[Bidagdha_TADIR]

Few days ago I performed an experiment (in the lab as part of my syllabus) to estimate the amount of aspirin in a commercial tablet using UV-Visible spectrophotometer.
According to the method, I first hydrolyzed sample containing aspirin into salicylic acid using NaOH solution. I also applied same procedure on aspirin standard solutions. Then I measured absorbances of the sample and the standard solutions.
Question is, why even use NaOH? Couldn't I just make a sample solution of aspirin in ethanol and some standard solutions of aspirin in ethanol and then take absorbances? It's not like using NaOH produced any color, so is there any chemical basis to using NaOH?

[Corribus]

You answered your own question: NaOH is necessary to hydrolyze acetylsalicylic acid to salicylic acid. The bigger question is why it's better to measure the UV absorption of salicylic acid than acetylsalicylic acid. What do you think?

You can find a clue near in the first figure on this page: http://www.odinity.com/synthesis-characterization-aspirin/

[Bidagdha_TADIR]

I didn't find anything near figure 1 that stuck to my eyes. But figure 7 caught my attention. It seems that salicylic acid gives a much sharper peak than aspirin (of course this opens up a lot more questions like why would aspirin have less sharp, less intense and less wavelength peak even though it has an additional double bond).

Have I spotted the answer, or am I lost?

[Corribus]

Yes, I was alluding to Figure 7, and you can see that Salicylic acid's peak is much more intense and also not as deep into the UV. So, it's a better peak to use for analysis.

[Furanone]

Is it because there is better stabilized resonance of the conjugate dienes with salicylic acid (where the hydroxyl group can deprotonate to =O) than with acetylsalicylic acid where the added acetyl group will interfere with the resonance at the central Oxygen at ester linkage?

[Bidagdha_TADIR]

Last year I synthesized aspirin in the laboratory but when I took UV spectrum there was basically no difference between the spectral data of aspirin and salicylic acid (the raw material), so everybody concluded that I didn't make aspirin.

I am not sure why salicylic acid gives better peak, I just remembered that according to Woodward-Fieser rule hydroxyl group is an auxochrome but ester group is not (at least that's what I remember). That could explain why aspirin spectrum is not so intense.

anyway, I got an explanation for why I am hydrolyzing aspirin into salicylic acid and that's good enough for now.