A: Important topics in this part:

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Absorption Spectroscopy
A Important topics in this part
Beer's Law deviations from Beers law instruments
for absorption measurements major absorbing
species quantitative analysis photometric
titration calculation of absorbance important
instrumental components application of absorption
technique
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1. Chemical deviations from Beers law
unbuffered solutions of the indicator Hin.
Beer's law A - long (T) -log (I/I0)
ebc
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• 2. Instrumental deviation
• Strict adherence to Beer's law is observed only
  with truly
• monochromatic radiation.
• Stray light
• b. Polychromatic radiation

Stray light A log (P0 Ps) / (P Ps)
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3. Deviation from Polychromatic Radiation
l
Molar absorptivity (absorption coefficient, e)
is wavelength dependent
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Instruments for absorption spectrometer Sources
Wavelength selector Sample container
Radiation detector Signal processor and
readout device.
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Light sources Deuterium and hydrogen lamps
160 - 375 nm Tungsten filament lamps 350 - 2500
nm Xenon lamp 250 - 600 nm, peaking at 500
nm. Sample container cuvettes quartz or fused
silica is required for work under 350 nm
silicate glasses can be used for 350 - 2000 nm
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Photo-detectors
Silicon Diode (P-N junction)
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Photo-detectors
Photodiode Array
Positional Encoding
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Photo-detectors
Charge Coupled Device
Positional Encoding
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Single-and Double-Beam Photometers
Shot noise (photon detector) Source flick noise
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Photodiode Array Detector
Fast response, it is suitable for kinetic studies
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Figure 13-16 Schematic of a probe-type
photometer. (Courtesy of Brinkman Instrument
Company, Division of Sybron Corporation,
Westbury, NY.)
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Effect of bandwidth on spectral detail. The
sample was a didymium glass.
Effect of bandwidth on spectrophotometric analyses

Loss of detailed spectral structures as
bandwidth gets wider
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Figure 13-9 Effect of slit width and bandwidth
on peak heights. Here, the sample was a solution
of praseodymium chloride.
Effect of slit width
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Slit width for narrow peaks peak absorbance
increases as slit width decreases wide slit
width results in wide and lower peaks
Why?
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Absorption Spectroscopy
Important topics in this part
Beer's Law deviations from Beers law instruments
for absorption measurements major absorbing
species quantitative analysis photometric
titration calculation of absorbance important
instrumental components application of absorption
technique
17
B Important points in this lecture
1. The magnitude of molar absorptivites (e) and
absorbing species.
2. Qualitative analysis 3. Quantitative
analysis 4. Photomatric titrations
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The Magnitude of Molar absorptivity is 0 to
around 105 e 8.7 x 1019 P A P
transition probability (0 to 1) A cross
sectional target area in square centimeters
(for organic molecules, this number is around
10-15 cm2).
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Absorbing species Four types of electronic
transitions 1. p, s and n electrons 2. d and
f electrons 3. charge transfer electrons 4.
electronic transitions in semiconductor
nanocrystals
Absorbing electrons 1). those that participate
directly in bond formation between atoms and are
thus associated with more than one atom 2). non
bonding or unshared outer electrons that are
largely localized.
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Electron distribution in sigma and pi molecular
orbitals.
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Types of molecular orbitals in formaldehyde
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Figure 14-3 Electronic molecular energy levels.
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p, s and non-bonding orbital four types of
transitions s s p p n s n p
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1). absorption maxima due to s s transitions
(around 125 nm) have never been observed in the
ordinary, accessible UV region. 2). n s
transitions (from 150 to 250 nm) depend
primarily upon the kind of atomic bond and to a
lesser extent upon the structure of the molecule.
Molar absorptivity 100 to 3000 L/ (molcm). 3).
p p, n p transitions ((from 200 to 700
nm) both transitions require the presence of an
unsaturated functional group to provide the p
orbital. Molar absorptivity for n p is low
10 to 100 L/(molcm) blue shift with increasing
solvent polarity due to increased solvation of
the unbonded electron pair. Molar absorptivity
for p p is high 1000 to 10,000 l/ (molcm).
Red shift with increasing solvent polarity
(usually, but not always).
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Figure 14-4 Ultraviolet spectra for typical
organic compounds.
The peaks are ordinarily broad because of
vibrational effects.
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Effect of conjugation of chromophores p
electrons are further delocalized by conjugation
The effect of this delocalization is to lower the
energy level of the p orbital absorption maxima
shifted to longer wavelength.
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Absorption by Aromatic Systems
Auxochrome is a function group that does not
itself sbsorb in the ultra-violet region but has
the effect of shifting chromophore peaks to
longer wavelength as well as increasing their
intensities.
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Figure 14-5 Ultraviolet absorption spectra for
1,2,4,5-tetrazine (a) in the vapor phase, (b) in
hexane solution, and (c) in aqueous solution.
Superposition of vibrational transitions.
Solvents could affect the vibration of solute
Molecules. Thus it could could diminish
these superposition of the vibrational
transitions.
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Absorbing species Four Types of Electronic
Transitions 1. p, s and n electrons 2. d and
f electrons 3. charge transfer electrons 4.
electronic transitions in semiconductor
nanocrystals
Absorbing electrons 1). those that participate
directly in bond formation between atoms and are
thus associated with more than one atom 2). non
bonding or unshared outer electrons that are
largely localized.
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Absorption Spectra of CdS Nanocrystals (Blue)
CdS
Conduction Band
Valence Band
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B Important points in this lecture
1. The magnitude of molar absorptivites (e) and
absorbing species.
2. Qualitative analysis 3. Quantitative
analysis 4. Photomatric titrations
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Quantitative analysis by absorption measurements
advantages 1). wide applicability 2).
good sensitivity ( 10-4 to 10-7 M) 3).
moderate to high selectivity 4). good
accuracy 1 to 3 relative uncertainties 5).
easy data acquisition.
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Procedures 1). selection of wavelength peak
wavelength (maximum sensitivity good adherence
to Beer's law less sensitive to
uncertainties) 2). Variables that influence
absorbance solvents pH of the solution
temperature high electrolyte concentrations
presence of interfere species 3). Cleaning the
cell lens paper soaked in methanol is used to
wipe cell surfaces 4). relationship between A
and c using standard solutions to obtain A vs. c
curve
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Data for standard addition method for the
determination of Fe3 as the SCN- complex.
Standard addition method
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Photometric titration Titration curve consists
two straight lines with different slopes.
Typical photometric titration curves. Molar
absorptivities of the substance titrated, the
product, and the titrant are given by ?s, ?p, ?t,
respectively.
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Photometric titration curve of 100 mL of a
solution that was 2.0 ? 10-3 M in Bi3 and Cu2.
Wavelength 745 nm.
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CHEM 4130 Exam I Preparation Homework problems
and Lecture Notes Ch. 1 Introduction basic
concepts (sensitivity, detection limit, bias,
precision, selectivity) and calculations
standard addition method and homework
questions. Ch. 5 signal and noise S/N
calculations of S/N Different noises types,
sources ways to improve S/N multiple
measurements for better S/N, hardware methods
and Homework questions.
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Ch. 6 An Introduction to Spectrometric
methods Basic concepts in electromagnetic
radiation concepts, calculations of energy,
wavenumber, frequency. diffraction of radiation
Snells law reflection refraction refractive
index photoelectric effect concepts and
application Homework questions. Ch. 7
Components of optical instrumentation Light
sources wavelength selection monochromator and
filter, working principle, second order
diffraction, calculations.
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Ch. 13 and Ch. 14 Molecular absorption Beers
law application, limitation and deviations
Instrumentation photon detector, effective slit
width double and single beam instrument basic
understanding of absorbing species different
transitions, conjugation effect Quantitative
analysis using absorption method Photometric
titration understanding of Figure 14-18.
Review Chapter 15 Homowork in Chapter 15 1, 2,
3, 4, 6, 7