Polyprotic
Definition and meaning of Polyprotic in chemistry.
A polyprotic acid is an acid that can donate more than one proton (hydrogen ion, H+) per molecule.
In more detail
Polyprotic acids release their protons in stepwise ionization steps, with each step characterized by its own acid dissociation constant (Ka1, Ka2, Ka3, etc.). The first proton is typically released most readily, with successive protons becoming progressively harder to remove because they must be extracted from increasingly negative ions. A diprotic acid like sulfuric acid (H2SO4) donates two protons, while a triprotic acid like phosphoric acid (H3PO4) donates three. Understanding polyprotic behavior is essential for calculating pH and equilibrium compositions in solutions.
Key facts
| Field | General Chemistry |
|---|---|
| Chemical formula example | H2SO4 (sulfuric acid, diprotic) |
| Another example | H3PO4 (phosphoric acid, triprotic) |
| Key characteristic | Ka1 >> Ka2 >> Ka3 (each successive proton harder to remove) |
Sulfuric acid (H2SO4) is a common diprotic acid. Its first ionization (H2SO4 to HSO4- + H+) is essentially complete, behaving as a strong acid (Ka1 is very large, on the order of 103), while its second ionization (HSO4- to SO42- + H+) is much weaker, with Ka2 ≈ 1.2 × 10-2.
Frequently asked questions
What is the difference between a monoprotic and polyprotic acid?
A monoprotic acid like HCl donates only one proton per molecule, while a polyprotic acid like H2SO4 can donate multiple protons in successive ionization steps.
Why do successive Ka values decrease?
Each successive proton is harder to remove because it must be extracted from a progressively more negatively charged ion, which electrostatically resists loss of the positive hydrogen ion.