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.