Transcript L27

Applications of UV-Vis
Spectroscopy
Lecture 27
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Absorption Involving d and f Orbitals
Many transition metals have colored solutions and are
also colored in the solid state. The transition metals
have some of their d orbitals empty where a d-d
transition can occur. The d-d transitions require
excitation energy in the UV-Vis region. The direct
interaction of the d electrons with ligands around the
transition metal results in a spectrum of broad band
nature. On the other hand, inner transition elements
show transitions by absorption of UV-Vis radiation (ff transitions). Since the electrons in the f orbitals are
far inside the metal orbitals and are screened by
electrons in orbitals of higher principal quantum
numbers, f-f electronic transitions will not be
affected by the nature of ligands or solvent around
the inner transition metals. Therefore, the spectra of
inner transition metals have narrow bands.
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The crystal field theory is usually used to explain
splitting of the d orbital energy so that a transition
from a lower energy d orbital electron can be excited
to a higher energy d orbital. The theory will be
described for a transition metal with six ligands or
molecules of water around it. An octahedral (only
this case will be discussed) arrangement of these
ligands is most appropriate where ligands will be
located at the z axis and at the x and y axis (will repel
electronic cloud in the dz 2and dx-2y 2orbitals and thus
will make these two orbitals to have higher
energies). The other four ligands will be arranged in
between axis( dxy ,dxz ,and dyz )which will increase
their energies but to a lower extent. The net result is
the splitting of the degenerate d orbital into two
groups of d orbitals of different energies (the energy
difference is referred to as D).
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Transition metal ions
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Charge Transfer Absorption
When a ligand permanently donates an electron to a
metal ion, a charge transfer is said to take place. The
net outcome of the process is an oxidation reduction
phenomenon occurring within the complex. An
example is the reaction of Fe3+ with thiocyanate
where the product is an excited species with neutral
thiocyanate and Fe3+. In less common situations, the
transfer of electrons can take place from the electron
deficient metal ion to the ligand. An example is the
Fe2+ or Cu+ complexes with 1,10-phenanthroline
where Fe2+ and Cu+ metal ions donate electrons to
1,10-phenanthroline. The complex will then have Fe3+
and Cu2+ ions. Charge transfer complexes are of
special interest their molar absorptivities are
unusually high; allowing very sensitive
determinations.
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Applications
of Absorbance
Qualitative Analysis
Measurement
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As seen earlier, the broad band absorption spectra
obtained in UV-Vis absorption spectroscopy is
usually featureless and lacks details that can be
used in qualitative analysis. Therefore, this
technique is mainly a quantitative technique.
Plotting Spectral Data
A plot of either the absorbance or %transmittance
against wavelength can be made. However, the most
common practice is to plot the absorbance versus
wavelength.
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Solvents
We have seen earlier that solvent polarity affects the
absorptivity of the analyte molecules; due to change
in transition energies. Usually, polar solvents are
used when possible. However, polar solvents like
water or alcohol tend to remove the fine spectral
details. Therefore, in cases where the fine spectral
details are really needed (as in qualitative analysis) a
non polar solvent like hexane should be used. In
addition, the solvent must be optically clear (does
not absorb incident radiation), well dissolve the
sample, and chemically pure.
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Quantitative Analysis
The basis for quantitative analysis in the UV-Vis relies on
Beer’s law. Several characteristics of quantitative
measurements using UV-Vis absorption spectroscopy can
be rationalized:
1. Applicability to all types of analytes as far as they absorb
in the UV-Vis region.
2. Moderate sensitivities in the range from 10-4 to 10-6 with
possibility to extend this range under certain conditions.
3. The relative standard deviation occurs within 1-3% which
reflects good precision.
4. Easy to perform and convenient.
5. Can be used for quantitative analysis in liquid
chromatographic separations.
6. Non absorbing species can also be determined if they are
derivatized with an absorbing species as the case of
metal ions when complexed to ligands.
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Procedural Details
1. Selection of Wavelength
The first step in a successful determination is to find the suitable
wavelength for the analysis. This is accomplished by plotting
the absorbance/wavelength curve. However, the following
points should also be considered:
1. If more than an absorption maximum is available, the
wavelength far from the instrument extremes should be
preferred
2. A wavelength at the maximum of a broad peak should be
preferred to another of a sharp peak
3. The peak with a maximum peak height is preferred
4. If interferences are present, the wavelength that is far away
from interferences should be selected
5. Working in the visible region should be preferred
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2. Cleaning and Handling the Cell
First, one should appreciate the use of good quality
matched cells that are free from wearing, etching,
and scratches. In addition, cleaning procedures of
external and internal cell surfaces are also
important. A suggested cleaning procedure
involves moistening a lens paper with methanol
and wiping the external surface, then leaving the
cell to evaporate. The interior of the cell is first
washed with water followed by methanol and the
solvent is also allowed to evaporate. Disposable
polypropylene cuvettes are incompatible with non
polar solvents and formulations having these
solvents should be avoided; or large errors will be
encountered.
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3. Calibration Curves
Usually, a plot of the absorbance of a series of standards is
plotted versus the concentration. The absorbance of
the unknown is then determined and the prepared
calibration plot is used for the determination of the
analyte concentration. If the absorbance of the analyte
was located outside the calibration plot, more standars
should be made or the analyte concentration must be
adjusted to occur on the calibration plot. We have seen
earlier that it is not allowed to theoretically extrapolate
or interpolate a calibration plot. It should also be
appreciated that the composition of standard solutions
must approximate that of the sample solution. In cases
where the sample composition is not clear, the method
of standard addition should be used. The slope of the
linear calibration plot is the molar absorptivity when
the path length is 1.00 cm. Larger slopes mean higher
sensitivities.
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4. Standard Addition method
The use of this method provides better correlations between
absorbances of standards and sample due to constant
matrix effects. The method involves addition of the same
sample volume to a set of tubes or containers. Variable
volumes of a standard are added to the tube set followed
by completion to a specific volume. Now, all tubes contain
the same amount of sample but different concentrations
of analyte. A plot is then made for the volume of standard
and absorbance. This plot will have an intercept (b) with
the y axis and a slope equals m. The concentration of the
analyte can be determined by the relation:
Cx = bCs/mVx
Where, Cx and Vx are concentration and volume of analyte
and Cs is the concentration of standard.
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One can only use two points to get the analyte
concentration using the relation:
Cx = AxCsVs/(At –Ax)Vx
Where, Ax is the absorbance of the analyte
solution brought to a final volume Vt and At is
the total absorbance of the solution (final
volume is Vt) containing same amount of
analyte and a volume of the standard (Vs).
However, this procedure assumes that both
points occur at the linear portion of the
calibration plot. In case where this is not
true, an error will be encountered.
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