What is the formula for pyrophosphoric acid?

The chemical formula for pyrophosphoric acid (also known as diphosphoric acid) is ${\text{H}}_4{\text{P}}_2{\text{O}}_7$.12

It is formed when two molecules of phosphoric acid interact, typically at high temperatures, resulting in the loss of one molecule of water. This process creates a compound containing a $\text{P–O–P}$ bond, which serves as the simplest member of a series of polyphosphoric acids.13