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.

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Helmenstine, Anne Marie, Ph.D. "Polyprotic Acid Definition in Chemistry." ThoughtCo, Feb. 16, 2021, thoughtco.com/definition-of-polyprotic-acid-605545. Helmenstine, Anne Marie, Ph.D. (2021, February 16). Polyprotic Acid Definition in Chemistry. Retrieved from https://www.thoughtco.com/definition-of-polyprotic-acid-605545 Helmenstine, Anne Marie, Ph.D. "Polyprotic Acid Definition in Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-polyprotic-acid-605545 (accessed September 21, 2021).