Question

sketch the general appearance of the curve for the titration of weak diprotic acid with NaOH. what chemistry governs the pH in each distinct region of the curve

Answer 1

From the graph, it is clear that there are two regions which involve sharp increase in pH. These regions correspond to equivalence point.

At first equivalence point, all of the diprotic acid (H₂X) present, gives one proton to the base (NaOH) and forms a monoprotic anion (HX^-). This is a neutralization reaction wherein one proton from the acid neutralizes the OH^- from base.

H₂X + NaOH ---> Na⁺ + HX^- + H₂O

If acid dissociation constant for this reaction is K_a1, then, at the mid point of this region, pH = pK_a1

After the acid completely loses one proton at the first equivalence point, it reacts further with the base and loses the seond proton. HX^- reats with NaOH (another neutralization reaction) to give water and X^2-. The completion of this reaction is indicated by the second equivalence point.

HX^- + NaOH ---> Na⁺ + X^2- + H₂O

If acid dissociation constant for this reaction is K_a2, then, at the mid point of this region, pH = pK_a2