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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.

ε ≤ 10
10 ≤ ε ≤ 1000
1000 ≤ ε ≤ 100000
ε ≥ 100000
forbidden transition
partly allowed transition
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.

Aλ = ελ.l.c
with ελ the molar absorption coefficient (
l the path length
c the concentration (mol.L-1)

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…).

Absorbance spectrum. Two vertical arrows are located on either side of the absorbance peak at 500 nm. Above is the hyperchromic effect, below is the hypochromic effect.

The hyperchromic effect

The hyperchromic effect corresponds to an increase in absorbance.

The hypochromic effect

The hypochromic effect corresponds to a decrease in absorbance.

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