Absorption bands intensity
Absorption bands intensity is related to the electronic transition probability, with a molar absorption coefficient ε λmax which is valid for one wavelength.
10 ≤ ε ≤ 1000
1000 ≤ ε ≤ 100000
ε ≥ 100000
partly allowed transition
highly allowed transition
Absorption bands intensity is measured with the absorbance Aλ = log I0/I
For a defined wavelength, the Beer-Lambert law links the absorbance to the concentration of species in solution.
with ελ the molar absorption coefficient (M-1.cm-1)
l the path length
c the concentration (mol.L-1)
Validity of the Beer-Lambert law
The Beer-Lambert law is valid only if the exciting light is monochromatic, with a homogeneous system, isotropic and transparent. Besides, absorbing species must be non-photosensitive and in relatively low concentration, especially to ensure a sufficient quantity of light being detected by the spectrophotometer. Other factors, such as aggregates formation or species change upon dilution, may be responsible for a loss in linearity. Such a loss induces a saturation of the A = f(c) curve: absorbance is no longer proportional to concentration.
For teaching laboratory spectrophotometers, the maximal value of absorbance is commonly around the unit. For more efficient spectrophotometers, the limit value is much higher.
The absorbance of a compound is proportional to the number of chromophores it possesses.
Additivity of the Beer-Lambert law
For a fixed wavelength, the absorbance of a mixture is the sum of the absorbances of each species.
As absorption bands position, their intensity can be modified by different factors (pH, solvent…).
The hyperchromic effect
The hyperchromic effect corresponds to an increase in absorbance.
The hypochromic effect
The hypochromic effect corresponds to a decrease in absorbance.