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Re: [Channel-talkchemistry] Absorbance

From: Laura Witham <lwnjr@yahoo.com>
Date: Wed Apr 13 2005 - 11:55:36 EDT

With the conversion from %T to Abs., to use the
formula A = -logT, make sure you divide the %T by 100
to get T (just a problem that students sometimes
make).

Laura Rotondo

--- DPT <alpha77@peoplepc.com> wrote:

---------------------------------
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 Wed Apr 13 12:00:10 2005

 
 

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