Pyrophosphoric acid

Editor-In-Chief: Henry A. Hoff

Pyrophosphoric acid, also known under the name diphosphoric acid, is a syrupy liquid of off-white color or a needle-like crystalline solid. Pyrophosphoric acid is colorless, odorless, hygroscopic and is soluble in water, diethyl ether, and ethyl alcohol. It is the anhydride of phosphoric acid and is produced from phosphoric acid by dehydration. Pyrophosphoric acid is one of many phosphorus oxoacids.

Pyrophosphoric acid is unstable in aqueous solution and in the absence of enzymic catalysis hydrolyzes extremely slowly into phosphoric acid in all but highly acidic media.


 * 2H3PO4 ⇌ H4P2O7 + H2O

Enzyme EC 3.6.1.1 catalyzes this hydrolysis. Specificity varies with the source and with the activating metal ion, e.g. Mg2+.

Pyrophosphoric acid is a medium strong inorganic acid. In a pyrophosphoric acid molecule, there are four hydrogens bonded to oxygens, and one, two, three, or all four can be lost as H+ ions. When all four are lost from pyrophosphoric acid, a pyrophosphate ion is formed. The anion, the salts, and the esters of pyrophosphoric acid are called pyrophosphates. Because pyrophosphoric acids can undergo four dissociations, there are four Ka values for it, as well as four corresponding pKa values. Similarly, pyrophosphate is a base with four Kb and, of course, four pKb values for regaining the H+ ions in reverse order.