><ref name=PGCH>{{PGCH|0506}}</ref> | ||
| REL = TWA 1 | | REL = TWA 1 mg/m<sup>3</sup> ST 3 mg/m<sup>3</sup><ref name=PGCH/> | ||
| PEL = TWA 1 | | PEL = TWA 1 mg/m<sup>3</sup><ref name=PGCH/> | ||
}}|Section8={{Chembox Related | }}|Section8={{Chembox Related | ||
| OtherFunction = {{ubl | | OtherFunction = {{ubl | ||
| Line 144: | Line 110: | ||
}} | }} | ||
'''Phosphoric acid''' (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) 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]] {{chem2|[[Hydrogen|H]]3[[phosphorus|P]][[oxygen|O]]4}}. It is commonly encountered as an 85% [[aqueous solution]], which is a colourless, odourless, and non-[[volatility (chemistry)|volatile]] syrupy liquid. It is a major industrial chemical, being a component of many fertilizers. | ||
The compound is an [[acid]]. Removal of all three {{chem2|H+}} ions gives the [[phosphate]] ion {{chem2|PO4(3−)}}. Removal of one or two protons gives [[phosphate|dihydrogen phosphate]] ion {{chem2|H2PO4−}}, and the [[phosphate|hydrogen phosphate]] ion {{chem2|HPO4(2−)}}, respectively. Phosphoric acid forms [[esters]], called [[organophosphate]]s.<ref name="Westheimer">{{Cite journal|last=Westheimer |first=F.H. |author-link=Frank Westheimer |title=Why nature chose phosphates |journal=[[Science (journal)|Science]] |volume=235 |issue=4793 |pages=1173–1178 (see pp. 1175–1176) |date=6 June 1987 |doi=10.1126/science.2434996 |bibcode=1987Sci...235.1173W|citeseerx=10.1.1.462.3441 |pmid=2434996 }}</ref> | The compound is an [[acid]]. Removal of all three {{chem2|H+}} ions gives the [[phosphate]] ion {{chem2|PO4(3−)}}. Removal of one or two protons gives [[phosphate|dihydrogen phosphate]] ion {{chem2|H2PO4−}}, and the [[phosphate|hydrogen phosphate]] ion {{chem2|HPO4(2−)}}, respectively. Phosphoric acid forms [[esters]], called [[organophosphate]]s.<ref name="Westheimer">{{Cite journal|last=Westheimer |first=F.H. |author-link=Frank Westheimer |title=Why nature chose phosphates |journal=[[Science (journal)|Science]] |volume=235 |issue=4793 |pages=1173–1178 (see pp. 1175–1176) |date=6 June 1987 |doi=10.1126/science.2434996 |bibcode=1987Sci...235.1173W|citeseerx=10.1.1.462.3441 |pmid=2434996 }}</ref> | ||
| Line 151: | Line 117: | ||
=={{Anchor|Manufacture}}Production== | =={{Anchor|Manufacture}}Production== | ||
Phosphoric acid is produced industrially by one of two routes, wet processes and dry.<ref>{{cite book |last1=Becker |first1=Pierre |title=Phosphates and phosphoric acid |date=1988 |publisher=Marcel Dekker |location=New York |isbn=978-0824717124}}</ref> | Phosphoric acid is produced industrially by one of two routes, wet processes and dry.<ref>{{cite book |last1=Becker |first1=Pierre |title=Phosphates and phosphoric acid |date=1988 |publisher=Marcel Dekker |location=New York |isbn=978-0824717124}}</ref><ref>{{cite book |last1=Gilmour |first1=Rodney |title=Phosphoric acid: purification, uses, technology, and economics |date=2014 |publisher=CRC Press |location=Boca Raton |url=https://books.google.com/books?id=lDMTAgAAQBAJ&pg=PP1 |isbn=9781439895108 |pages=44–61}}</ref><ref>{{cite journal |last1=Jupp |first1=Andrew R. |last2=Beijer |first2=Steven |last3=Narain |first3=Ganesha C. |last4=Schipper |first4=Willem |last5=Slootweg |first5=J. Chris |title=Phosphorus recovery and recycling – closing the loop |journal=Chemical Society Reviews |date=2021 |volume=50 |issue=1 |pages=87–101 |doi=10.1039/D0CS01150A|pmid=33210686 |doi-access=free}}</ref> | ||
=== Wet process === | === Wet process === | ||
In the wet process, | In the wet process, phosphate-containing minerals such as calcium [[hydroxyapatite]] or [[fluorapatite]] are treated with [[sulfuric acid]].<ref>{{Greenwood&Earnshaw2nd|pages=520–522}}</ref> | ||
:{{chem2|Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 [[CaSO4]] + H2O}} | :{{chem2|Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 [[CaSO4]] + H2O}} | ||
:{{Chem2|Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 [[CaSO4]] + HF}} | :{{Chem2|Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 [[CaSO4]] + HF}} | ||
[[ | By-products include [[calcium sulfate]] ({{Chem2|CaSO4}}) and [[hydrogen fluoride]] (HF). The HF gas may be recovered by streaming it into a [[Wet scrubber|wet (water) scrubber]] producing [[hydrofluoric acid]]. {{Chem2|CaSO4}} is better known as gypsum, which is commonly used in the construction industry, however the {{Chem2|CaSO4}} produced from phosphoric acid production can contain trace levels of radioactive elements such as [[radium]]. This makes it unsuitable for commercial use, and it is called [[phosphogypsum]] to distinguish it. It is typically stored indefinitely. | ||
In both cases the phosphoric acid solution usually contains 23–33% {{chem2|P2O5}} (32–46% {{chem2|H3PO4}}). It may be concentrated to produce ''commercial-'' or ''merchant-grade'' phosphoric acid, which contains about 54–62% [[Phosphorus pentoxide|{{chem2|P2O5}}]] (75–85% {{chem2|H3PO4}}). Further removal of water yields ''superphosphoric acid'' with a {{chem2|P2O5}} concentration above 70% (corresponding to nearly 100% {{chem2|H3PO4}}). The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities. | |||
=== Dry process=== | === Dry process=== | ||
To produce food-grade phosphoric acid, phosphate ore is first reduced with [[Coke (fuel)|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.<ref>{{cite journal |doi=10.1021/acscentsci.0c00332|title=Let's Make White Phosphorus Obsolete |year=2020 |last1=Geeson |first1=Michael B. |last2=Cummins |first2=Christopher C. |journal=ACS Central Science |volume=6 |issue=6 |pages=848–860 |pmid=32607432 |pmc=7318074 }}</ref> | To produce food-grade phosphoric acid, phosphate ore is first reduced with [[Coke (fuel)|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.<ref>{{cite journal |doi=10.1021/acscentsci.0c00332 |doi-access=free|title=Let's Make White Phosphorus Obsolete |year=2020 |last1=Geeson |first1=Michael B. |last2=Cummins |first2=Christopher C. |journal=ACS Central Science |volume=6 |issue=6 |pages=848–860 |pmid=32607432 |pmc=7318074 }}</ref> The thermal process produces phosphoric acid with a very high concentration of {{chem2|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 {{chem2|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.<ref>[https://www.epa.gov/sites/default/files/2020-07/documents/phosphoric-acid-phosphatic-fertilizers_ip_07-1993.pdf Phosphoric Acid and Phosphatic Fertilizers: A profile]</ref> 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.<ref>{{Cite journal |last1=Minpeng |last2=Chen |last3=Fu |last4=Sun |last5=Xu |last6=Xia |last7=Ji-ning |title=The Phosphorus Flow in China : A Revisit from the Perspective of Production |journal=Global Environmental Research |volume=19 |issue=1 |pages=19–25 |s2cid=201655549 |url=https://airies.wikiplus.net/attach.php/6a6f75726e616c5f31392d31656e67/save/0/0/19_1-4.pdf}}</ref> | |||
== Purification == | |||
Phosphoric acid produced from [[phosphate rock]] or thermal processes often requires purification. A common purification method is [[liquid–liquid extraction]], which involves the separation of phosphoric acid from water and other impurities using organic solvents, such as [[tributyl phosphate]] (TBP), [[methyl isobutyl ketone]] (MIBK), or [[n-octanol|''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 [[Polyethyleneamines|polyethyleneimines]]. Nanofiltration has been shown to significantly reduce the concentrations of various impurities, including [[cadmium]], [[Aluminium|aluminum]], [[iron]], and [[Rare-earth element|rare earth elements]]. The laboratory and industrial pilot scale results showed that this process allows the production of food-grade phosphoric acid.<ref>{{cite journal |last1=Wet Process Phosphoric Acid Purification |title=Wet Process Phosphoric Acid Purification Using Functionalized Organic Nanofiltration Membrane |journal=Separations |date=2022 |volume=9 |issue=4 |page=100 |doi=10.3390/separations9040100 |doi-access=free }}</ref> | |||
[[Fractional crystallization (chemistry)|Fractional crystallization]] can achieve higher 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.<ref>{{Cite web |url=https://www.sulzer.com/-/media/files/products/separation-technology/crystallisation/brochures/fractional_crystallization_e.pdf?sc_lang=en |title=Fractional Crystallization |access-date=30 January 2024 |archive-date=30 January 2024 |archive-url=https://web.archive.org/web/20240130154521/https://www.sulzer.com/-/media/files/products/separation-technology/crystallisation/brochures/fractional_crystallization_e.pdf?sc_lang=en |url-status=dead }}</ref> | |||
==Acidic properties== | ==Properties== | ||
===Acidic properties=== | |||
In aqueous solution phosphoric acid behaves as a triprotic acid. | In aqueous solution phosphoric acid behaves as a triprotic acid. | ||
:{{chem2|H3PO4 ⇌ H2PO4- + H+}}, p''K''<sub>a1</sub> = 2.14 | :{{chem2|H3PO4 ⇌ H2PO4- + H+}}, p''K''<sub>a1</sub> = 2.14 | ||
:{{chem2|H2PO4- ⇌ HPO4(2-) + H+}}, p''K''<sub>a2</sub> = 7.20 | :{{chem2|H2PO4- ⇌ HPO4(2-) + H+}}, p''K''<sub>a2</sub> = 7.20 | ||
:{{chem2|HPO4(2-) ⇌ PO4(3-) + H+}}, p''K''<sub>a3</sub> = 12.37 | :{{chem2|HPO4(2-) ⇌ PO4(3-) + H+}}, p''K''<sub>a3</sub> = 12.37 | ||
The difference between successive p''K'' values is sufficiently large so that salts of either monohydrogen phosphate, {{chem2|HPO4(2-)}} or dihydrogen phosphate, {{chem2|H2PO4-}}, can be prepared from a solution of phosphoric acid by adjusting the pH to be mid-way between the respective p''K'' values. | The difference between successive [[pKa|p''K''<sub>a</sub>]] values is sufficiently large so that salts of either monohydrogen phosphate, {{chem2|HPO4(2-)}} or dihydrogen phosphate, {{chem2|H2PO4-}}, can be prepared from a solution of phosphoric acid by adjusting the [[pH]] to be mid-way between the respective p''K''<sub>a</sub> values. | ||
===Aqueous solutions=== | |||
Aqueous solutions up to 62.5% {{chem2|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% {{chem2|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 of 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]] 2H<sub>3</sub>PO<sub>4</sub>•H<sub>2</sub>O, freezing at 29.32 °C.<ref>{{cite journal |last1=Ross |first1=William H. |last2=Jones |first2=Russell M. |title=The Solubility and Freezing-Point Curves of Hydrated and Anhydrous Orthophosphoric Acid |journal=Journal of the American Chemical Society |date=August 1925 |volume=47 |issue=8 |pages=2165–2170 |doi=10.1021/ja01685a015|bibcode=1925JAChS..47.2165R }}</ref><ref>{{cite web |title=Purified Phosphoric Acid H3PO4 Technical Information Bulletin |url=http://www.waterguardinc.com/files/90712047.pdf |publisher=[[PotashCorp]] |access-date=11 February 2023}}</ref> 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 [[Supercooling|supercool]] before crystallization occurs, and may be relatively resistant to crystallisation even when stored below the freezing point.<ref name="Ross">{{Cite journal |last1=Ross |first1=Wm. H. |last2=Jones |first2=R. M. |last3=Durgin |first3=C. B. |date=October 1925 |title=The Purification of Phosphoric Acid by Crystallization. |url=https://pubs.acs.org/doi/abs/10.1021/ie50190a031 |journal=Industrial & Engineering Chemistry |language=en |volume=17 |issue=10 |pages=1081–1083 |doi=10.1021/ie50190a031 |issn=0019-7866|url-access=subscription }}</ref> | |||
===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]]: | |||
:{{chem2|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.<ref>{{Citation |last1=Korte |first1=Carsten |title=Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers |date=2016 |url=http://link.springer.com/10.1007/978-3-319-17082-4_8 |work=High Temperature Polymer Electrolyte Membrane Fuel Cells |pages=169–194 |editor-last=Li |editor-first=Qingfeng |place=Cham |publisher=Springer International Publishing |language=en |doi=10.1007/978-3-319-17082-4_8 |isbn=978-3-319-17081-7 |access-date=2023-02-12 |last2=Conti |first2=Fosca |last3=Wackerl |first3=Jürgen |last4=Lehnert |first4=Werner |editor2-last=Aili |editor2-first=David |editor3-last=Hjuler |editor3-first=Hans Aage |editor4-last=Jensen |editor4-first=Jens Oluf|url-access=subscription }}</ref> | |||
As the concentration is increased [[Phosphoric acids and phosphates|higher acids]] are formed, culminating in the formation of [[polyphosphoric acid]]s.<ref>{{cite journal |last1=Jameson |first1=R. F. |title=151. The composition of the "strong" phosphoric acids |journal=Journal of the Chemical Society (Resumed) |date=1 January 1959 |pages=752–759 |doi=10.1039/JR9590000752}}</ref> 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.<ref name="Ross" /><ref name="Greenwood" /> | |||
==Uses== | ==Uses== | ||
{{see also|Phosphorus#Food additive}} | |||
The dominant use of phosphoric acid is for [[fertilizer]]s, consuming approximately 90% of production.<ref name=Ullmann>{{Ullmann|first1=Klaus|last1=Schrödter|first2=Gerhard|last2=Bettermann|first3=Thomas |last3=Staffel|first4=Friedrich|last4=Wahl|first5=Thomas|last5=Klein|first6=Thomas|last6=Hofmann|title=Phosphoric Acid and Phosphates|year=2008|doi=10.1002/14356007.a19_465.pub3}}</ref> | The dominant use of phosphoric acid is for [[fertilizer]]s, consuming approximately 90% of production.<ref name=Ullmann>{{Ullmann|first1=Klaus|last1=Schrödter|first2=Gerhard|last2=Bettermann|first3=Thomas |last3=Staffel|first4=Friedrich|last4=Wahl|first5=Thomas|last5=Klein|first6=Thomas|last6=Hofmann|title=Phosphoric Acid and Phosphates|year=2008|doi=10.1002/14356007.a19_465.pub3}}</ref> | ||
The remaining 10% is primarily used in soaps and detergents: | |||
{| class="wikitable" | {| class="wikitable" | ||
|- | |- | ||
| Line 184: | Line 176: | ||
| [[Water treatment]] || 164 || SHMP, [[Sodium triphosphate|STPP]], [[Tetrasodium pyrophosphate|TSPP]], [[Monosodium phosphate|MSP]] ({{chem2|NaH2PO4}}), DSP | | [[Water treatment]] || 164 || SHMP, [[Sodium triphosphate|STPP]], [[Tetrasodium pyrophosphate|TSPP]], [[Monosodium phosphate|MSP]] ({{chem2|NaH2PO4}}), DSP | ||
|- | |- | ||
| [[Toothpaste]]s || 68 || [[Dicalcium phosphate|DCP]] ({{chem2|CaHPO4}}), IMP, SMFP | | [[Toothpaste]]s || 68 || [[Dicalcium phosphate|DCP]] ({{chem2|CaHPO4}}), IMP, [[SMFP]] | ||
|- | |- | ||
| Other applications || 287 || [[Sodium triphosphate|STPP]] ({{chem2|Na3P3O9}}), TCP, APP, DAP, [[zinc phosphate]] ({{chem2|Zn3(PO4)2}}), [[aluminium phosphate]] ({{chem2|AlPO4}}), {{chem2|H3PO4}} | | Other applications || 287 || [[Sodium triphosphate|STPP]] ({{chem2|Na3P3O9}}), TCP, APP, DAP, [[zinc phosphate]] ({{chem2|Zn3(PO4)2}}), [[aluminium phosphate]] ({{chem2|AlPO4}}), {{chem2|H3PO4}} | ||
|} | |} | ||
Food-grade phosphoric acid (additive [[E number|E338]]<ref name=fgovuk>{{cite web|url=http://www.food.gov.uk/policy-advice/additivesbranch/enumberlist#h_7|title=Current EU approved additives and their E Numbers|date=14 March 2012|publisher=Foods Standards Agency|access-date=22 July 2012|archive-date=21 August 2013|archive-url=https://web.archive.org/web/20130821045312/http://food.gov.uk/policy-advice/additivesbranch/enumberlist#h_7|url-status=live}}</ref>) is used to acidify foods and beverages such as various [[cola]]s and jams, providing a tangy or sour taste. The phosphoric acid also serves as a [[preservative]].<ref>{{Cite web|title=Why is phosphoric acid used in some Coca‑Cola drinks?{{!}} Frequently Asked Questions {{!}} Coca-Cola GB|url=https://www.coca-cola.co.uk/our-business/faqs/why-is-phosphoric-acid-used-in-coca-cola-drinks-diet-coke-coke-zero|access-date=2021-08-31|website=www.coca-cola.co.uk|language=en-GB|archive-date=2 August 2021|archive-url=https://web.archive.org/web/20210802114054/https://www.coca-cola.co.uk/our-business/faqs/why-is-phosphoric-acid-used-in-coca-cola-drinks-diet-coke-coke-zero|url-status=live}}</ref> Soft drinks containing phosphoric acid, which would include [[Coca-Cola]], are sometimes called [[phosphate soda]]s or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion.<ref>{{Cite journal|title=Dietary advice in dental practice|journal=British Dental Journal|volume=193|issue=10|pages=563–568|date=23 November 2002|doi=10.1038/sj.bdj.4801628|pmid=12481178|last1=Moynihan|first1=P. J.|doi-access=free}}</ref> Phosphoric acid also has the potential to contribute to the formation of [[Kidney stone disease|kidney stones]], especially in those who have had kidney stones previously.<ref name=Qa2014>{{cite journal |last1= Qaseem |first1= A |last2= Dallas |first2= P |last3= Forciea |first3= MA |last4= Starkey |first4= M |last5= Denberg |first5= TD |display-authors= 4 |title= Dietary and pharmacologic management to prevent recurrent nephrolithiasis in adults: A clinical practice guideline from the American College of Physicians |journal= [[Annals of Internal Medicine]] |date= 4 November 2014 |volume= 161 |issue= 9 |pages= 659–67 |doi= 10.7326/M13-2908 |pmid=25364887|doi-access= | Food-grade phosphoric acid (additive [[E number|E338]]<ref name=fgovuk>{{cite web|url=http://www.food.gov.uk/policy-advice/additivesbranch/enumberlist#h_7|title=Current EU approved additives and their E Numbers|date=14 March 2012|publisher=Foods Standards Agency|access-date=22 July 2012|archive-date=21 August 2013|archive-url=https://web.archive.org/web/20130821045312/http://food.gov.uk/policy-advice/additivesbranch/enumberlist#h_7|url-status=live}}</ref>) is used to acidify foods and beverages such as various [[cola]]s and jams, providing a tangy or sour taste. The phosphoric acid also serves as a [[preservative]].<ref>{{Cite web|title=Why is phosphoric acid used in some Coca‑Cola drinks?{{!}} Frequently Asked Questions {{!}} Coca-Cola GB|url=https://www.coca-cola.co.uk/our-business/faqs/why-is-phosphoric-acid-used-in-coca-cola-drinks-diet-coke-coke-zero|access-date=2021-08-31|website=www.coca-cola.co.uk|language=en-GB|archive-date=2 August 2021|archive-url=https://web.archive.org/web/20210802114054/https://www.coca-cola.co.uk/our-business/faqs/why-is-phosphoric-acid-used-in-coca-cola-drinks-diet-coke-coke-zero|url-status=live}}</ref> Soft drinks containing phosphoric acid, which would include [[Coca-Cola]], are sometimes called [[phosphate soda]]s or phosphates. Phosphoric acid in soft drinks has the potential to cause dental erosion.<ref>{{Cite journal|title=Dietary advice in dental practice|journal=British Dental Journal|volume=193|issue=10|pages=563–568|date=23 November 2002|doi=10.1038/sj.bdj.4801628|pmid=12481178|last1=Moynihan|first1=P. J.|doi-access=free}}</ref> Phosphoric acid also has the potential to contribute to the formation of [[Kidney stone disease|kidney stones]], especially in those who have had kidney stones previously.<ref name=Qa2014>{{cite journal |last1= Qaseem |first1= A |last2= Dallas |first2= P |last3= Forciea |first3= MA |last4= Starkey |first4= M |last5= Denberg |first5= TD |display-authors= 4 |title= Dietary and pharmacologic management to prevent recurrent nephrolithiasis in adults: A clinical practice guideline from the American College of Physicians |journal= [[Annals of Internal Medicine]] |date= 4 November 2014 |volume= 161 |issue= 9 |pages= 659–67 |doi= 10.7326/M13-2908 |pmid=25364887|doi-access= |s2cid= 3058172 }}</ref> | ||
Specific applications of phosphoric acid include: | Specific applications of phosphoric acid include: | ||
* in anti-rust treatment by [[phosphate conversion coating]] or [[Passivation (chemistry)|passivation]] | * in anti-rust ([[iron]] and [[steel]] [[oxidation]]) treatment by [[phosphate conversion coating]] or [[Passivation (chemistry)|passivation]],<ref>{{Cite web |date=February 2021 |title=Phosphates - Metal Finishing |url=https://phosphatesfacts.org/wp-content/uploads/2021/02/Phosphates-Metal-Finishing.pdf |publisher=Phospates for Americas}}</ref> including the [[Parkerization (metallurgy)|Parkerization]] process | ||
* as an external standard for [[phosphorus-31 nuclear magnetic resonance]] | * as an external standard for [[phosphorus-31 nuclear magnetic resonance]] | ||
* in [[phosphoric acid fuel cell]]s | * in [[phosphoric acid fuel cell]]s | ||
* in [[activated carbon]] production<ref>{{Cite journal | last1 = Toles | first1 = C. | last2 = Rimmer | first2 = S. | last3 = Hower | first3 = J. C. | doi = 10.1016/S0008-6223(96)00093-0 | title = Production of activated carbons from a washington lignite using phosphoric acid activation | journal = Carbon | volume = 34 | issue = 11 | pages = 1419 | year = 1996 }}</ref> | * in [[activated carbon]] production<ref>{{Cite journal | last1 = Toles | first1 = C. | last2 = Rimmer | first2 = S. | last3 = Hower | first3 = J. C. | doi = 10.1016/S0008-6223(96)00093-0 | title = Production of activated carbons from a washington lignite using phosphoric acid activation | journal = Carbon | volume = 34 | issue = 11 | pages = 1419 | year = 1996 | bibcode = 1996Carbo..34.1419T }}</ref> | ||
* in [[compound semiconductor]] processing, to etch [[Indium gallium arsenide]] selectively with respect to [[indium phosphide]]<ref>[http://terpconnect.umd.edu/~browns/wetetch.html Wet chemical etching.] {{Webarchive|url=https://web.archive.org/web/20120925075407/http://terpconnect.umd.edu/~browns/wetetch.html |date=25 September 2012 }} umd.edu.</ref> | * in [[compound semiconductor]] processing, to etch [[Indium gallium arsenide]] selectively with respect to [[indium phosphide]]<ref>[http://terpconnect.umd.edu/~browns/wetetch.html Wet chemical etching.] {{Webarchive|url=https://web.archive.org/web/20120925075407/http://terpconnect.umd.edu/~browns/wetetch.html |date=25 September 2012 }} umd.edu.</ref> | ||
* in [[microfabrication]] to etch [[silicon nitride]] selectively with respect to [[silicon dioxide]]<ref name="Wolf">{{cite book |title =Silicon processing for the VLSI era: Volume 1 – Process technology |last = Wolf |first = S. |author2=R. N. Tauber |year=1986 |page=534 |isbn=978-0-9616721-6-4}}</ref> | * in [[microfabrication]] to etch [[silicon nitride]] selectively with respect to [[silicon dioxide]]<ref name="Wolf">{{cite book |title =Silicon processing for the VLSI era: Volume 1 – Process technology |last = Wolf |first = S. |author2=R. N. Tauber |year=1986 |page=534 |publisher = Lattice Press |isbn=978-0-9616721-6-4}}</ref> | ||
* as a pH adjuster in cosmetics and skin-care products<ref>{{cite web|publisher = Paula's Choice|title = Ingredient dictionary: P|work = Cosmetic ingredient dictionary|access-date = 16 November 2007|url = http://www.cosmeticscop.com/learn/cosmetic_dictionary.asp?id=21&letter=P | * in microfabrication to etch [[aluminium]] | ||
* as a sanitizing agent in the dairy, food, and brewing industries<ref>{{cite web|url=http://www.fivestarchemicals.com/wp-content/uploads/StarSanTech-HB2.pdf|title= | * as a pH adjuster in cosmetics and skin-care products<ref>{{cite web|publisher = Paula's Choice|title = Ingredient dictionary: P|work = Cosmetic ingredient dictionary|access-date = 16 November 2007|url = http://www.cosmeticscop.com/learn/cosmetic_dictionary.asp?id=21&letter=P|archive-url = https://web.archive.org/web/20080118084632/http://www.cosmeticscop.com/learn/cosmetic_dictionary.asp?id=21&letter=P|archive-date = 18 January 2008|df = dmy-all}}</ref> | ||
* as a sanitizing agent in the dairy, food, and brewing industries<ref>{{cite web|url=http://www.fivestarchemicals.com/wp-content/uploads/StarSanTech-HB2.pdf|title=Star San|publisher=Five Star Chemicals|access-date=17 August 2015|archive-date=8 February 2016|archive-url=https://web.archive.org/web/20160208134008/http://www.fivestarchemicals.com/wp-content/uploads/StarSanTech-HB2.pdf|url-status=live}}</ref> | |||
* in [[Chemical-mechanical polishing|chemical polishing]] ([[etching]]) of metals like aluminium | |||
== Safety == | == 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.<ref name=":0">{{Cite web|url=http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=US&language=en&productNumber=345245&brand=ALDRICH&PageToGoToURL=http%3A%2F%2Fwww.sigmaaldrich.com%2Fcatalog%2Fproduct%2Faldrich%2F345245%3Flang%3Den|title=Phosphoric Acid, 85 wt.% SDS|date=5 May 2016|website=Sigma-Aldrich|access-date=16 January 2017|archive-date=18 January 2017|archive-url=https://web.archive.org/web/20170118123759/http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do?country=US&language=en&productNumber=345245&brand=ALDRICH&PageToGoToURL=http%3A%2F%2Fwww.sigmaaldrich.com%2Fcatalog%2Fproduct%2Faldrich%2F345245%3Flang%3Den|url-status=live}}</ref> | |||
==See also== | ==See also== | ||
* Phosphate [[fertilizer]]s, such as [[ammonium phosphate]] fertilizers | * Phosphate [[fertilizer]]s, such as [[ammonium phosphate]] fertilizers | ||
* [[Chiral phosphoric acid]] | |||
==References== | ==References== | ||
{{Reflist | {{Reflist}} | ||
==Cited sources== | ==Cited sources== | ||
| Line 224: | Line 214: | ||
{{Authority control}} | {{Authority control}} | ||
{{Hydrogen compounds}} | |||
{{DEFAULTSORT:Phosphoric Acid}} | {{DEFAULTSORT:Phosphoric Acid}} | ||
[[Category:Flavors]] | [[Category:Flavors]] | ||
[[Category:Food acidity regulators]] | [[Category:Food acidity regulators]] | ||
| Line 232: | Line 222: | ||
[[Category:Phosphates]] | [[Category:Phosphates]] | ||
[[Category:Phosphorus oxoacids]] | [[Category:Phosphorus oxoacids]] | ||
[[Category:Acid catalysts]] | [[Category:Acid catalysts]] | ||
[[Category:E-number additives]] | [[Category:E-number additives]] | ||
[[Category:Phosphorus(V) compounds]] | [[Category:Phosphorus(V) compounds]] | ||
[[Category:Over-the-counter drugs in the United States]] | |||
Latest revision as of 06:54, 8 March 2026
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
|
Template:Chembox Footer/tracking
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula H
3PO
4. 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 H
2PO−
4, and the hydrogen phosphate ion HPO2−
4, respectively. Phosphoric acid forms esters, called organophosphates.[1]
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[edit | edit source]
Phosphoric acid is produced industrially by one of two routes, wet processes and dry.[2][3][4]
Wet process[edit | edit source]
In the wet process, phosphate-containing minerals such as calcium hydroxyapatite or fluorapatite are treated with sulfuric acid.[5]
- Ca
5(PO
4)
3OH + 5 H
2SO
4 → 3 H
3PO
4 + 5 [[CaSO
4]] + H
2O - Ca
5(PO
4)
3F + 5 H
2SO
4 → 3 H
3PO
4 + 5 [[CaSO
4]] + HF
By-products include calcium sulfate (CaSO
4) and hydrogen fluoride (HF). The HF gas may be recovered by streaming it into a wet (water) scrubber producing hydrofluoric acid. CaSO
4 is better known as gypsum, which is commonly used in the construction industry, however the CaSO
4 produced from phosphoric acid production can contain trace levels of radioactive elements such as radium. This makes it unsuitable for commercial use, and it is called phosphogypsum to distinguish it. It is typically stored indefinitely.
In both cases the phosphoric acid solution usually contains 23–33% P
2O
5 (32–46% H
3PO
4). It may be concentrated to produce commercial- or merchant-grade phosphoric acid, which contains about 54–62% P
2O
5 (75–85% H
3PO
4). Further removal of water yields superphosphoric acid with a P
2O
5 concentration above 70% (corresponding to nearly 100% H
3PO
4). The phosphoric acid from both processes may be further purified by removing compounds of arsenic and other potentially toxic impurities.
Dry process[edit | edit source]
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.[6] The thermal process produces phosphoric acid with a very high concentration of P
2O
5 (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 P
2O
5 (about 26–52%) and a higher level of impurities. The wet process is the most common method of producing phosphoric acid for fertilizer use.[7] 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.[8]
Purification[edit | edit source]
Phosphoric acid produced from phosphate rock or thermal processes often requires purification. A common purification method is liquid–liquid extraction, which involves the separation of phosphoric acid 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.[9]
Fractional crystallization can achieve higher 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.[10]
Properties[edit | edit source]
Acidic properties[edit | edit source]
In aqueous solution phosphoric acid behaves as a triprotic acid.
- H
3PO
4 ⇌ H
2PO−
4 + H+
, pKa1 = 2.14 - H
2PO−
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, H
2PO−
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[edit | edit source]
Aqueous solutions up to 62.5% H
3PO
4 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% H
3PO
4 (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 of 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.[11][12] 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.[13]
Self condensation[edit | edit source]
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 H
3PO
4 ⇌ H
2O + H
4P
2O
7
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.[14]
As the concentration is increased higher acids are formed, culminating in the formation of polyphosphoric acids.[15] 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.[13][16]
Uses[edit | edit source]
The dominant use of phosphoric acid is for fertilizers, consuming approximately 90% of production.[17]
The remaining 10% is primarily used in soaps and detergents:
| Application | Demand (2006) in thousands of tons | Main phosphate derivatives |
|---|---|---|
| Soaps and detergents | 1836 | STPP |
| Food industry | 309 | STPP (Na 5P 3O 10), SHMP, TSP, SAPP, SAlP, MCP, DSP (Na 2HPO 4), H 3PO 4 |
| Water treatment | 164 | SHMP, STPP, TSPP, MSP (NaH 2PO 4), DSP |
| Toothpastes | 68 | DCP (CaHPO 4), IMP, SMFP |
| Other applications | 287 | STPP (Na 3P 3O 9), TCP, APP, DAP, zinc phosphate (Zn 3(PO 4) 2), aluminium phosphate (AlPO 4), H 3PO 4 |
Food-grade phosphoric acid (additive E338[18]) 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.[19] 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.[20] Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.[21]
Specific applications of phosphoric acid include:
- in anti-rust (iron and steel oxidation) treatment by phosphate conversion coating or passivation,[22] including the Parkerization process
- as an external standard for phosphorus-31 nuclear magnetic resonance
- in phosphoric acid fuel cells
- in activated carbon production[23]
- in compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide[24]
- in microfabrication to etch silicon nitride selectively with respect to silicon dioxide[25]
- in microfabrication to etch aluminium
- as a pH adjuster in cosmetics and skin-care products[26]
- as a sanitizing agent in the dairy, food, and brewing industries[27]
- in chemical polishing (etching) of metals like aluminium
Safety[edit | edit source]
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.[28]
See also[edit | edit source]
- Phosphate fertilizers, such as ammonium phosphate fertilizers
- Chiral phosphoric acid
References[edit | edit source]
- ↑ Westheimer, F.H. (6 June 1987). "Why nature chose phosphates". Science. 235 (4793): 1173–1178 (see pp. 1175–1176). Bibcode:1987Sci...235.1173W. CiteSeerX 10.1.1.462.3441. doi:10.1126/science.2434996. PMID 2434996.
- ↑ Becker, Pierre (1988). Phosphates and phosphoric acid. New York: Marcel Dekker. ISBN 978-0824717124.
- ↑ Gilmour, Rodney (2014). Phosphoric acid: purification, uses, technology, and economics. Boca Raton: CRC Press. pp. 44–61. ISBN 9781439895108.
- ↑ Jupp, Andrew R.; Beijer, Steven; Narain, Ganesha C.; Schipper, Willem; Slootweg, J. Chris (2021). "Phosphorus recovery and recycling – closing the loop". Chemical Society Reviews. 50 (1): 87–101. doi:10.1039/D0CS01150A. PMID 33210686.
- ↑ Template:Greenwood&Earnshaw2nd
- ↑ Geeson, Michael B.; Cummins, Christopher C. (2020). "Let's Make White Phosphorus Obsolete". ACS Central Science. 6 (6): 848–860. doi:10.1021/acscentsci.0c00332. PMC 7318074. PMID 32607432.
- ↑ Phosphoric Acid and Phosphatic Fertilizers: A profile
- ↑ Minpeng; Chen; Fu; Sun; Xu; Xia; Ji-ning. "The Phosphorus Flow in China : A Revisit from the Perspective of Production" (PDF). Global Environmental Research. 19 (1): 19–25. S2CID 201655549.
- ↑ Wet Process Phosphoric Acid Purification (2022). "Wet Process Phosphoric Acid Purification Using Functionalized Organic Nanofiltration Membrane". Separations. 9 (4): 100. doi:10.3390/separations9040100.
- ↑ "Fractional Crystallization" (PDF). Archived from the original (PDF) on 30 January 2024. Retrieved 30 January 2024.
- ↑ Ross, William H.; Jones, Russell M. (August 1925). "The Solubility and Freezing-Point Curves of Hydrated and Anhydrous Orthophosphoric Acid". Journal of the American Chemical Society. 47 (8): 2165–2170. Bibcode:1925JAChS..47.2165R. doi:10.1021/ja01685a015.
- ↑ "Purified Phosphoric Acid H3PO4 Technical Information Bulletin" (PDF). PotashCorp. Retrieved 11 February 2023.
- ↑ 13.0 13.1 Ross, Wm. H.; Jones, R. M.; Durgin, C. B. (October 1925). "The Purification of Phosphoric Acid by Crystallization". Industrial & Engineering Chemistry. 17 (10): 1081–1083. doi:10.1021/ie50190a031. ISSN 0019-7866.
- ↑ Korte, Carsten; Conti, Fosca; Wackerl, Jürgen; Lehnert, Werner (2016), Li, Qingfeng; Aili, David; Hjuler, Hans Aage; Jensen, Jens Oluf (eds.), "Phosphoric Acid and its Interactions with Polybenzimidazole-Type Polymers", High Temperature Polymer Electrolyte Membrane Fuel Cells, Cham: Springer International Publishing, pp. 169–194, doi:10.1007/978-3-319-17082-4_8, ISBN 978-3-319-17081-7, retrieved 12 February 2023
{{citation}}: CS1 maint: work parameter with ISBN (link) - ↑ Jameson, R. F. (1 January 1959). "151. The composition of the "strong" phosphoric acids". Journal of the Chemical Society (Resumed): 752–759. doi:10.1039/JR9590000752.
- ↑ Cite error: Invalid
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- ↑ "Current EU approved additives and their E Numbers". Foods Standards Agency. 14 March 2012. Archived from the original on 21 August 2013. Retrieved 22 July 2012.
- ↑ "Why is phosphoric acid used in some Coca‑Cola drinks?| Frequently Asked Questions | Coca-Cola GB". www.coca-cola.co.uk. Archived from the original on 2 August 2021. Retrieved 31 August 2021.
- ↑ Moynihan, P. J. (23 November 2002). "Dietary advice in dental practice". British Dental Journal. 193 (10): 563–568. doi:10.1038/sj.bdj.4801628. PMID 12481178.
- ↑ 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. S2CID 3058172.
- ↑ "Phosphates - Metal Finishing" (PDF). Phospates for Americas. February 2021.
- ↑ Toles, C.; Rimmer, S.; Hower, J. C. (1996). "Production of activated carbons from a washington lignite using phosphoric acid activation". Carbon. 34 (11): 1419. Bibcode:1996Carbo..34.1419T. doi:10.1016/S0008-6223(96)00093-0.
- ↑ Wet chemical etching. Archived 25 September 2012 at the Wayback Machine umd.edu.
- ↑ Wolf, S.; R. N. Tauber (1986). Silicon processing for the VLSI era: Volume 1 – Process technology. Lattice Press. p. 534. ISBN 978-0-9616721-6-4.
- ↑ "Ingredient dictionary: P". Cosmetic ingredient dictionary. Paula's Choice. Archived from the original on 18 January 2008. Retrieved 16 November 2007.
- ↑ "Star San" (PDF). Five Star Chemicals. Archived (PDF) from the original on 8 February 2016. Retrieved 17 August 2015.
- ↑ "Phosphoric Acid, 85 wt.% SDS". Sigma-Aldrich. 5 May 2016. Archived from the original on 18 January 2017. Retrieved 16 January 2017.
Cited sources[edit | edit source]
- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1439855119.