[Rory]

hi all,
In class today we were synthesizing aspirin by mixing salicylic acid and acetic anhydride with a sulfuric acid catalyst. One of the things I have to do for the prac writeup is describe how to prove that the sulfuric acid isn't a reactant - I have no idea how to prove this. 
You can't do it by testing pH or titration or anything because both a reactant and a by-product are acidic, so they would interfere. And gravimetric analysis won't work because it's impossible to measure the mass of sulfuric mass both before and after the reaction. i thought maybe doing the reaction without the sulfuric acid present to show that the reaction still occurs (albeit very slowly) without it present. Any ideas or comments?

Cheers, Rory

[movies]

How about looking for the S=O stretch in the IR?

The more direct experimental way would be to use a catalytic amount of H₂SO₄ and see if you get catalytic turnover, although that doesn't give direct evidence for the presence of the catalyst after the reaction.

[HP]

You may precipitate H2SO4 with BaO solution(barite water) as BaSO4. Yse stechiometric Ba2+ to SO42- to avoid possible side reaction with acethyl salicilic acid thow i think its not very prone.After filtration BaSO4 and drying you may gravimetrically prove H2SO4 as catalist.