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 (K_a). Each successive K_a 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 pK_a. 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 K_a values of these protons.