Peroxydiphosphoric acid

Peroxydiphosphoric acid ((HO₂P(O)O−OP(O)(OH)₂) is an oxyacid of phosphorus. Its salts are known as peroxydiphosphates. It is closely related to peroxymonophosphoric acid. These compounds (and their salts) are in turn related to the peroxydisulfuric acid and peroxysulfuric acid. These compounds are all colorless.^[1] In terms of structure, peroxydiphosphoric acid features two tetrahedral phosphorus centers linked by a peroxide group.

Peroxydiphosphoric acid

Names
Other names peroxodiphosphoric acid
Identifiers
CAS Number	potassium salt: 15593-49-4
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29284
ChemSpider	103142
PubChem CID	115278
CompTox Dashboard (EPA)	DTXSID701337409
InChI InChI=1S/H4O8P2/c1-9(2,3)7-8-10(4,5)6/h(H2,1,2,3)(H2,4,5,6) Key: NUGJFLYPGQISPX-UHFFFAOYSA-N InChI=1S/H4O8P2/c1-9(2,3)7-8-10(4,5)6/h(H2,1,2,3)(H2,4,5,6)
SMILES OP(=O)(O)OOP(=O)(O)O
Properties
Chemical formula	H4P2O8
Molar mass	193.97 g/mol
Related compounds
Related compounds	peroxymonophosphoric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Preparation

Both peroxyphosphoric acids were first synthesized and characterized in 1910 by Julius Schmidlin and Paul Massini,^[2] where peroxydiphosphoric acid was obtained in poor yields from the reaction between diphosphoric acid and highly-concentrated hydrogen peroxide.

H₄P₂O₇ + H₂O₂ → H₄P₂O₈ + H₂O

Peroxydiphosphoric acid can be prepared by the reaction between phosphoric acid and fluorine, with peroxymonophosphoric acid being a by-product.^[1]

2 H₃PO₄ + F₂ → H₄P₂O₈ + 2 HF

The compound is not commercially available and must be prepared as needed.^[1] Potassium peroxodiphosphate, which is commercially available, can be obtained by electrolysis of phosphate solutions.^[3][1]

Properties

Peroxydiphosphoric acid is a tetraprotic acid, with acid dissociation constants given by pK_a1 ≈ −0.3, pK_a2 ≈ 0.5, pK_a3 = 5.2 and pK_a4 = 7.6.^[4] In aqueous solution, it disproportionates upon heating to peroxymonophosphoric acid and phosphoric acid.^[5]

H₄P₂O₈ + H₂O ⇌ H₃PO₅ + H₃PO₄