Peroxydiphosphoric acid (H
4P
2O
8) is an
oxyacid
An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...
of
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
. Its salts are known as peroxydiphosphates. It is one of two peroxyphosphoric acids, along with
peroxymonophosphoric acid
Peroxymonophosphoric acid () is an oxyacid of phosphorus. It is a colorless viscous oil. Its salts are called peroxymonophosphates. Another peroxyphosphoric acid is peroxydiphosphoric acid, .
Preparation
Peroxyphosphoric acids were first synt ...
.
History
Both peroxyphosphoric acids were first synthesized and characterized in 1910 by Julius Schmidlin and Paul Massini,
where peroxydiphosphoric acid was obtained in poor yields from the reaction between
diphosphoric acid and highly-concentrated
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3%†...
.
:
H4P2O7 + H2O2 -> H4P2O8 + H2O
Preparation
Peroxydiphosphoric acid can be prepared by the reaction between phosphoric acid and fluorine, with peroxymonophosphoric acid being a by-product.
:
2H3PO4 + F2 -> H4P2O8 + 2HF
The compound is not commercially available and must be prepared as needed.
Peroxodiphosphates can be obtained by
electrolysis
In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from n ...
of phosphate solutions.
Properties
Peroxydiphosphoric acid is a
tetraprotic acid
In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a sequ ...
, with
acid dissociation constants given by pK
a1 ≈ −0.3, pK
a2 ≈ 0.5, pK
a3 = 5.2 and pK
a4 = 7.6.
In aqueous solution, it
disproportionates
In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can ...
upon heating to peroxymonophosphoric acid and
phosphoric acid
Phosphoric acid (orthophosphoric acid, monophosphoric acid or phosphoric(V) acid) is a colorless, odorless phosphorus-containing solid, and inorganic compound with the chemical formula . It is commonly encountered as an 85% aqueous solution, w ...
.
[{{Cite book, last=Kolditz, first=Lothar, title=Anorganische Chemie, publisher=Deutscher Verlag der Wissenschaften, year=1983, volume=1, location=Berlin, pages=437]
:
H4P2O8 + H2O <=> H3PO5 + H3PO4
References
Phosphorus oxoacids
Mineral acids