3.9: Titration of Polyprotic Base with a Strong Acid

A polyprotic bronsted base like sodium carbonate reacts with water in two successive steps.

In its titration with hydrochloric acid, the initial pH estimated from the first dissociation constant of sodium carbonate is 11.6. The solution slowly turns acidic, demonstrating two equivalence points on the titration curve.

At the first equivalence point, all the carbonate ions are converted to bicarbonate ions. The second equivalence point corresponds to the complete conversion of bicarbonate ions to carbonic acid, which in an acidic solution dissociates to carbon dioxide and water.

While the region before the first equivalence point contains the carbonate/bicarbonate buffer,  the area between the first and second equivalence points holds the bicarbonate/carbonic acid buffer.

The indicators for detecting the first and second endpoints are phenolphthalein and methyl orange, respectively.

Here, the methyl orange exhibits a gradual color change resulting in poor endpoint detection. So, a mixture of methyl orange and xylene cyanol is used, which sharpens the second endpoint observation.