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Dissociation of Polyprotic Acids

Some acids have two or more protons that can be released upon dissociation. Such acids are called polyprotic acids. For example, phosphoric acid (H3PO4) can lose up to three protons in aqueous solution.
Sulfuric acid is a polyprotic acid that can lose two protons in solution. The first ionization is complete because sulfuric acid is a strong acid.

image\Ch 9 sec H, g1.png

The second proton comes off as depicted by the next equation. In this case, an equilibrium exists because hydrogen sulfate ion (HSO4) is not as strong as H2SO4.

image\Ch 9 sec H, g2.png

It is clear from this example that the second dissociation constant is always lower than the first dissociation constant. Thus, it is easier to remove a proton from an uncharged species H2SO4 than from a charged species (HSO4).
Based on the Bronsted-Lowry definition of acids and bases, an acid is a proton donor and base is a proton acceptor. A species (charged or uncharged) that can gain or lose a proton is called amphoteric or amphiprotic species.
For example, (bicarbonate ion) HCO3 can donate a proton acting as an acid. The same species can accept a proton acting as a base. Hence, amphoteric species can act as an acid or a base depending on the surrounding conditions.

image\Ch sec H, g3.png

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