The oxidation number of phosphorus in Pyrophosphoric acid is +5.
The total number of electrons that an atom either gains or loses to form a chemical bond with another atom is known as the oxidation number. Pyrophosphoric acid is a tetraprotic acid with the formula H4P2O7.
Pyrophosphoric acid H4P2O7
Let oxidation state of phosphorus is x
(4 x 1 + (-2) x 7+2x)=0