Titrations in Diprotic Systems

The principles developed for titrations of monoprotic acids and bases are readily extended to titrations of polyprotic acids and bases. We will examine two cases. 

The volume at the second equivalence point must be 2Ve, because the second reaction requires the same number of moles of HCI as the first reaction. 

Before acid is added, the solution contains just weak base, B, whose pH is governed by the reaction 

At any point between A (the initial point) and C (the first equivalence point), we have a buffer containing B and BH+. Point B is halfway to the equivalence point, so fB] = [BH+]. 

The pH is calculated from the Henderson-Hasselbalch equation for the weak acid, BH+, 

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