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Extinction Coefficient from Time series data

  1. May 26, 2016 #1
    I have some time series data of the absorbance of Br2 formation using UV Vis spectroscopy and I need to figure out the extinction coefficient/ absorptivity.
    The overall reaction is
    BrO3-+5Br- +6H+-->3Br2+3H2O
    which is expcted to go to completion
    I know that the equation relating absorbance to concentration is
    A=εcl
    and I have times series A measurements and can calculate the initial concentrations of BrO3, Br -and the expected concentration of Br2 from the solutions I made. I just need to find ε.

    I first attempted to plot the absorbance v. time and find the slope where it was most linear but I don't know how valid this approach is.
     
  2. jcsd
  3. May 26, 2016 #2

    Borek

    User Avatar

    Staff: Mentor

    It is about as good as it can be.

    I would record an additional point after waiting for some time to make sure the amount of Br2 produced is just stoichiometric. That would give a good calibration point.

    Besides, if all you are after is a time series (for kinetic measurements), all you are interested in is the rate of changes - are you sure you need absolute values for that?
     
  4. May 26, 2016 #3
    Maybe I'm thinking about this wrong. I need to know the extinction coefficient for Br2 because in another reaction I measure its loss over time. So I dd an initial run for the formation of Br2 in order to determine its extinction coefficient. Then in a second run I added a compound that reacts with it and measured the absorbance again. I am interested in the rate of loss so does that mean the extinction coefficient from the initial run is not an exact value?
     
  5. May 26, 2016 #4

    Borek

    User Avatar

    Staff: Mentor

    If you are using the same cuvette, wavelength and the same instrument you don't need extinction coefficient but a calibration curve. Linear regression on the data is typically a way to go.

    What I don't get about your setup is why you use time series instead of just making a series of samples of different concentrations?
     
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