3.11:
Composition of Polyprotic Acid Solutions as a Function of pH
On titration with sodium hydroxide, polyprotic acids like maleic acid with two ionizable protons demonstrate two equivalence points on the titration curve.
During the titration, the different species in the solution are H2M, HM−, and M2−, whose relative concentration is represented by alpha-knot, alpha-one, and alpha-two, respectively.
The change in concentration of the species is obtained by plotting the alpha values as a function of the base volume.
Initially, the alpha-knot is 0.7, and alpha-one is 0.3, suggesting 70% H2M and 30% HM−. As the base is added, pH increases with alpha-knot decreasing to nearly zero at the first equivalence point. Simultaneously, the fraction of HM− increases and approaches unity.
On further addition of the base, while the alpha-one decreases to near zero, the fraction of M2−, alpha-two, increases to around unity at the second equivalence point.
The fraction of different species depends only on the pH and is independent of the total concentration of the solution.
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.