Explain molar absorptivity to a dumb physicist.

In summary: The molar absorptivity of the 2,4,6-trichlorophenol at the irradiating wavelength is 3.6x10-5 cm3/mol.
  • #1
christianjb
529
1
OK chem-wizards...

I'm looking at a paper which states

"...the integrated absorptivity of the stretching vibrations of a water molecule is 4.89 cm micro mol^-1"

(I'm transcribing a 'mu' character as micro in the above. If mu doesn't stand for micro, then let me know!)

OK, so some questions:

1) Is the paper quoting an integrated 'molar absorptivity'?

2) The integration is over the frequency axis, but is this frequency in wave-numbers?

3) Is this defn. for molar absorptivity correct?

Molar Absorptivity,? = A/ c l

( where A= absorbance, c = sample concentration in moles/liter
& l = length of light path through the cuvette in cm.)

taken from http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/UV-Vis/uvspec.htm

4) According to this Wikipedia article, absorbance is calculated as a base 10 logarithm of I/I0. Is that defn. universally used?

http://en.wikipedia.org/wiki/Absorbance

5) I can't understand the units. If A=absorption and alpha = absorption coefficient and L=sample length, then A=alpha L, so that means that alpha is in units of inverse length.

If e=absorptivity, and c is the concentration in mols per liter and e=A/cL=alpha/c, then e is in units of mols^-1 liters ^-1 cm^-1. Thus, I'd expect the integrated absorptivity to be in mols^-1 liters^ -1 cm ^-2, given that the integral is over wavenumbers which have units of cm^-1.

6) Basically what I want is to convert the number in the paper into a value for the absorption coefficient.

Thanks in advance for any help!
 
Last edited by a moderator:
Physics news on Phys.org
  • #2
Oh, I should add, this is from an IR experiment.
 
  • #3
The absorbance, A, is the exponent that is seen in the Beer-Lambert equation.

[tex]I = I_0 ~e^{-\kappa x} = I_0 ~e^{-A} [/tex]

The extinction coefficient, K , is often proportional to the concentration of the solution over a wide range of values, allowing us to write [itex]\kappa=M \alpha [/itex], where M is the molar concentration in Mol/Liter and [itex]\alpha[/itex] is what is usually called the molar absortivity.

Since A is dimensionless, [itex]\alpha[/itex] has units of cm2/mol (ignoring a multiplier of 100 or 1000 depending on actual units used) and the integrated molar absorptivity [itex] \int { \alpha dk} [/itex], where [itex]k=2\pi/\lambda[/itex] has units of cm/mol.
 
Last edited:
  • #4
Thanks! I will take a good look at this later.
 
  • #5
Again, thanks. I finally got back to looking at this tonight and your explanation was enough for me to make the correct conversions in the paper I'm writing.
 
  • #6
Hi,

Can anyone tell me what is the molar absorptivity at the irradiating wavelength for the 2,4,6-trichlorophenol? I use wavelength 293nm to detect the concentration of this chemical in a UV-Vis Spectrophotometer. Thank you.

Cheers
Collin
 

What is molar absorptivity?

Molar absorptivity, also known as molar extinction coefficient, is a measure of how strongly a substance absorbs light at a specific wavelength. It is a physical property that is unique to each substance and is used to quantify the concentration of a substance in a solution.

How is molar absorptivity calculated?

Molar absorptivity is calculated by dividing the absorbance of a solution by the concentration and the path length of the light through the solution. This can be expressed as A/(c*l), where A is the absorbance, c is the concentration, and l is the path length in centimeters. The unit for molar absorptivity is typically M-1 cm-1.

What is the relationship between molar absorptivity and concentration?

Molar absorptivity and concentration have a direct relationship. This means that as the concentration of a substance increases, the molar absorptivity also increases. This is because a higher concentration of a substance will result in more light being absorbed.

How does molar absorptivity differ from absorbance?

Molar absorptivity and absorbance are related but different concepts. Absorbance is a measure of how much light is absorbed by a substance at a specific wavelength, while molar absorptivity is a measure of the substance's ability to absorb light. Absorbance is dependent on the concentration of the solution, while molar absorptivity is a constant for a particular substance at a specific wavelength.

Why is molar absorptivity important in scientific research?

Molar absorptivity is a crucial parameter in many scientific experiments, particularly in the fields of chemistry and biochemistry. It is used to determine the concentration of a substance in a solution, as well as to identify and characterize unknown substances. Molar absorptivity is also used in spectroscopy techniques, such as UV-Vis spectroscopy, to quantify the amount of light absorbed by a sample.

Similar threads

  • Biology and Chemistry Homework Help
Replies
11
Views
2K
  • Chemistry
Replies
5
Views
4K
  • Biology and Chemistry Homework Help
Replies
1
Views
1K
Replies
3
Views
1K
  • Biology and Chemistry Homework Help
Replies
4
Views
4K
Replies
8
Views
1K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
2K
  • Introductory Physics Homework Help
Replies
5
Views
1K
Replies
1
Views
1K
  • Advanced Physics Homework Help
Replies
1
Views
1K
Back
Top