phosphoric acid#pH and composition of a phosphoric acid aqueous solution

Phosphoric acid is produced industrially by one of two routes, wet processes and dry.{{cite book |last1=Becker |first1=Pierre |title=Phosphates and phosphoric acid |date=1988 |publisher=Marcel Dekker |location=New York |isbn=978-0824717124}}{{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}}{{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}}

In the wet process, a phosphate-containing mineral such as calcium hydroxyapatite and fluorapatite are treated with sulfuric acid.{{Greenwood&Earnshaw2nd|pages=520–522}}

:{{chem2|Ca5(PO4)3OH + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + H2O}}

:{{Chem2|Ca5(PO4)3F + 5 H2SO4 → 3 H3PO4 + 5 CaSO4 + HF}}

Calcium sulfate (gypsum, {{Chem2|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% {{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 (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.

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.{{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 }} 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.[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] 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.{{Cite journal |last=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}}

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.{{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 }}

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.[https://www.sulzer.com/-/media/files/products/separation-technology/crystallisation/brochures/fractional_crystallization_e.pdf?sc_lang=en Fractional Crystallization]

In aqueous solution phosphoric acid behaves as a triprotic acid.

:{{chem2|H3PO4 ⇌ H2PO4- + H+}}, pK_a1 = 2.14

:{{chem2|H2PO4- ⇌ HPO4(2-) + H+}}, pK_a2 = 7.20

:{{chem2|HPO4(2-) ⇌ PO4(3-) + H+}}, pK_a3 = 12.37

The difference between successive pK_a 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 pK_a values.

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 2H₃PO₄•H₂O, freezing at 29.32 °C.{{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}}{{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}} 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.{{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}}

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.{{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}}

As the concentration is increased higher acids are formed, culminating in the formation of polyphosphoric acids.{{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}} 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.

{{also|Phosphorus#Food additive}}

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

Food-grade phosphoric acid (additive E338{{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}}) 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.{{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}} 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.{{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}} Phosphoric acid also has the potential to contribute to the formation of kidney stones, especially in those who have had kidney stones previously.{{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 }}

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{{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 }}
• in compound semiconductor processing, to etch Indium gallium arsenide selectively with respect to indium phosphide[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.
• in microfabrication to etch silicon nitride selectively with respect to silicon dioxide{{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}}
• in microfabrication to etch aluminium
• as a pH adjuster in cosmetics and skin-care products{{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}}
• as a sanitizing agent in the dairy, food, and brewing industries{{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}}

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.{{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}}

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.{{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}}

A link has been shown between long-term regular cola intake and osteoporosis in later middle age in women (but not men).{{cite journal|vauthors=Tucker KL, Morita K, Qiao N, Hannan MT, Cupples LA, Kiel DP | title=Colas, but not other carbonated beverages, are associated with low bone mineral density in older women: The Framingham Osteoporosis Study| journal=American Journal of Clinical Nutrition| volume=84| pages=936–942| issue = 4| date = 1 October 2006| pmid = 17023723 | doi=10.1093/ajcn/84.4.936| doi-access=free}}

• Phosphate fertilizers, such as ammonium phosphate fertilizers
• Chiral phosphoric acid

{{Reflist|30em}}

• {{cite book | editor= Haynes, William M. | year = 2011 | title = CRC Handbook of Chemistry and Physics | edition = 92nd | publisher = CRC Press | isbn = 978-1439855119| title-link = CRC Handbook of Chemistry and Physics }}

{{Commons category|Phosphoric acid|lcfirst=yes}}

• [http://www.npi.gov.au/substances/phosphoric-acid/index.html National pollutant inventory – Phosphoric acid fact sheet]
• [https://www.cdc.gov/niosh/npg/npgd0506.html NIOSH Pocket guide to chemical hazards]

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