3.11: Composition of Polyprotic Acid Solutions as a Function of pH
Polyprotic acids of the type H2M constitute two ionizable protons. As a result, on titration with a base, they exhibit two equivalence points in the titration curve. During titration, the species H2M, HM−, and M2− will be present in the solution at different points. The fractions of H2M, HM−, and M2− present at the various instances of the titration are denoted by α0, α1, and α2, respectively.
A graph with the alpha values is plotted against the volume of base added during titration. Here, a value of 0.7 for the α0 at the beginning of the titration suggests that 70% of the solution is H2M, with the remaining 30% as HM−. On adding the base, the fraction of H2M, α0, decreases to nearly zero at the first equivalence point. Simultaneously, α1, representing the fraction of HM−, increases and approaches unity. As more base is added, the fraction of HM− decreases and reaches zero at the second equivalence point while the fraction of M2−, represented by α2, approaches unity.
