Question

Why is the mixture of .033 M of H₂SO₃ and .033 M of NaSO₃ not good buffers?

Answer 1

characteristics of a good buffer includes following things.

a) combination of weak acid and its conjugate base salt

b) combination of weak base and its conjugate acid salt

c) dissociation of either base or acid in aqueous medium must be non-facile. dissociation constant must be very low.

d) conjugate base of acid/conjugate acid of base must be hard in nature and strong enough to limit the changes in pH when either base or acid is added to the system.

thus

using a combination of sulfurous acid and its sodium salt is not a good buffer. because, sulfurous acid is readily dissociated in an aqueous medium. its pKa value is 1.81. it is acidic in nature. it loses H+ when comes in the aqueous phase.

moreover conjugate base of strong acid is not hard and strong in nature. thus the conjugate base of a sulfurous acid, i.e, sulphite ion is not a good base. thus if we add acid to the solution , it won't abstract the acid, and thus H+ will remain in aqueous phase and thus it will lead to change in pH.

but a good buffer won't allow any drastic change in pH.