Phosphorous acid 1966205 215233453 2008-05-27T08:59:07Z DOI bot 6652755 Citation maintenance. Please [[User:DOI_bot/bugs|report any bugs]] you see. Any concerns? Please [[User_talk:Smith609|Contact the bot's operator]]. [[WP:DOI|Try this bot]] yourself! {{Chembox new | ImageFile = Phosphonic-acid-3D-balls.png | ImageSize = 150px | ImageName = Phosphonic acid | ImageFile1 = Phosphonic-acid-2D-dimensions.png | ImageSize1 = 150px | ImageName1 = Phosphonic acid | IUPACName = phosphonic acid<br />hydridodihydroxidooxidophosphorus | OtherNames = phosphonic acid | Section1 = {{Chembox Identifiers | CASNo = 13598-36-2 | RTECS = SZ6400000 }} | Section2 = {{Chembox Properties | Formula = H<sub>3</sub>PO<sub>3</sub> | MolarMass = 82.00 g/mol | Appearance = colorless solid | Density = 1.65 g/cm<sup>3</sup> | Solubility = miscible | MeltingPt = 70.1 °C | BoilingPt = decomposes }}<!-- | [[Acid dissociation constant|Acidity]] (p''K''<sub>a1</sub>) | 2.0 |- | [[Acid dissociation constant|Acidity]] (p''K''<sub>a2</sub>) | 6.7 |- --> | Section3 = {{Chembox Structure | MolShape = tetrahedral | CrystalStruct = | Dipole = }} | Section7 = {{Chembox Hazards | ExternalMSDS = | MainHazards = skin irritant | NFPA-H = 2 | NFPA-R = 1 | NFPA-F = 0 | RPhrases = 22-35 | SPhrases = 26-36/37/39-45 }} | Section8 = {{Chembox Related | Function = [[?]] | OtherFunctn = | OtherCpds = [[phosphoric acid|H<sub>3</sub>PO<sub>4</sub>]] (i.e., PO(OH)<sub>3</sub>)<br />[[hypophosphorous acid|H<sub>3</sub>PO<sub>2</sub>]] (i.e., H<sub>2</sub>PO(OH)) }} }} '''Phosphorous acid''' is the [[Compound (chemistry)|compound]] described by the [[chemical formula|formula]] H<sub>3</sub>PO<sub>3</sub>. It is one of the [[oxoacid]]s of [[phosphorus]], other important members being [[phosphoric acid]] (H<sub>3</sub>PO<sub>4</sub>) and [[hypophosphorous acid]] (H<sub>3</sub>PO<sub>2</sub>). Note that only the reduced phosphorus compounds are spelled with an "ous" ending. Other names for this acid are orthophosphorous acid and dihydroxyphosphine oxide. HP(O)(OH)<sub>2</sub> is the product of the hydrolysis of its [[acid anhydride]], P<sub>4</sub>O<sub>6</sub>: ::P<sub>4</sub>O<sub>6</sub> + 6 H<sub>2</sub>O → 4 HP(O)(OH)<sub>2</sub> An analogous relationship connects H<sub>3</sub>PO<sub>4</sub> and P<sub>4</sub>O<sub>10</sub>. ==Tautomerization== H<sub>3</sub>PO<sub>3</sub> is better described with the structural formula HP(O)(OH)<sub>2</sub>. This species exists in equilibrium with a minor tautomer P(OH)<sub>3</sub>. The latter is called phosphorous acid, whereas the dihydroxy form is called phosphonic acid. Many of the reduced phosphorus acids are subject to similarly complicated equilibria involving shifts of H between O and P. In the solid state, HP(O)(OH)<sub>2</sub> is tetrahedral with one shorter P=O bond of 148 [[picometer|pm]] and two longer P-O(H) bonds of 154 pm. ==Preparation== Although commercially available, the acid is most commonly prepared by hydrolysis of [[phosphorus trichloride]] with water or steam: ::PCl<sub>3</sub> + 3 H<sub>2</sub>O → HP(O)(OH)<sub>2</sub> + 3 [[HCl]] Potassium phosphite is a convenient precursor to phosphorous acid: :K<sub>2</sub>HPO<sub>3</sub> + 2 HCl → 2 KCl + H<sub>3</sub>PO<sub>3</sub> In practice aqueous potassium phosphite is treated with excess hydrochloric acid. By concentrating the solution and precipitations with alcohols, the pure acid can be separated from the salt. ==Acid-base properties== Phosphorous acid is a diprotic acid, since the hydrogen bonded directly to the central phosphorus atom is not readily ionizable. Chemistry examinations often test students' appreciation of the fact that all three hydrogen atoms are ''not'' acidic under aqueous conditions, in contrast with phosphoric acid. HP(O)<sub>2</sub>(OH)<sup>−</sup> is a moderately strong acid. ::HP(O)(OH)<sub>2</sub> → HP(O)<sub>2</sub>(OH)<sup>−</sup> + H<sup>+</sup> p''K''<sub>a</sub> = 1.3<ref>CRC Handbook of Chemistry and Physics, 87th Ed. 8-42</ref> ::HP(O)<sub>2</sub>(OH)<sup>−</sup> → HPO<sub>3</sub><sup>2−</sup> + H<sup>+</sup> p''K''<sub>a</sub> = 6.7 The monodeprotonated species, HP(O)<sub>2</sub>(OH)<sup>−</sup>. is called the [[phosphite]] [[ion]]. The [[IUPAC nomenclature of organic chemistry|IUPAC (mostly organic)]] name is '''phosphonic acid'''. This nomenclature is commonly reserved for [[Substituent|substituted]] derivatives, that is, organic group bonded to phosphorus, not simply an ester. For example, (CH<sub>3</sub>)PO(OH)<sub>2</sub> is "methylphosphonic acid", which may of course form "methylphosphonate" [[ester]]s. Both phosphorous acid and its deprotonated forms are good [[reducing agent]]s, although not necessarily quick to react. They are oxidized to [[phosphoric acid]] or its salts. It reduces solutions of noble metal cations to the metals. ==Uses== ===Conversion to phosphine=== Phosphine, being a flammable and toxic gas, is inconvenient to store. Fortunately this useful species is readily prepared by thermal decomposition of phosphorous acid, which degrades at about 180°C: ::4 HP(O)(OH)<sub>2</sub> → PH<sub>3</sub> + 3 H<sub>3</sub>PO<sub>4</sub> Since phosphoric acid is a syrupy non-volatile liquid, the gaseous PH<sub>3</sub> is readily separated. ===In agriculture=== A large quantity of phosphorous acid is used as phosphatic [[fertilizer]]. <ref>{{cite journal | title = Phosphite (Phosphorous acid ): Its Relevance in the Environment and Agriculture and Influence on Plant Phosphate Starvation Respose | author = Allison E. McDonald; Bruce R. Grant; William C. Plaxton | journal = Journal of Plant Nutrition | volume = 24 | issue = 10 | pages = 1505–1519 | year = 2001 | url = | doi = 10.1081/PLN-100106017 }}</ref> Pure phosphorous acid is also used for preparing phosphite salts, such as [[monopotassium phosphite]] or [[aluminum phosphonite]]. These salts, as well as aqueous solutions of pure phosphorous acid, have shown effectiveness in controlling a variety of microbial plant diseases—in particular, treatment using either trunk injection or [[foliar]] containing phosphorous acid salts is indicated in response to infections by ''[[phytophthora]]'' and ''[[pythium]]''-type plant pathogens (both within class ''[[oomycetes]]'', known as [[water mold]]s), such as [[Phytophthora cinnamomi|dieback/root rot]] and [[downy mildew]].<ref>[http://organiclabs.com/images/logos/110_EXEL%20LG%20Back%20NEW.doc Organic Labs. Product label for 'Exel LG,' Retrieved April 9, 2007.]</ref> Anti-microbial products containing salts of phosphorous acid are marketed in Australia as 'Yates Anti-Rot'; and in the [[United States of America]], for example, aluminum salts of phosphorous acid (known generically as 'Fosetyl-Al') are sold under the trade name 'Aliette'.<ref>[http://msds.orica.com/pdf/shess-en-cds-010-000000020540.pdf Yates, a Division of Orica Australia Pty Ltd. “MSDS ('Yates Anti Rot Phosacid Systemic Fungicide').” Version 1. SH&E Shared Services, Orica. Homebush, NSW (Australia): April 4, 2005 (retrieved from www.orica.com April 9, 2007).]</ref><ref>[[s:Image:Fosetyl-al red-facts 1994.pdf|US EPA. “Fosetyl-Al (Aliette): Reregistration Eligibility Decision (RED) Fact Sheet.” Office of Pesticide Programs, US EPA. Washington, DC (USA): 1994 ''(retrieved from www.epa.gov April 9, 2007)''.]]</ref> ===As a chemical reagent=== Phosphorous acid is used in chemical reactions as a [[reducing agent]] that is somewhat less vigorous than the related [[hypophosphorous acid]].<ref>[http://www.bartleby.com/61/93/P0259300.html “Phosphorous acid.” The American Heritage® Dictionary of the English Language, 4th ed. Boston: Houghton Mifflin, 2000 (retrieved from www.bartleby.com April 9, 2007).]</ref> ==References== {{reflist}} ===Other references=== *Holleman, A. F.; Wiberg, E. “Inorganic Chemistry.” Academic Press: San Diego, 2001. ISBN 0-12-352651-5. *D. E. C. Corbridge. “Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology.” 5th ed. Elsevier: Amsterdam. ISBN 0-444-89307-5. [[Category:Phosphites]] [[Category:Acids]] [[Category:Phosphorus compounds]] [[cs:Kyselina fosforitá]] [[de:Phosphonsäure]] [[eo:Fosfonata acido]] [[fr:Acide phosphoreux]] [[it:Acido fosforoso]] [[pl:Kwas fosfonowy]] [[pt:Ácido fosforoso]] [[ru:Фосфористая кислота]] [[zh:亞磷酸]]