Hypophosphoric acid

{{Distinguish|Hypophosphorous acid}} {{Chembox | ImageFile = Hypophosphoric acid.svg | ImageSize = | ImageAlt = | IUPACName = Hypodiphosphoric acid | OtherNames = Diphosphoric acid |Section1={{Chembox Identifiers | CASNo = 7803-60-3 | PubChem = 24536 | UNII = CY8Z35442Z | ChEBI = 29263 | SMILES = OP(O)(P(O)(O)=O)=O | ChemSpiderID = 22943 | StdInChI = InChI=1S/H4O6P2/c1-7(2,3)8(4,5)6/h(H2,1,2,3)(H2,4,5,6) | StdInChIKey = TVZISJTYELEYPI-UHFFFAOYSA-N}} |Section2={{Chembox Properties | Formula = H₄P₂O₆ | MolarMass = 161.98 g/mol | Appearance = White solid (dihydrate) | Density = | MeltingPtC = 54 | pKa = 2.2, 2.8, 7.3, 10.0<ref name="Greenwood"/> | ConjugateBase = Hypophosphate}} |Section3={{Chembox Hazards | MainHazards = | FlashPt = | AutoignitionPt = }} }}

'''Hypophosphoric acid''' is a mineral acid with the formula H₄P₂O₆, with phosphorus in a formal oxidation state of +4. In the solid state it is present as the dihydrate, H₄P₂O₆·2H₂O. In hypophosphoric acid the phosphorus atoms are identical and joined directly with a P−P bond. '''Isohypophosphoric acid''' is a structural isomer of hypophosphoric acid in which one phosphorus has a hydrogen directedly bonded to it and that phosphorus atom is linked to the other one by an oxygen bridge to give a phosphorous acid/phosphoric acid mixed anhydride. The two phosphorus atoms are in the +3 and +5 oxidation states, respectively.

==Preparation and reactions== Hypophosphoric acid can be prepared by the reaction of red phosphorus with sodium chlorite at room temperature.<ref name = "Corbridge6th">Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, {{ISBN|978-1-4398-4088-7}}</ref> :2&nbsp;P + 2 NaClO₂ + 2 H₂O → Na₂H₂P₂O₆ + 2&nbsp;HCl

A mixture of hypophosphoric acid, phosphorous acid (H₃PO₃) and phosphoric acid (H₃PO₄) is produced when white phosphorus oxidises in air when partially immersed in water.<ref name = "Corbridge6th"/>

The tetrasodium salt Na₄P₂O₆·10H₂O crystallises at pH 10 and the disodium salt, Na₂H₂PO₆·6H₂O at pH 5.2.<ref name="Greenwood">{{Greenwood&Earnshaw1st|pages=515–516}}</ref> The disodium salt can be passed through an ion exchange column to form the acid dihydrate, H₄P₂O₆·2H₂O.<ref name = Corbridge6th/>

The anhydrous acid can be formed by vacuum dehydration over P₄O₁₀ or by the reaction of H₂S on lead hypophosphate, Pb₂P₂O₆.<ref name="Greenwood"/>

Hypophosphoric acid is tetraprotic with dissociation constants p''K''_a1 = 2.2, p''K''_a2 = 2.8, p''K''_a3 = 7.3 and p''K''_a4 = 10.0.<ref name="Greenwood"/>

On standing the anhydrous acid undergoes rearrangement and disproportionation to form a mixture of isohypophosphoric acid, HPO(OH)-O-PO(OH)₂; pyrophosphoric acid H₄P₂O₇ and pyrophosphorous acid.<ref name="Greenwood"/>

Hypophosphoric acid is unstable in hot hydrochloric acid, in 4&nbsp;M HCl it hydrolyses to give H₃PO₃ + H₃PO₄.<ref name="Greenwood"/>

==Structure== Hypophosphorus acid contains oxonium ions and is best formulated [H₃O⁺]₂ [H₂P₂O₆]²⁻. The acid is isostructural with the diammonium salt which contains the [HOPO₂PO₂OH]²⁻ anion with a P−P bond length of 219 pm.<ref name = Corbridge6th/>

The HOPO₂PO₂OH²⁻ anion in Na₂H₂P₂O₆·6H₂O has a symmetric, staggered ethane-like structure with a P−P bond of length 219&nbsp;pm. Each phosphorus atom has two P−O bonds with length 151&nbsp;pm, and a P−OH bond length of 159&nbsp;pm.<!--I added this reference because there is a misprint in Greenwood, Corbridge is not specific on which P−O bond is referred to and Housecroft and sharpe also repeat the greenwood misprint--><ref name="CollinWillis1971">{{cite journal|last1=Collin|first1=R. L.|last2=Willis|first2=M.|title=The crystal structure of disodium dihydrogen hypophosphate hexahydrate (Na₂H₂P₂O₆·6H₂O) and disodium dihydrogen pyrophosphate hexahydrate (Na₂H₂P₂O₇·6H₂O)|journal=Acta Crystallographica Section B|volume=27|issue=2|year=1971|pages=291–302|issn=0567-7408|doi=10.1107/S0567740871002127|doi-access=}}</ref>

==Hypophosphate salts== Many hypophosphate salts are known, for example, K₄P₂O₆·8H₂O, Ca₂P₂O₆·2H₂O, K₃HP₂O₆·3H₂O, K₂H₂P₂O₆·2H₂O, KH₃P₂O₆.

On standing in air, hypophosphates tend to oxidise to pyrophosphates containing the {{chem|P|2|O|7|4−}} ion where P has a formal oxidation state of +5. Hypophosphates are stable to alkali hydroxides. In fused sodium hydroxide they convert rapidly to the orthophosphate containing {{chem|PO|4|3−}}.<ref name = "Corbridge6th"/>

==Polyhypophosphates== Polyhypophosphates are known containing linear anions, for example Na₅P₃O₈ containing {{chem2|O(PO2}})₃O⁵⁻ with a P−P−P chain and Na₆P₄O₁₀·2H₂O containing {{chem2|O(PO2}})₄O⁶⁻, with a P−P−P−P chain. The cyclic anion {{chem|(PO|2|)|6|6−}}, (hypohexametaphosphate<ref name="Wiberg">{{Holleman&Wiberg|page=715}}</ref>) where each phosphorus atom has an oxidation state of +3 is formed when a suspension of red phosphorus in KOH is oxidised with bromine.<ref name = "Corbridge6th"/>

== See also == *Dithionic acid, the sulfur equivalent.

==References== {{Reflist}}

Category:Phosphorus oxoacids