# Re: [Channel-talkchemistry] Absorbance

From: DPT <alpha77@peoplepc.com>
Date: Tue Apr 12 2005 - 14:59:42 EDT
Beer-Lambert Law:          Absorption = ebc =
{e=molar absorbtivity [L/(mol*cm)]}
{b=path lenght of sample [cm], in other words size in length of sample}
{c=concentration of sample {mol/L]}

Please note that Absorption is unitless or can be measured as a precentage!  (Like Tansmission% compared to Absorption%)  Unless I'm misunderstanding the formula you gave in your message of, wavelength in cm, times cell length in cm , time, concentration in mol/Liter leaves units of (cm^2*mol) per Liter.  Unless I misunderstand, I think your formula is incorrect.

So relating the formula you gave to the above equation: c = concentration [in mol/Liter]
b = cell length [cm]
and       e =  molar absorbtivity [Lit/(mol*cm)]

How does "e" related to the wavelength?

First off we know that absorption reduces the intensity of detected light (because the sample "absorbes" some of those little photons).

Another way to find absorption is if we know the intensity of light before it passes though our sample, then

transmisstion =(Intensity of light after)/(Intensity of light before)

and

absorbance = -log base 10 of transmission

Also, I assume you mean you gather the # of photons per some period of time?  But when was this measurement made?  If it was made before the light passed through our sample then it is a measure of the transmission, and using the formula above will give you the absorbtion (converting the measurements into precentages).  If this measurement is after the light passes through the sample, then it measures the absorption directly.  But for this measurement to mean anything, you need to know the transmission before hand, so that you can tell how many photons "decided" not to leave the sample.

I hope my explaination are clear and understandable to you.  Please explain corrections to what I said, or if I am missing something in your statment of the formula.

Dan

-----Original Message-----
From: Jen Wells
Sent: Apr 11, 2005 7:19 PM
To: channel-talkchemistry@learner.org
Subject: [Channel-talkchemistry] Absorbance

I tried to do the Homework assignment for the Reactions to Chemistry Workshop 2, question 4 where I was to go to a site on-line and calculate absorbance to create a graph of absorbance vs. concentration.  I checked his site, but am still stumped on how to calculate absorbance!!  Math was never my strong point.  Could somebody tell me (in plain English) how to calcuate the absorbance.  I know the formula is wavlength x cell length x concentration.  I gathered # photons detected, time,  & intensity of detected light.  Now what do I do with all of this!!
Jen
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Received on Tue Apr 12 15:01:29 2005