A polyprotic acid contains more than one ionizable hydrogen and undergoes a stepwise ionization process. If the acid dissociation constants of the ionizable protons differ sufficiently from each other, then the titration curve for such polyprotic acid generates a distinct equivalence point for each of its ionizable hydrogens. Therefore, titration of a diprotic acid results in the formation of two equivalence points, whereas the titration of a triprotic acid results in the formation of three equivalence points on the titration curve.
Carbonic acid, H2CO3, is an example of a weak diprotic acid. The first ionization of carbonic acid yields hydronium ions and bicarbonate ions in small amounts.
The bicarbonate ion can also act as an acid. It ionizes and forms hydronium ions and carbonate ions in even smaller quantities.
The Ka1 is larger than the Ka2 by a factor of 104. Therefore, when H2CO3 is titrated with a strong base like NaOH, it produces two distinct equivalence points for each ionizable hydrogen.
Phosphoric acid, a triprotic acid, ionizes in three steps:
When H3PO4 is titrated with a strong base like KOH, it produces three equivalence points for each ionizable hydrogen. However, as HPO42− is a very weak acid, the third equivalence point is not easily discernible on the titration curve.
This text is adapted from Openstax, Chemistry 2e, Section 14.5: Polyprotic Acids.