The titration of a polyprotic base such as sodium carbonate with a strong acid such as hydrochloric acid results in two equivalence points on the titration curve. At the first equivalence point, the carbonate ions in the base are completely converted to bicarbonate ions. The second equivalence point corresponds to the complete conversion of bicarbonate ions to carbonic acid, which dissociates into carbon dioxide and water. The region before the first equivalence point corresponds to the carbonate/bicarbonate buffer, and the area between the first and second equivalence points corresponds to the bicarbonate/carbonic acid buffer. While phenolphthalein is used to detect the first endpoint, a mixture of methyl orange and xylene cyanol is used to sharpen the second endpoint.
