Determine the pH at the first equivalence point. In a driprotic acid titration, calculate the equilibrium constant, Ka,and then convert to the logarithmic constant, pKa. A diprotic acid has a second equilibrium constant, Ka2, because of the additional hydrogen proton and second logarithmic constant, denoted as pKa2. These values are found in various chemistry texts for most acids (see Resources). Calculate the pH at the first equivalence point as half the sum of the pKa1 and pKa2.
pH of first equivalence point = (pKa1 + pKa2) / 2
Identify the pH between the first equivalence point and the second equivalence point as equal to the pKa2 value. This is referred to as the second half equivalence point.
pH halfway between first and second equivalence point = pKa2
Realize that halfway between the first and second equivalence point, half of the second protons from the diprotic acid have been removed. Thus, the predominant species in the acid base titration is a salt formed from the combination of the ion bonded to the hydrogen proton in the acid, and the ionic atom in the base. As the titration progress to the second equivalence point, the salt will become the main species present in the solution. Therefore, at the second equivalence point, the pH of the corresponding salt will identify the final pH of the solution.