Environmental Engineering Reference
In-Depth Information
The structural interpretation of more complex molecules may be frustrating to
beginners, but keep in mind that it takes time and practice for anyone to get to a
comfortable level. Fortunately, the structural elucidation is only the routine work for
a skilled organic chemist. For most environmental work, qualitative analysis for
structural analysis is of less importance. Most environmental applications are done
at the quantitative level for concentration measurement, which does not require the
skilled techniques in spectrum interpretation. The quantitative principles are dis-
cussed in the following section.
8.1.4 Quantitative Analysis with Beer-Lambert's Law
In spectroscopy, the Beer-Lambert's law, also known as Beer's law is an empirical
relationship in relating the absorption of light to the amount of chemical when light
is traveling through the sample. The Beer
'
s law
states that
absorbance
(
A
)is
proportional to the concentration (
C
) of the light-absorbing chemicals in the
sample according to the following:
A ¼ e
lC
ð8
:
4Þ
where
e¼molar absorptivity (L/mol/cm), l ¼
length of light path in a sample cell,
and
C¼
concentration of the chemical (mol/L).
Molar absorptivity
is an intrinsic
property of the chemical species, which is very large for strongly absorbing
compounds (
e ¼ 10100
).
The Beer's law (Eq. 8.4) is also additive if there are several absorbing species
(1, 2,
e >
10
;
000
) and very small if absorption is weak (
...
,
n
) in the solution, thus we have
A ¼ e
1
lC
1
þ e
2
lC
2
þ e
3
lC
3
þþe
n
lC
n
ð8
:
5Þ
Occasionally, transmittance
(
T
) is used, which is defined as the fraction of the
original light that has passed through the sample:
Tð%Þ¼P
6Þ
where P
0
and
P
are the power of incident and transmitted light, respectively
(Fig. 8.13). Therefore, absorbance (
A
) and transmittance are related by
A ¼log
10
T ¼ logP
0
=
=
P
0
100%
ð8
:
P
ð8
:
7Þ
(a)
(b)
l
Sample cell
A
P
0
P
A =
ε
lC
Absorbing
solution of
concentration
C
C
Figure 8.13
UV-VIS light absorption and the Beer's law: (a) Light absorption in a sample cell.
(b) The Beer's law as shown in a calibration curve
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