An acid that can donate more than one proton per molecule. Sulfuric acid (H₂SO₄) is diprotic; phosphoric acid (H₃PO₄) is triprotic. Each ionization step has its own Ka value (Ka1 > Ka2 > Ka3). Successive proton losses become progressively more difficult.
A polyprotic acid can donate more than one proton per molecule. Diprotic acids (H₂SO₄, H₂CO₃, H₂S) have two Ka values; triprotic acids (H₃PO₄, H₃AsO₄, H₃Cit) have three. Successive Ka values decrease: Ka1 >> Ka2 >> Ka3 (each successive proton removed from an increasingly negative anion). H₂SO₄ is unusual: Ka1 = very large (strong), Ka2 = 1.2 × 10⁻² (moderate weak). At intermediate pH values, buffer regions occur between each pair of dissociation steps.
Uses & Applications
Polyprotic acids are used in: multi-stage buffer systems (phosphate buffer with three pKa values: 2.1, 7.2, 12.4; useful across wide pH range), analytical titrations (polyanions show multiple equivalence points allowing simultaneous determination), biochemistry (phosphate buffers for biological systems, citrate buffers for food applications), and industrial processes (phosphoric acid in food acidulants, semiconductor cleaning).
Safety Information
Polyprotic strong acids (H₂SO₄, H₃PO₄) are corrosive at all concentrations. Concentrated H₂SO₄ is also a dehydrating agent and reacts violently with organic matter. H₃PO₄ at high concentrations forms a highly viscous syrup that is difficult to work with safely. Always add acid to water, never water to acid.
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
An acid that can donate more than one proton per molecule. Sulfuric acid (H₂SO₄) is diprotic; phosphoric acid (H₃PO₄) is triprotic. Each ionization step has its own Ka value (Ka1 > Ka2 > Ka3). Successive proton losses become progressively more difficult.
Polyprotic acids are used in: multi-stage buffer systems (phosphate buffer with three pKa values: 2.1, 7.2, 12.4; useful across wide pH range), analytical titrations (polyanions show multiple equivalence points allowing simultaneous determination), biochemistry (phosphate buffers for biological syst…
Polyprotic strong acids (H₂SO₄, H₃PO₄) are corrosive at all concentrations. Concentrated H₂SO₄ is also a dehydrating agent and reacts violently with organic matter. H₃PO₄ at high concentrations forms a highly viscous syrup that is difficult to work with safely. Always add acid to water, never water …
The formula or notation for Polyprotic Acid is: Polyprotic acid: H₃PO₄ ⇌ H₂PO₄⁻ + H⁺ (Ka1); H₂PO₄⁻ ⇌ HPO₄²⁻ + H⁺ (Ka2)
Editorial standards: Chemical data is sourced from peer-reviewed literature,
CAS Registry, NIST WebBook, and PubChem. Safety information reflects guidance from OSHA, ECHA,
and IAEA. For educational purposes only — always consult official SDS documentation and qualified
professionals before handling chemicals.
