11-19. Sketch the general appearance of the curve for the titration of a weak diprotic acid withNaOH. What chemistry governs the pHin each distinct region of the curve?

The titration curve for the titration of a weak diprotic acid with NaOH

A diprotic acid is titrated with a known quantity of NaOH solution.

A polyprotic acid contributes protons in the same way as an Arrhenius acid donates a proton$\left(\mathrm{H}+\right)$ . A polyprotic acid, on the other hand, differs from a monoprotic acid in that it contains more than one acidic$\mathrm{H}+$and can thus contribute numerous protons. It does not entirely dissociate as a weak polyprotic acid. Some examples of weak polyprotic acids are as follows:

• ${\mathrm{H}}_3{\mathrm{PO}}_4$Is a triphosphate acid.
• ${\mathrm{H}}_2{\mathrm{CO}}_3$as a diprotic acid
• ${\mathrm{H}}_2{\mathrm{SO}}_3$as a diprotic acid

The titration curve for the titration of a weak diprotic acid with NaOH looks like this:

As we can see on the titration curve, there are six regions on it:

• Before any base is added, the solution only contains the weak diprotic acid and the pH is calculated with the hydrolysis reaction of the acid.
• When the base is added, the solution contains an acid-base buffer and the pH is calculated using the Henderson-Hasselbalch equation.
• At the first equivalence point, the solution contains the intermediate form of an acid which determines the pH.
• After the first equivalence point the solution contains another acid-base buffer and the pH is calculated using the Henderson-Hasselbalch equation.
• At the second equivalence point, the solution contains the second intermediate form of an acid which determines the pH.
• After the equivalence point, all the acid has been neutralized and the pH is determined by the excess of the base.