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Homework Help: Beer-Lambert equation

  1. Jan 19, 2009 #1
    1. The problem statement, all variables and given/known data
    I need to use the Lambert-Beer equation to plot optical density of a lactate solution.


    2. Relevant equations
    [tex]A=\varepsilon\ell C[/tex]
    (The path length is 1 cm)

    3. The attempt at a solution
    This is one of my four samples (if you can show me where I'm going wrong with this then I can apply the correction to the others):
    2.22 [tex]\mu[/tex]mol in 1.5ml, therefore there is 0.296 [tex]\mu[/tex]mol in 0.2ml
    This info seems to match to the example graph I have.
    But using the formula above, where [tex]\varepsilon[/tex]=6.22 for micromol/ml, I get:
    6.22 x (0.296/0.2) micromol/ml = 9.2056
    This doesn't match up with the graph I have! Any help appreciated! Please!:redface:
     
    Last edited: Jan 19, 2009
  2. jcsd
  3. Jan 19, 2009 #2

    Ygggdrasil

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    Beer's law is accurate only for values of A from about 0.1 to 1.0. Experimentally, it is difficult to accurately measure absorption values outside this range.
     
  4. Jan 19, 2009 #3
    That's the main problem, the results from the example graph are at a maximum of 0.4 for the absorption at 340nm (y-axis) when plotted against amount in micromoles up to 0.4 on the x-axis.
    I can get a fairly close approximation (the graph itself is supposed to be an estimate) if I use the following formula:
    (amount in micromoles x amount in ml) x 6.22. But this equation makes no sense; or at least I can't see the reasoning behind it.
     
  5. Jan 19, 2009 #4

    Ygggdrasil

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    The slope of the best fit line to the graph should give you ε.
     
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