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Beer-Lambert Law

The Beer-Lambert Law states that the absorbance of light by a medium is directly proportional to its concentration as well as path length (the distance travelled by the light).

The law can be expressed mathematically as:

A = ϵcl = log( I0/I)

Where:

 A = Absorbance (measure of the amount of light absorbed). 

· I0 = Intensity of incident Light

· I = Intensity of transmitted Light

· ϵ =  Molar absorptivity Constant 

· c =  the concentration of the solute

· l =  the path length of the sample (the distance light travels through the solution)

Units:

· A = unitless

· ϵ =  cm2/mol

· c =  mol/L

· l =  cm

Molar absorptivity Constant (ϵ) --- A constant that indicates how strongly a substance absorbs light at a particular wavelength. It is characteristic of a substance and is independent of change in its concentration and path length.

Significance of Molar absorptivity Constant (ϵ)  --- It helps to predict the probability of electronic transitions

Value of ϵ higher than 104 --- Allowed Transitions

Value of ϵ lower than 104 --- Forbidden Transitions

 

· Applications of Beer-Lambert Law: it is used for quantification of concentration of various solutes/analytes in analytical chemistry, biochemistry, and also the environmental science.

Limitations of Beer-Lambert Law: The law holds true only for very dilute solutions and also for homogeneous solutions

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