Polyprotic Acid Definition in Chemistry

Sulfuric acid is a polyprotic acid, able to donate two hydrogen ions to aqueous solution.
Sulfuric acid is a polyprotic acid, able to donate two hydrogen ions to aqueous solution. LAGUNA DESIGN / Getty Images

A polyprotic acid is an acid that can donate more than one proton or hydrogen atom per molecule to an aqueous solution. In contrast, a monoprotic acid (e.g., HCl) can only donate one proton per molecule.

Polyprotic Acid Examples

Sulfuric acid (H2SO4) is a polyprotic acid because it can donate two hydrogen atoms to an aqueous solution. Specifically, sulfuric acid is a diprotic acid because it has two available hydrogen atoms.

Orthophosphoric acid (H3PO4) is a triprotic acid. Successive deprotonations yield H2PO4-, HPO42-, and PO43-. In this acid, the positions of the original three hydrogen atoms are equivalent on the molecule, but removal of subsequent protons becomes less energetically favorable.