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Benzoic acid Molar Extinction Coefficient

  1. Oct 30, 2010 #1
    Hi guys! First time post here.

    I was wondering where on the internet you could find such a value? I have tried searching for sites that may have a list of molar extinction coefficient values for various compounds but have found them to be quite hard to access. Would any of you with perhaps considerably more experience in this direct me to the right place? Many thanks. :smile:
  2. jcsd
  3. Oct 30, 2010 #2
    What solvent did you use? I found a table in the following paper that had log ε values for benzoic acid in different solvents:

    Herbert E. Ungnade, Robert W. Lamb
    J. Am. Chem. Soc., 1952, 74 (15), pp 3789–3794

    log ε for Benzoic Acid in:

    Cyclohexane: 2.96
    Chloroform: 2.95-2.96
    0.01 N HCl: 2.96
    Water: 2.96
    Methanol: 2.81
    Ethanol: 2.84
    Isopropanol: 2.84
    t-butanol: 2.88
    Dioxane: 2.92

    I don't know about particular websites that have this type of information, though. I'm sure there's a way to calculate it; I just don't know how.
  4. Oct 31, 2010 #3


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    Staff: Mentor

    These things are much easier determined experimentally than calculated.

    DDTea - what wavelength is this information for?
  5. Oct 31, 2010 #4
    0.1 M HCl was added to the decresing volumes of Benzoic acid each time before being made up to 50cm3 with distilled water. Then, each of these sultions of differing volumes of benzoic acid were analysed in a spectrophotometer for absorbance values.
  6. Oct 31, 2010 #5
    Whoops Borek, that was a pretty silly omission on my part wasn't it? I'd assume it was for the lambda-max value, corresponding to the pi-->pi* transition, but I don't know which wavelength they used...

    Unfortunately, from my home computer, I cannot access that article! So this is from table 11-9 of Organic Structural Determination by Lambert et. al. *

    Benzoic acid in:

    Water: λmax = 230nm, εmax = 10 000
    95% Ethanol: λmax = 226nm, εmax = 9 800

    In less polar solvents, I would expect λmaxand εmax to decrease slightly due to solvatochromic effects.

    In any case, for this particular experiment, you can calculate εmax using the Beer-Lambert law, where:

    A = εmaxcL

    where A = absorbance value, c = concentration (molarity), L = pathlength of the light beam (the width of your cuvette containing your sample).

    The idea when using the Beer-Lambert law is to use concentrations that are within the "linear range" for the molecule and the spectrophotometer: i.e., to only analyze samples where a linear relationship can be made between absorbance and concentration. You're going to have to make a plot of A vs. c. The slope will be your experimentally-determined εmax for the wavelength you used.

    I thought you were wondering how εmax could be calculated from quantum mechanics :P

    *Joseph B. Lambert et. al. Organic Structural Determination. Prentice-Hall, Inc.: 1998. ISBN: 0-13-258690-8
    Last edited: Oct 31, 2010
  7. Oct 31, 2010 #6
    Sorry, that misunderstanding may have been not providing enough detail on my part. Although I have already worked out the εmax to be at 0.0011 from my gradient of y=mx+c. As I am working out the end concentration of an unknown product, this has led to calculating my concentration to 632.72 from the A=εmaxcL equation. I assume I divide this concentration by 1000 twice to get a value that is closer to my other end concentration values that I have worked out, which are values that are x10-4 M?
  8. Oct 31, 2010 #7
    Sorry, ignore my previous question. I have realised that this whole confusion was mostly caused by one very stupid mistake that I should've seen. :redface:
    But as for the values, thank you. They have been very useful.
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