Polyprotic acid

Phosphoric acid (H
3PO
4) is a triprotic acid. It can lose up to three hydrogen atoms to become phosphate (PO3−
4).

A polyprotic acid is an acid that has more than one acidic hydrogen atom. "Polyprotic" means "many protons", because the hydrogen ion is (normally) a single proton.

Polyprotic acids are also grouped according to the exact number of hydrogen atoms. Diprotic acids like sulfuric and carbonic acid can give up two hydrogens; triprotic acids like phosphoric acid and citric acid can give up three. Acids with more than three acidic hydrogens are normally just called "polyprotic" instead of more specific names like "tetraprotic" or "pentaprotic".