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PKa of Phenols

  1. Nov 8, 2004 #1
    Hi all
    I am doing this lab in my organic course right now, and I have a few questions. The phenol I am working on is pKa of p-chlorophenol.
    I obtained a UV spectrum of phenol in acid, phenol in base, phenol in buffer with higher pH and phenol in buffer with lower pH.

    1)In the UV spectrum, there is a isosbestic or crossover point by the 4 solutions used above, would changing of the pH and concentration alter this point ?

    2)On my UV spectrum the absorbances of phenol and phenolate is different, is it because one of them are conjugate acid and other is conjugate base?

    3)Finally a stupid question, Why is not right to wash the glassware with acetone ? I know it is a very strong and reactive solvent than phenol, but what properties makes it so ?

    I am sorry if these question are stupid, but my prof didn't went over any of these stuff.

    Thanks in advance.
  2. jcsd
  3. Nov 8, 2004 #2


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    Let me try to answer your questions in a brief basis.
    1. I didn't recall what isosbestic point is. If
  4. Nov 8, 2004 #3
    Isosbestic point is a point on the UV spectrum where all speectrum intersects.

    By the way , I think you are trying to post a link ? it didn't show up.

    Thank you.
  5. Nov 8, 2004 #4


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    This message is not supposed to be like this, I inadvertently pushed the submit button, I think.
    1. The isosbestic point should be the signal belonging to the [itex]\displaystyle \pi[/itex] system which does not essentially change. I don't think that a serious shift will be observed.
    2. Are you sure you've prepared your solutions with exactly same concentration? It is directly proportional to the absorbance maxima obtained.
    3. Acetone is UV-active chemical, so if it is let to dry inside the cell, some traces may be present in the second analysis, and will cause a serious intereference. Remember, we are talking about very dilute solutions; concentrations about [itex]\displaystyle 10^{-5}~M[/itex] are required to see a nice spectrum.

    A final note: These questions are not stupid, but your prof must have spent some more time to explain (at least, imply) these. Maybe he just wanted you to think and find them by yourself, I don't want to offend him.
    Last edited: Nov 8, 2004
  6. Nov 8, 2004 #5
    OOps, I´m sorry chem_tr, I was writing my post and I didn´t see that you had already posted. I agree with you.

    But just a note, it theese cases, maximum absorbances of acid and basic forms can be different (at the same total concentrations). That´s just because "epsilon" coefficients of Lambert-Beer law (I´m sorry, I don´t know how to call it in English) of the two forms are different.
    Last edited: Nov 8, 2004
  7. Nov 8, 2004 #6


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    No problem, it is nice to see somebody agrees with me :smile:
  8. Nov 8, 2004 #7
    Thank you chem_tr and altered-gravity.
    Yeah, after you explained it, I realized that he did mention something similar in his notes, I just didn't make the right connections. Now it makes a lot more sense !

  9. Nov 9, 2004 #8


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    Just an addition to Altered-Gravity's post:

    [itex]\displaystyle \epsilon[/itex] is known as molar absorption coefficient.
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