Chemistry Reference
In-Depth Information
Loss of intensity due to point (1) can be compensated by use of an appro-
priate blank solution since phase boundary effects should be the same in the
test and blank solutions.
The scattering effects in point (2) can be minimised by careful sample
preparation, i.e. ensuring the sample dissolves completely in the chosen
solvent, that there are no air bubbles adhering to the sample cell, and that
there are no fingerprints, dust, mascara, dandruff or other unwanted mate-
rial on the outside of
the cell which will affect the accuracy of
the
absorbance measurements.
Losses in intensity due to point (3) are what we are interested in
measuring.
Beer's and Lambert's laws
The quantitative aspects of spectrophotometry are based on two very similar
laws. The first is Beer's law (Figure 7.16), which states that 'the intensity of a
beam of parallel, monochromatic light decreases exponentially with the
concentration of the absorbing molecules'. Beer's law (named after German
chemist August Beer) can be expressed mathematically as
I
I
0
e
k
c
(7.1)
where
I
0
is intensity of light incident on the sample,
I
is intensity of light
transmitted by the sample,
k
is a constant and
c
is the concentration of the
sample.
l
0
l
Figure 7.16
A diagram of Beer's law.
Taking logarithms,
I
0
log
—
k
c
I
