[dun13203171]

Ive been looking at a method for analysing a mixture of aspirin and caffeine dissolved in methanol which has had had sodium hydroxide added to it.

One of the questions in the books asks why cant aspirin by analysed using this method but there is no answer in the book.

Now I'm not sure if the answer is as simple as, "paracetamol absorbs at a different wavelength to that specified for caffeine and aspirin."

Or is it because in basic conditions, paracetamol undergoes hydrolysis, or it undergoes oxidation. My immediate thought was it forms a salt, but I would guess that makes it even more souble and don't see why this couldn't be analysed via UV.

what do you guys think

thanks

[Arkcon]

Oh, yes, definitely.










Those are two good explanations.  One or both could be correct.  Or there may be another reaction interfering.  You'll need to use the resources you have, to prove which one, to get credit for this answer.

[horse]

I'm a bit confused by the question, where does paracetamol come into the question? Are you saying that paracetamol can be analysed with this method, but aspirin can't? Also, is the book saying that somehow the conditions (dissolved in methanol with NaOH and the presence of caffeine) makes it impossible to analyse aspirin via UV-Vis, whereas with paracetamol you can? And what exactly are you looking to analyse with the UV-Vis spectrum?

But anyhows, the first thing that would pop into my head too is that aspirin is more acidic than paracetamol, and NaOH isn't as basic in methanol as water, so its possible paracetamol isn't acidic enough to be turned into a salt in those conditions whereas aspirin is. I still don't know whats being analysed with the UV-Vis spectrum though. Have you checked out if methanol is a polar enough solvent to enable NaOH hydrolysis of paracetamol? That could narrow it down.

[dun13203171]

Basically the question is why can't paracetamol be analysed using this method.

I.e the method being using UVvis to determine the conc of aspirin and caffeine dissolved in Methanol + NaOH...so if paracetamol was also added to this mixture, why could it not be analysed by UV

Thanks for your reply

[Arkcon]

OK.  What UV wavelengths are given as the lambda max for paracetamol?

Draw paracetamol's structure:  Does it have functional groups that behave the same way as aspirin?  Does it have functional groups or a molecular structure that breaks it apart in NaOH?

If you don't feel like doing that work, aspirin, paracetamol, and others are common pharmaceuticals.  The pharmacopeia, is a government listing of analytical methods.  If you can find it, you can use it to see how its analyzed.  Warning:  these government databases offer no explanations, they just give the method to be followed.  But if its possible, its there.

[Ibrido]

Hey,

I'm getting mad over this problem  . I have an idea, but I think you're also (inadvertedly) not explicitating something.

May I ask you to report the test exactly as it is in the book? Thanks!