Engineering Chemistry S Datta

INSTRUMENTAL METHODS OF ANALYSIS 519
Quantitative application of UV spectroscopy
Absorption of light in both the ultraviolet and visible regions of the electromagnetic
spectrum takes place when the energy of light matches that required to induce in the molecule
and electronic transition and its associated vibrational and rotational transitions subsequently.
Beer-Lambert’s law
When a beam of light is passed through a transparent cell containing a solution of an
absorbing substance, then the intensity of the incident light may be reduced.
1
I t
2
I o
3
4
5
Fig. 24.4 Reduction of the intensity of light by reflection at cell faces (rays 1-2),
absorption (ray 4) and scattering by particles (ray 5).
I absorbed
= I o
– I t
I o
⇒ intensity of the incident light.
I t
⇒ intensity of the transmitted light.
Generally UV experiments are done in quartz cells as glass absorbs UV.
The transmittance (T) is:
T = I t
and % T is given by % T = 100 I t
Io
Io
In 1760 Lambert put forward the relation between I t
and I o
for various thicknesses (b) of
the substance. This relationship is given in shape of a law, which is known as Lambert’s law.
The law is stated as follows:
The rate of decrease in intensity of light with thickness is proportional to the
intensity of the incident light.
When the law is expressed mathematically
– d I ∝ I
db
or – d I = k1 I
db t
[k 1
⇒ proportionality constant]
or – d I
= k
I 1
db
t
Integrating both sides:
– I t
= k 1
b + c
when b = 0, c = – ln I 0