Chapter 26
Molecular Absorption Spectrometry
• Molecular spectroscopic methods are among the most
widely used of all instrumental analytical methods.
• Molecular spectroscopy is used for the identification and
determination of a huge number of inorganic, organic
and biochemical species.
• Molecular ultraviolet/visible absorption spectroscopy is
employed primarily for quantitative analysis.
• Infrared absorption spectroscopy is one of the most
powerful tools for determining the structure of both
inorganic and organic compounds.
ULTRAVIOLET/VISIBLE MOLECULAR
ABSORTION SPECTROSCOPY
In the UV/visible region, many types of
inorganic compounds absorb radiation directly.
Others can be converted to absorbing species by
means of a chemical reaction.
Absorption measurements in the UV/visible
region of the spectrum provide qualitative and
quantitative information about organic,
inorganic, and biochemical molecules.
Absorption by Organic Compounds: Two types of
electrons are responsible for the absorption of ultraviolet
and visible radiation by organic molecules: (1) shared
electrons that participate directly in bond formation and
(2) unshared outer electrons that are largely localized on
atoms such as oxygen, the halogens, sulfur, and nitrogen.
The shared electrons in single bonds are so firmly held
that absorption occurs only with photons more energetic
than normal UV photons. Electrons involved in double
and triple bonds of organic molecules are more loosely
held and are therefore more easily excited than electrons
in single bonds. Thus, species with unsaturated bonds
generally absorb in the UV. Unsaturated organic functional
groups that absorb in the UV/visible region are known as
chromophores.
Absorption by Inorganic species: In general, the
ions and complexes of elements in the first two
transition series absorb broad bands of visible
radiation in at least one of their oxidation states
and are, as a consequence, colored. Absorption
involves transitions between filled and unfilled dorbitals of the metal ion with energies that depend
on the bonded ligands. The energy differences
between these d-orbitals and thus the position of
the corresponding absorption maximum depend
on the position of the element in the periodic
table, its oxidation state, and the nature of the
ligand bonded to it.
Charge-Transfer Absorption: For quantitative
purposes,
charge-transfer
absorption
is
particularly
important
because
molar
absorptivities are unusually large, a circumstance
that leads to high sensitivity. Many inorganic and
organic complexes exhibit this type of absorption
and are therefore called charge-transfer
complexes.
A charge-transfer complex consists of an
electron-donor group bonded to an electron
acceptor. When this product absorbs radiation,
and electron from the donor is transferred to an
orbital that is largely associated with the acceptor.
The excited state is thus the product of a kind of
internal oxidation/reduction process.
Qualitative Analysis
Qualitative
applications
of
UV/visible
spectroscopy are limited because the spectra of
most compounds in solution consist of one or, at
most, a few broad bands with no fine structure
that would be desirable for unambiguous
identification. The spectral position of an
absorption band is, however, an indication of the
presence or absence of certain structural features
or functional groups in a molecule. Usually,
UV/visible absorption spectroscopy is only used
for confirmation in conjunction with a more
useful qualitative technique, such as NMR, IR,
and mass spectrometry.
Fundamental Studies
Spectrophotometry in the UV/visible region is one of
the major tools for studying chemical equilibria and
kinetics. Wavelengths are chosen to allow monitoring of
one or more reactants, products, or intermediate species.
The concentrations are then obtained by using Beer’s
law with known or previously determined molar
absorptivities. A wide variety of reaction types have
been studied in this way.
From Beer’s law, the final concentrations of reactants
and products are obtained and equilibrium constants
determined from known stoichiometric relationships.
In kinetic studies, spectrophotometry is used to monitor
the appearance of a product or intermediate, or the
disappearance of a reactant.
Quantitative Analysis
Ultravilet/visible spectrophotometry is one of the
most powerful and widely used tools for
quantitative analysis. Important characteristics of
UV/visible spectrophotometry include wide
applicability to organic, inorganic, and
biochemical systems; good sensitivity; detection
limits of 10-4 to 10-7 M; moderate to high
selectivity; reasonable accuracy and precision
(relative errors in the 1 to 3% range and with
special techniques, as low as a few tenths of a
percent); and speed and convenience. In addition,
spectrophotometric
methods
are
readily
automated.
Standards and the Calibration Curve: In most
spectrophotometric methods, calibration is
achieved by the method of external standards.
Here, a series of standard solutions of the analyte
is used to construct a calibration curve of
absorbance versus concentration or to produce a
linear regression equation. The slope of the
calibration curve or regression equation is the
product of absorptivity and pathlength. Thus,
using external standards is way of determining
the proportionality factor between absorbance
and concentration under the same conditions and
with the same instrument as is used for the
samples.
The Standard Addition Method: The
difficulties associated with production of
standards with an overall composition closely
resembling that of the sample can be formidable.
Under such circumstances, the method of
standard additions may prove useful. In the
single-point standard addition method, a known
amount of analyte is introduced into a second
aliquot of the sample and the difference in
absorbance is used to calculate the analyte
concentration of the sample. Alternatively,
multiple additions can be made to several
aliquots of the sample and multiple standard
addition calibration curve obtained.
Analysis of Mixtures: The total absorbance of a
solution at any given wavelength is equal to the
sum of the absorbances of the individual
components in the solution. This relationship
makes it possible in principle to determine the
concentration of the individual components in a
mixture even if there is strong overlap in their
spectra. There is no wavelength at which the
absorbance is due to just one of these
components. To analyze the mixture, molar
absorptivities are first determined at wavelengths
1 and 2.
…continued…
The wavelengths selected are ones at which the
two spectra differ significantly. Thus, at 1, the
molar absorptivity of component M is much
larger than that for component N. The reverse is
true for 2. To complete the analysis, the
absorbance of the mixture is determined at the
same two wavelengths. From the known molar
absorptivities and pathlength, the following
equations hold:
A1 = M1bcM + N1bcN
A2 = M2bcM + N2bcN
Spectrophotometric Titrations
Ultraviolet/visible
spectrophotometric
and
photometric measurements are useful for locating
the end points of titrations. The method requires
that one or more of the reactants or products
absorb radiation or that an absorbing indicator be
present. In spectrophotometric titrations, the
spectrophotometer serves as the detector that
monitors the transmittance or absorbance or the
solution at a suitable wavelength during the
addition of increments or the titrant. Acid/base
titration can be monitored spectrophoto-metrically
by adding a small amount of an indicator that is
colored in either the acidic or basic form.
Titration Curves: The plot of absorbance versus
titrant volume is called a spectrophotometric titration
curve; the shapes depend on the species that absorbs
radiation. Normally, the absorbances are corrected for
dilution by the titrant by multiplying the measured
values by (VT + VA)/VA, where VA and VT are the
volumes of the analyte solution and titrant,
respectively. Ideally, the end point is located by a sharp
change in absorbance; often, conditions are arranged so
that two straight-line regions of differing slopes
intersect at the end point. If the reaction is not
quantitative near the equivalence point, the linear
segments before and after the end point can be
extrapolated to locate the end pint; adherence to Beer’s
law is a necessity.
Applications of Spectrophotometric Titrations:
Spectrophotometric or photometric titrations have
been applied to many types of reactions. Most
standard oxidizing agents have characteristic
absorption
spectra
and
thus
produce
photometrically detectable end points. Although
standard acids or bases do not absorb, the
introduction of acid/base indicators permits the
use of spectophotometric end points in
neutralization titration. The photometric end point
has also been used to great advantage in titration
with EDTA and other complexing agents.
…continued…
At 745 nm, the cations, the reagent, and the
bismuth complex formed in the first part of the
titration do not absorb, but the copper complex
does. Thus, the solution exhibits no absorbance
until essentially all the Bi(III) has been titrated.
With the first formation of the Cu(II) complex,
an increase in absorbance occurs. The increase
continues until the copper end point is reached.
Additional reagent causes no further absorbance
change. Clearly, two well-defined end points
result.
INFRARED ABSORPTION SPECTROSCOPY
Infrared absorption spectroscopy is also widely
employed
in
analytical
chemistry
for
identification. Its scope is nearly as broad as that
of UV/visible methods. In the IR region,
absorption of radiation can give information about
the identity of compounds, the presence or
absence of functional groups, and the structure of
molecules. IR absorption is one of the premier
techniques for qualitative analysis and functional
group identification.
Molecules That Absorb Infrared Radiation
With the exception of homonuclear diatomic
molecules, such as O2, Cl2, and N2, all
molecules, organic and inorganic, absorb
infrared radiation. Absorption of IR radiation
involves transitions among the vibrational
energy levels of the lowest excited electronic
energy levels of molecules. The number of ways
a molecule can vibrate is related to the number
of bonds it contains and thus the number of
atoms making up the molecule. The number of
vibrations is large even for a simple molecule.