The Beer-Lambert law or Beer's law is the linear relationship between absorbance and concentration of an absorbing species. The general Beer-Lambert law is usually written as:. When working in concentration units of molarity, the Beer-Lambert law is written as:. The latter is particularly convenient. In words, this relationship can be stated as " e is a measure of the amount of light absorbed per unit concentration".
Molar absorbtivity is a constant for a particular substance, so if the concentration of the solution is halved so is the absorbance, which is exactly what you would expect. Let us take a compound with a very high value of molar absorbtivity, say , L mol -1 cm -1 , which is in a solution in a 1 cm pathlength cuvette and gives an absorbance of 1. Now let us take a compound with a very low value of e , say 20 L mol -1 cm -1 which is in solution in a 1 cm pathlength cuvette and gives an absorbance of 1.
The answer is now obvious - a compound with a high molar absorbtivity is very effective at absorbing light of the appropriate wavelength , and hence low concentrations of a compound with a high molar absorbtivity can be easily detected.
It is either 20 or , L mol -1 cm Answer : I am guessing that you think the higher value is correct, because copper sulphate solutions you have seen are usually a beautiful bright blue colour. However, the actual molar absorbtivity value is 20 L mol -1 cm -1! The bright blue colour is seen because the concentration of the solution is very high. It is found at exceedingly low concentrations. You may not be surprised to learn that the molar absorbtivity of b -carotene is , L mol -1 cm -1!
Guanosine has a maximum absorbance of nm. What is the concentration of guanosine? What is the extinction coefficient? The proportion of the light absorbed will depend on how many molecules it interacts with.
Suppose you have got a strongly colored organic dye. If it is in a reasonably concentrated solution, it will have a very high absorbance because there are lots of molecules to interact with the light.
However, in an incredibly dilute solution, it may be very difficult to see that it is colored at all. The absorbance is going to be very low. Suppose then that you wanted to compare this dye with a different compound. Unless you took care to make allowance for the concentration, you couldn't make any sensible comparisons about which one absorbed the most light. The absorption coefficient of a glycogen-iodine complex is 0.
Suppose this time that you had a very dilute solution of the dye in a cube-shaped container so that the light traveled 1 cm through it. The absorbance is not likely to be very high. On the other hand, suppose you passed the light through a tube cm long containing the same solution. More light would be absorbed because it interacts with more molecules. Again, if you want to draw sensible comparisons between solutions, you have to allow for the length of the solution the light is passing through.
Both concentration and solution length are allowed for in the Beer-Lambert Law. Remember that the absorbance of a solution will vary as the concentration or the size of the container varies. Molar absorptivity compensates for this by dividing by both the concentration and the length of the solution that the light passes through.
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