Lecture 3

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Transcript Lecture 3

CM2007
Lecture 3
Background Correction
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A baseline spectrum of the solvent must be obtained in order
to subtract from the spectrum of the solvent + analyte.
The baseline spectrum is normally recorded by placing a cell,
filled with the appropriate solvent (minus analyte) into the
spectrophotometer.
Baseline spectrum is recorded before the analyte spectrum
using a single beam instrument.
Double beam instruments record both spectra simultaneously.
However, the intensity of the dual beams must be the same, the
cells must posses exactly the same absorbtivity, and the
solvents must be exactly the same
Application of UV/Vis to
Quantitative Analysis
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Solvents need to be considered in UV/Vis Spectroscopy
Parameters to consider
Transparency
Solvent effect on absorbing species, e.g., polar solvents
obliterate fine structure.
Compounds exhibit different absorption maxima in various
solvents
Absorption maxima shift depending on the solvent
E.g., acetaldehyde absorbs most strongly at 287nm in heptane
and at 278nm in water.
Solvent Effects on Acetaldehyde
The Origin and Position of Absorption Bands
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Bathochromic (red) Shift = shift of max to longer wavelength
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Hypsochromic (blue) Shift = shift of max to shorter wavelength
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Hyperchromic Shift = intensity increase of the band.
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Hypochromic Shift = intensity decrease of the band
Inorganic Spectra
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UV/Vis can be used to quantitatively determine any
absorbing species.
Also reagents can be used to react selectively with nonabsorbing species to give products which absorb strongly
in the UV/Vis.
Non-absorbing inorganic species can be determined using
complexing agents.
E.g., thiocyanate ion for Fe, Co and Mo. Peroxide anion
for Ti, V and Cr. Iodide for Bi, Pd and Te.
Also important are organic chelating agents that form
stable, coloured complexes with cations.
E.g., o=phenanthroline for Fe, dimethylgloxime for Ni,
diethyldithiocarbamate for Cu and
diphenyldithiocarbazone for Pb
Experimental Considerations
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Wavelength selection: make measurements at a wavelength
corresponding to the absorption maxima.
Variables which influence absorption are solvent, pH,
temperature, electrolyte concentration and interferences.
Cleaning and handling of cells
Materials used to make cells/cuvette. In order of preference try
to use matched quartz cell, glass cells and as a last resort use
plastic.
Prepare calibration curve to determine the relationship
between absorbance and concentration.
Analysis of Mixtures
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Total absorbance of a solution at a given wavelength is
equal to the sum of absorbances of all the components
present.
No wavelength exists at which the absorbance of the
mixture is due to one of the components.
The absorbance of a mixture at two wavelngths ’ and ’’
may be expressed as:
A’ = em’ cm l + en’ cnl
A’’ = em’’ cm l + en’’ cnl
The molar absorbtivities em’, en’, em’’ and en’’ can be
evaluated either from individual standards of M and N or
from the slopes of the Beer-Lambert plots
The absorbances A’ and A’’ and the cell length l can be
determined experimentally. Therefore the individual
concentrations can be determined
Analysis of Mixtures
Isobestic Point
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Often one absorbing species, X, is converted to another
absorbing species, Y, during the course of a reaction.
This transformation leads to a very obvious and
characteristic behaviour.
If the spectrum of pure X and pure Y cross each other at any
wavelength, then any spectrum recorded during this reaction
will cross at the same point
The observation of an isobestic point during a reaction is
good evidence that only two principal species are present.
E.g., methyl red changes between red (Hin) and yellow (In-)
near pH = 5.5

(CH 3)2N
N

OOC
H
N
+
HIn
(red)
OOC
pK2 ~5.1
(CH 3)2N
N
N
HIn
(yellow)
Isobestic Point
Infrared Spectroscopy
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IR spectrum encompasses wavelengths 800 – 1,000,000nm or
0.8 - 1000μm
Analytical IR techniques normally only exploit radiation in the
range 2500 – 16,000nm (2.5 - 16μm)
Molecules oscillate in a predictable manner around molecular
bonds. (bending, stretching and vibrating)
IR is used for qualitative structural identification of
compounds.
By historical convention IR spectra are displayed in a different
manner to UV/Vis.
The y-axis is of an IR spectrum is plotted in terms of
percentage transmittance.
IR Spectra
% Transmittance
IR Spectra
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The x-axis is not displayed in terms of either wavelength of
frequency but ‘wavenumber’.
Wavenumbers represent the reciprocal of wavelength (1/λ)
and have units of cm-1.
It should be noted that increasing wavenumbers correspond to
increasing frequency and, therefore, to progressively more
energetic radiation.
The identification of absorption peaks can be further used to
identify a class of molecule, e.g., alcohol, aldehyde, ketone,
ether, ester.
Characteristic absorption bands for molecular vibration are
tabulated in ‘correlation charts’ to aid structural identification
of spectra
Bond
Compound Type
Frequency range, cm-1
2960-2850(s) stretch
Alkanes
C-H
1470-1350(v) scissoring and bending
CH3 Umbrella Deformation
1380(m-w) - Doublet - isopropyl, t-butyl
3080-3020(m) stretch
C-H
Alkenes
1000-675(s) bend
C-H
Aromatic Rings
3100-3000(m) stretch
Phenyl Ring Substitution Bands
870-675(s) bend
Phenyl Ring Substitution Overtones
2000-1600(w) - fingerprint region
3333-3267(s) stretch
C-H
Alkynes
700-610(b) bend
C=C
Alkenes
1680-1640(m,w)) stretch
CC
Alkynes
2260-2100(w,sh) stretch
C=C
Aromatic Rings
1600, 1500(w) stretch
C-O
Alcohols, Ethers, Carboxylic acids, Esters
1260-1000(s) stretch
C=O
Aldehydes, Ketones, Carboxylic acids, Esters
1760-1670(s) stretch
Monomeric -- Alcohols, Phenols
3640-3160(s,br) stretch
Hydrogen-bonded -- Alcohols, Phenols
3600-3200(b) stretch
Carboxylic acids
3000-2500(b) stretch
O-H
3500-3300(m) stretch
N-H
Amines
1650-1580 (m) bend
C-N
Amines
1340-1020(m) stretch
CN
Nitriles
2260-2220(v) stretch
NO2
Nitro Compounds
1660-1500(s) asymmetrical stretch
1390-1260(s) symmetrical stretch
Interpretation of Spectra
3350 -- OH stretching vibrational frequency
2950 -- CH aliphatic asymmetrical stretching vibrational band. The less
intense band at 2860 is the symmetrical stretching vibrational band.
1425 -- CH2 characteristic absorption
1065 -- CO absorption
The compound is cyclohexanol.
3100 -- The broad intense absorption band seen here is characteristic of a carboxylic
acid dimer.
2960 -- CH aliphatic assymmetric stretch
2870 -- CH aliphatic symmetic stretching vibrational band.
1415 -- Absorption in this region is due to CH3. Note the weak band just below 1400.
This is the methyl bending vibrational band.
1290 -- Due to coupling of the in-plane OH bending and CO stretching of the dimer.
950 -- OH out-of-plane bending of the dimer.
The compound is octanoic acid
CM2007
Tutorial
Tutorial Questions
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Which wavelength range encompasses the UV/Vis spectrum?
Draw a schematic diagram of a spectrophotometer.
State the Beer Lambert law and define the parameters
What are the three possible deviation from the Beer Lambert
law? Real, Instrumental and chemical.
What are the typical radiant sources used to provide
broadband light in UV/Vis spectrophotometry?
How can the spectral range of a tungsten filament lamp be
extended in the UV?
Describe how a monochromator works to provide
monochromatic light.
Tutorial Questions
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Describe the different categories of cells/cuvettes available as
sample holders.
What is the basis and principles of IR spectroscopy?
What are the differences between IR and UV/Vis spectra?
Describe the different sample prep for the analysis of a solid
using IR. Why is KBr used?
http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM
What is the term used to describe a shift in λmax to a longer
wavelength.
Tutorial Questions
16/ A mixture of zinc sulfate and cobalt tetrachloride yields an
absorbance reading of 0.22 at a λ = 600nm. The
concentration of cobalt tetrachloride is known to be 1.0x10-2
M in the mixture and has a molar absorbtivity coefficient of
= 11 L mol-1 cm-1. What is the absorbance reading of cobalt
tetra chloride if the path length of the cell is 1cm? What is
the absorbance reading for zinc sulphate? Given the data for
zinc sulphate below, plot the data on graph paper and
determine the concentration of zinc sulphate in the mixture
and calculate its molar absorbtivity coefficient. If 600nm =
λmax for zinc sulphate, what is its likely colour?
Tutorial Questions
Concentration (x 10-2M)
Absorbance
0.2
0.04
0.4
0.08
0.6
0.12
0.8
0.16
1.0
0.20
Tutorial Questions
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Atotal = (ecl)x + (ecl)y
The absorbance reading of cobalt tetra chloride is calculated as
follows
A = 11L mol-1 cm-1 x 1.0 x 10-2M x 1cm = 0.11
Therefore the absorbance of copper sulfate = A = 0.22 – 0.11
= 0.11
Tutorial Questions
0.25
Absorbance
0.20
0.15
0.10
0.05
0.00
0.0
0.2
0.4
0.6
0.8
Copper Sulphate (x 10-2M)
1.0
1.2
Tutorial Questions
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Concentration of copper sulphate from the graph = 5.5 x 10-3
M and the molar absorbtivity coefficient = 20 L mol-1 cm-1.
Likely colour = blue due to the absorbance of light in the red
region of the spectrum.