Sodium Carbonate is a diequivalent base and it can be assayed by titration with a standard strong acid to the first equivalence point (pH ~ 8.3) using Phenolphthalein Indicator, or to the second equivalence point (pH ~ 3.7) using Methyl Orange Indicator. Reactions involved using Hydrochloric Acid as titrant are:

Na2CO3 + HCl → NaHCO3 + NaCl (first endpoint)

NaHCO3 + HCl → H2CO3 + NaCl (second endpoint)