Hypophosphoric acid

Hypophosphoric acid
Names
	IUPAC name Hypodiphosphoric acid
	Other names Diphosphoric acid
Identifiers
CAS Number	7803-60-3;
3D model (JSmol)	Interactive image;
ChEBI	CHEBI:29263;
ChemSpider	22943;
PubChem CID	24536;
UNII	CY8Z35442Z;
CompTox Dashboard (EPA)	DTXSID0064886 ;
	InChI InChI=1S/H4O6P2/c1-7(2,3)8(4,5)6/h(H2,1,2,3)(H2,4,5,6) Key: TVZISJTYELEYPI-UHFFFAOYSA-N;
	SMILES OP(O)(P(O)(O)=O)=O;
Properties
Chemical formula	H4P2O6
Molar mass	161.98 g/mol
Appearance	White solid (dihydrate)
Melting point	54 °C (129 °F; 327 K)
Acidity (pKa)	2.2, 2.8, 7.3, 10.0
Conjugate base	Hypophosphate
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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.^[2]

2 P + 2 NaClO₂ + 2 H₂O → Na₂H₂P₂O₆ + 2 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.^[2]

The tetrasodium salt Na₄P₂O₆·10H₂O crystallises at pH 10 and the disodium salt, Na₂H₂PO₆·6H₂O at pH 5.2.^[1] The disodium salt can be passed through an ion exchange column to form the acid dihydrate, H₄P₂O₆·2H₂O.^[2]

The anhydrous acid can be formed by vacuum dehydration over P₄O₁₀ or by the reaction of H₂S on lead hypophosphate, Pb₂P₂O₆.^[1]

Hypophosphoric acid is tetraprotic with dissociation constants pK_a1 = 2.2, pK_a2 = 2.8, pK_a3 = 7.3 and pK_a4 = 10.0.^[1]

On standing the anhydrous acid undergoes rearrangement and disproportionation to form a mixture of isohypophosphoric acid, HPO(OH)-O-PO₂(OH); pyrophosphoric acid H₂P₂O₇ and pyrophosphorous acid.^[1]

Hypophosphoric acid is unstable in hot hydrochloric acid, in 4 M HCl it hydrolyses to give H₃PO₃ + H₃PO₄.^[1]

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.^[2]

The HOPO₂PO₂OH²⁻ anion in Na₂H₂P₂O₆·6H₂O has a symmetric, staggered ethane-like structure with a P−P bond of length 219 pm. Each phosphorus atom has two P−O bonds with length 151 pm, and a P−OH bond length of 159 pm.^[3]

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 P
₂O⁴⁻
₇ 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 PO³⁻
₄.^[2]

Polyhypophosphates

Polyhypophosphates are known containing linear anions, for example Na₅P₃O₈ containing O(PO₂)₃O⁵⁻ with a P−P−P chain and Na₆P₄O₁₀·2H₂O containing O(PO₂)₄O⁶⁻, with a P−P−P−P chain. The cyclic anion (PO
₂)⁶⁻
₆, (hypohexametaphosphate^[4]) where each phosphorus atom has an oxidation state of +3 is formed when a suspension of red phosphorus in KOH is oxidised with bromine.^[2]

References

^ ^a ^b ^c ^d ^e ^f Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 515–516. ISBN 978-0-08-022057-4.
^ ^a ^b ^c ^d ^e ^f Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, ISBN 978-1-4398-4088-7
^ Collin, R. L.; Willis, M. (1971). "The crystal structure of disodium dihydrogen hypophosphate hexahydrate (Na₂H₂P₂O₆·6H₂O) and disodium dihydrogen pyrophosphate hexahydrate (Na₂H₂P₂O₇·6H₂O)". Acta Crystallographica Section B. 27 (2): 291–302. doi:10.1107/S0567740871002127. ISSN 0567-7408.
^ Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 715, ISBN 0-12-352651-5

[Greenwood-1] ^ ^a ^b ^c ^d ^e ^f Greenwood, Norman N.; Earnshaw, Alan (1984). Chemistry of the Elements. Oxford: Pergamon Press. pp. 515–516. ISBN 978-0-08-022057-4.

[Corbridge6th-2] ^ ^a ^b ^c ^d ^e ^f Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, ISBN 978-1-4398-4088-7

[CollinWillis1971-3] Collin, R. L.; Willis, M. (1971). "The crystal structure of disodium dihydrogen hypophosphate hexahydrate (Na₂H₂P₂O₆·6H₂O) and disodium dihydrogen pyrophosphate hexahydrate (Na₂H₂P₂O₇·6H₂O)". Acta Crystallographica Section B. 27 (2): 291–302. doi:10.1107/S0567740871002127. ISSN 0567-7408.

[Wiberg-4] Holleman, Arnold Frederik; Wiberg, Egon (2001), Wiberg, Nils (ed.), Inorganic Chemistry, translated by Eagleson, Mary; Brewer, William, San Diego/Berlin: Academic Press/De Gruyter, p. 715, ISBN 0-12-352651-5

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