What is the theory behind the titration of HCl against Na2CO3?

Answer 1

The titration is carried out this way because you can't get a solution of carbonic acid with which to react with a base.

So the titration curve goes downhill, with two steep vertical portions. These represent the formation of (HCO3)- and (H2O + CO2) respectively.

Phenolphthalein will detect the first endpoint, and methyl orange the second, at twice the added volume of HCl.

Answer 2

Theory Behind Titration