Molar Extinction Coefficient
 This topic has 4 replies, 4 voices, and was last updated 11 years, 7 months ago by r6barfly.

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January 30, 2007 at 10:26 am #6865Miss_MeParticipant
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 beerlambert law (A=ECL). However, how would I calculate the extinction coefficient from a graph where concentration is on the xaxis and optical density on the yaxis? 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!!
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January 30, 2007 at 7:55 pm #68383cbourneParticipant
Hi there
If you solve Beer’s law for E you get = A/cl
In your graph you have c on the xaxis, and A is on the yaxis
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 xaxis is in molar, then your units for E are M1
You can then take any given A reading, divide by E, and get MYou can also calculate the extinction coefficient if you can find it check out Protein Science (1995) 4:24112423; Pace et al "How to measure and predict the molar adsorption coefficient of a protein".
Good luck!

January 31, 2007 at 8:22 am #68405sdekivitParticipant
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 xaxis the concentration and on the yaxis 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

January 31, 2007 at 8:25 am #68406sdekivitParticipantquote cbourne:Hi there
If your c on the xaxis is in molar, then your units for E are M1
You can then take any given A reading, divide by E, and get Mnot 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

March 19, 2009 at 5:21 am #89769r6barflyParticipant
So if I have this data:
Absorbance @ 405nm:
.875Concentration (M) of Cr(NO3)3 (aq):
.25MSince 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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