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I need to monitor the concentration of ethyl acetate in an Fischer esterification reaction. What techniques or spectroscopies can detect the concentration of the compound? And which one do you recommend based on precision and less sample preparation?

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  • $\begingroup$ How much ethyl acetate is present in the mixture ? $\endgroup$ – ankit7540 May 25 '16 at 14:58
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    $\begingroup$ Hold on, why EA? I assume that the esterification you are doing is between the citric acid and ethanol, so shouldn't you be looking to monitor the ethyl citrate product? I can't imagine where EA would come from unless you added it from the beginning. $\endgroup$ – IT Tsoi May 25 '16 at 16:05
  • $\begingroup$ @ITTsoi I'm sorry, you're right. I meant acetic acid. It was a typo. $\endgroup$ – Pooya May 25 '16 at 21:04
  • $\begingroup$ @ankit7540 The concentration of ethyl acetate is considerable (>10% ww) $\endgroup$ – Pooya May 25 '16 at 21:05
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You might consider taking Aliquots of the reaction (or if you have the equipment directly sampling via a cannula) into a GC-FID or quantitative GC-MS. This way you could monitor the concentrations of all species in solution given the right GC method. Areas under each peak will give you relative amounts, you would need to use an appropriate known amount of an internal standard that remains constant to get absolute numbers.

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You don't mention the scale of the reaction, however 1H NMR will be the easiest and most effective method for monitoring the progress of the reaction, and give very accurate results.

If done on a small scale, you could take 50uL aliquots, and dilute in deuterated solvent. If you are doing this on a large scale, you simply take 0.4mL aliquots, and either add a small amount of deuterated solvent for lock, or simply run it unlocked. If running it neat, you can return your sample to the reaction vessel to continue the reaction.

Monitor the reaction via the concentration of the methylene groups for ethanol and ethyl acetate. The ethanol CH2 comes around 3.7ppm, and the ethyl acetate CH2 comes around 4.1ppm. Easy peasy.

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Great answers above using specific techniques.

I would use in-situ Raman spectroscopy.

Sample preparation : No sample preparation needed. The whole reaction vessel can be moved to the sample point of Raman spectrometer (if the Raman spectrometer allows it). The incoming laser beam will not affect the reaction since none of the chemicals mentioned are light sensitive.

Advantage : No need to open the reaction vessel and take aliquot after some time duration. You can continuously monitor the change (~ every 1-5 minutes depending upon spectrometer throughput).

Quantitative information: 2 approaches:

  1. Using an internal standard: Raman peak from some species whose concentration stays same throughout can be used as a relative internal standard. Concentration of other species can be found relative to this standard.

  2. Using abolute Raman cross-section: Absolute Raman cross-section of any species if known can be used and concentration can be found absolutely in the reaction vessel. If concentration of one species in known accurately others can be found relative to it.

*If you need more info send a comment.


Since custom Raman spectrometer is hard to find, it is more practical to use an instrument which is available in your department and is free to use, which is more likely to be an NMR instrument.

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