Boiling H2CO3..

Question

Hi!

I have the following trick question, and I am not sure how to answer it:

In the titration of HCO3- with HCl, why does it give a safer answer when choosing to boil the analyte around the equivalence point?

I guess it is because around equivalence point we get H2CO3, but we boil it so we can get rid of it by maximizing the formation of H2O and CO2, because H2CO3 might protolyse back into HCO3-...?

Answer 1

Yep. The heating/boiling allows you to drive off the CO₂ gas that is being generated during the titration. Not only will it revert to HCO₃^- but also to H₂CO₃ (carbonic acid) even though H₂CO₃ doesn't really exist except in aqueous solution. But that's another point. I think your explanation is a correct one.

===================================================================

Further discussion:

The H₂CO₃ formed would be a weak acid and will be mostly in the unionized form. When you get close to the equivalence point, you not only have the weak acid but also the conjugate base, thus forming a buffer. This might then give you a false endpoint. Driving off the CO₂ will help to prevent this.