Titration of Sodium Carbonate—A Diprotic Base

Titration curves for 100 mL 0.1 M weak bases of different Ki, values versus 0.1 M HCl. 

Sodium carbonate is a Br0nsted base that is used as a primary standard for the standardization of strong acids. It hydrolyzes in two steps  

Sodium carbonate can be titrated to give end points corresponding to the stepwise additions of protons to form HCO3- and CO2. (Carbonic acid, H2CO3, dissociates in acid solution to CO2—its acid anhydride—and H2O in acid solution.) The Kb values should differ by at least 10 to obtain good separation of the equivalence point breaks in a case such as this. 

The dashed line is for HCOs only, for which [H ] = /K iK 2 for the horizontal portion. 

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