Molar Extinction Coefficient

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    • #6865
      Miss_Me
      Participant

      Hi.

      Please can someone help me here, as I am confused and I don’t want to end up more confused! Thanks in advance!

      The extinction coefficient is present in the beer-lambert law (A=ECL). However, how would I calculate the extinction coefficient from a graph where concentration is on the x-axis and optical density on the y-axis? Would I have to work out a gradient? And so, where do I go next to work out the extinction coefficient??

      I’ve tried reading up on the extiction coefficient but I don’t seem to find anything which would help me work the extiction coefficient from a graph.

      Thankss!!
      🙂

    • #68383
      cbourne
      Participant

      Hi there-

      If you solve Beer’s law for E you get = A/cl
      In your graph you have c on the x-axis, and A is on the y-axis
      So the slope of your line (rise over run) is A / c
      You’ll have to compensate if you’ve used a path length besides 1cm, otherwise you can ignore l.
      If your c on the x-axis is in molar, then your units for E are M-1
      You can then take any given A reading, divide by E, and get M

      You can also calculate the extinction coefficient- if you can find it check out Protein Science (1995) 4:2411-2423; Pace et al "How to measure and predict the molar adsorption coefficient of a protein".

      Good luck!

    • #68405
      sdekivit
      Participant

      omg:

      A = ecl with e = molar extinction coefficient, c = concentration and l = length of the light through the sample

      to calculate e you chose on the x-axis the concentration and on the y-axis the absorbance/extinction or whatever you call it.

      –> than the slope of that grapgh will be equal to delta y/delta x = A/c = e*l

      –> thus your slope in the curve is equal to 2 constants: the molar extinction coefficient and the path of the light through the sample –> e * l

    • #68406
      sdekivit
      Participant
      quote cbourne:

      Hi there-

      If your c on the x-axis is in molar, then your units for E are M-1
      You can then take any given A reading, divide by E, and get M

      not true: absorbance/extinction has no unit (because it is also equal to I/I(0) –> intensity transmitted/blanc transmittance), thus when c in molarity and l in cm, than the molar extinction coefficient will have the unist M^-1 * cm^-1

    • #89769
      r6barfly
      Participant

      So if I have this data:

      Absorbance @ 405nm:
      .875

      Concentration (M) of Cr(NO3)3 (aq):
      .25M

      Since you’re saying that if l = 1cm (standard curvette), then E = A/C
      so then:
      .875/.25 = E
      E = 3.5?

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