3.10:
Titration of Polyprotic Acids with a Strong Base
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.
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 constant (Ka). Each successive Ka is weaker than the previous one. In the titration of a polyprotic acid like sulfurous acid with a strong base such as sodium hydroxide, the base first neutralizes the initial ionizable proton, forming an intermediate species (e.g., hydrogen sulfite ions). This step's titration curve resembles that of a weak monoprotic acid, with a half-equivalence point where the pH equals the first pKa. As the titration continues, an additional base neutralizes the second ionizable proton. This process requires twice the amount of base for complete neutralization, leading to another half-equivalence point and an equivalence point in the basic region. This pattern extends to other polyprotic acids, such as triprotic phosphoric acid, which will have three equivalence points. The number of equivalence points in the titration curve of a weak polyprotic acid corresponds to the number of ionizable protons, provided there's a significant difference between the Ka values of these protons.