Applications of Aqueous Equilibria
Interactive Student Tutorial

 15.9 Polyprotic Acid–Strong Base Titrations

When a protonated amino acid H2A+ is titrated with a strong base, the titration curve shows more than one equivalence point.

  • The initial pH is that of a weak acid. Calculating the initial pH is the same as calculating the pH of any weak diprotic acid using an equilibrium table.
  • Halfway to the first equivalence point, H2A+ is converted to HA by the addition of OH. The Henderson-Hasselbalch equation may be used to calculate the pH of the solution.
  • At the first equivalence point, the principal reaction is proton transfer between HA molecules.

    and

  • Halfway between the first and second equivalence points, half of the HA has been converted to A because of the neutralization reaction

    and pH = pKa2.

  • At the second equivalence point, the principal reaction is the reaction of the basic salt A with water.

The pH can be calculated from the equilibrium equation for the principal reaction.

  • After the second equivalence point, the pH is dominated by additional OH and can be calculated by stoichiometry.