3.10
Titration of Polyprotic Acids with a Strong Base
Titration of a polyprotic acid, which contains multiple ionizable protons, involves distinct dissociation steps, each with its own dissociation consta…
Polyprotic acids contain multiple ionizable protons, each dissociating differently, with each successive acid dissociation constant, being weaker than the previous one.
For instance, consider an example of sulfurous acid with two protons.
When titrated with a strong base like NaOH, the first proton is removed, generating a hydrogen sulfite ion.
The titration curve mirrors those of a weak monoprotic acid with a strong base with an equivalence point, and the pH of the solution at the half-equivalence point equals pKa1.
Further base addition neutralizes the second proton. Here, the same base amount is needed as the initial sulfurous acid concentration, indicating two moles of the base are required to neutralize one mole of acid.
The titration curve for this process has a second half-equivalence point and a second equivalence point in the basic region.
Similarly, the titration of triprotic phosphoric acid with a strong base reveals three equivalence points.
In weak polyprotic acid titrations, equivalence points equal the number of ionizable protons, assuming Ka values differ by over ten thousand fold.
Q1: What are ionizable protons in polyprotic acids?
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.
Q2: How does the titration curve change when neutralizing the first proton of a polyprotic acid?
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.
Q3: Why does a polyprotic acid require more base for complete neutralization?
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.
Q4: What determines the number of equivalence points in a polyprotic acid titration?
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.
Q5: Where does the second equivalence point appear on a polyprotic acid titration curve?
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.
Q6: How does sulfurous acid titration differ from phosphoric acid titration?
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.
Q7: What is the relationship between half-equivalence points and pKa values in polyprotic acid titrations?
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.