Peroxophosphoric acid

Description

Two of these acids have been prepared, a peroxymonophosphoric acid H3PO5 and a peroxydiphosphoric acid H4P2O8. Note that the latter is related to pyrophosphoric acid. Both are formed with 30% hydrogen peroxide, the first with phosphorous pentoxide:
P2O5 + 2H2O2 +H2O?2H3PO5
The reaction is very violent, but it can be moderated by using acetonitrile as a solvent. At higher concentrations, the peroxydiphosphoric acid can be produced. This is a stronger oxidizing agent than peroxymonosulphuric acid. With silver salts, it forms oxygen plus silver phosphate Ag3PO4. It converts aniline into nitroso- and nitrobenzene, and manganous salts quickly in the cold into permanganates. If pyrophosphoric acid is treated with hydrogen peroxide the diperoxy-acid H4P2O8 is formed along with the monoacid. The two behave differently with potassium iodide: the monoacid separates iodine at once, and the other acid only slowly. In water, the two acids go over into one another, the mono- predominating in acid and the di-(peroxy-) in alkaline solution.