Titration of Polyprotic Acids with a Strong Base

Polyprotic acids contain multiple ionizable protons, each dissociating with its own dissociation constant (Ka). Each successive Ka is weaker than the previous one, meaning the first proton dissociates most readily, followed by progressively weaker dissociations. This stepwise process is fundamental to understanding polyprotic acid behavior during titration.

When a strong base like NaOH neutralizes the first proton of a polyprotic acid such as sulfurous acid, the titration curve resembles that of a weak monoprotic acid. At the half-equivalence point, the pH equals pKa1. This initial neutralization produces an intermediate species, like hydrogen sulfite ions, before the second proton is removed.

A polyprotic acid requires proportionally more base because each ionizable proton must be neutralized separately. For a diprotic acid like sulfurous acid, twice the amount of base is needed compared to a monoprotic acid of equal concentration. This is because two moles of base neutralize one mole of the diprotic acid.

The number of equivalence points equals the number of ionizable protons in the polyprotic acid, provided the Ka values differ by over ten thousand fold. Triprotic phosphoric acid, for example, has three equivalence points corresponding to its three ionizable protons. This relationship holds for weak polyprotic acids with sufficiently separated Ka values.

The second equivalence point appears in the basic region of the titration curve. At this point, both ionizable protons have been neutralized. The titration curve also shows a second half-equivalence point between the first and second equivalence points, where pH equals pKa2.

Sulfurous acid is diprotic, producing two equivalence points when titrated with a strong base, while phosphoric acid is triprotic, producing three equivalence points. Both follow the same principle: each equivalence point corresponds to neutralization of one ionizable proton, with successive Ka values becoming progressively weaker.

At each half-equivalence point during polyprotic acid titration, the pH equals the corresponding pKa value. At the first half-equivalence point, pH equals pKa1; at the second, pH equals pKa2. This relationship allows determination of dissociation constants directly from the titration curve.