Chemistry Reference
In-Depth Information
Effect of pH on spectra
If a graph of the extent of light absorption (measured as the quantity
termed 'absorbance', defined later on p. 174) is plotted against the wave-
length, then the complete absorption spectrum of a molecule can be
obtained (Figure 7.5). The wavelength at which the absorbance (
A
) is
highest is called the k
max
(read as 'lambda max') and is a characteristic of a
particular chromophore. The k
max
of a compound is sometimes used in the
British Pharmacopoeia
for identification of
drugs and unknown
compounds.
A
k
k
max
Figure 7.5
A plot of light absorbance vs
k
.
The wavelength at which the k
max
occurs should be a constant for a
given compound but, like many 'constants' in science, k
max
can move
around and change. This is not entirely bad news, as a large amount of
useful information can be obtained about a compound simply by observing
any shifts that occur in k
max
when, for example, the compound is ionised.
A shift in k
max
towards longer wavelength is referred to as a
bathochromic
or red shift, because red is the colour at the long-wavelength
end of the visible spectrum. A bathochromic shift usually occurs due to the
action of an
auxochrome
. This is a functional group attached to the chro-
mophore which does not absorb light energy itself but which influences the
wavelengths of light absorbed by the chromophore.
Examples of auxochromes include the
SH
groups. These functional groups possess lone pairs of non-bonded electrons
that can interact with the p electrons of the chromophore and allow light of
longer wavelength to be absorbed. A good example of this effect is to
compare the k
max
values of benzene and aniline (also called phenylamine or
aminobenzene), shown in Figure 7.6.
a
NH
2
,
a
OH and
a
