ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18

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ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 18
DR. AUGUSTINE OFORI AGYEMAN Assistant professor
of chemistry Department of natural
sciences Clayton state university
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CHAPTER 18 ELECTROMAGNETIC RADIATION
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ELECTROMAGNETIC RADIATION
- Also known as radiant heat or radiant energy -
One of the ways by which energy travels through
space - Consists of perpendicular electric and
magnetic fields Examples heat energy in
microwaves light from the sun X-ray radio waves
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ELECTROMAGNETIC RADIATION
Three Characteristics of Waves Wavelength (?) -
Distance for a wave to go through a complete
cycle (distance between two consecutive peaks or
troughs in a wave) Frequency (?) - The number
of waves (cycles) per second that pass a given
point in space Speed (c) - All waves travel at
the speed of light in vacuum (3.00 x 108 m/s)
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ELECTROMAGNETIC RADIATION
?1
node
amplitude
?1 4 cycles/second
?2
?2 8 cycles/second
peak
?3
?3 16 cycles/second
trough
one second
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ELECTROMAGNETIC RADIATION
Wavelength (m)
10-11
103
Radio frequency FM Shortwave AM
Gamma rays
Ultr- violet
Infrared
Microwaves
Visible
X rays
Frequency (s-1)
104
1020
Visible Light VIBGYOR Violet, Indigo, Blue,
Green, Yellow, Orange, Red 400 750 nm - White
light is a blend of all visible wavelengths -
Can be separated using a prism
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ELECTROMAGNETIC RADIATION
- Inverse relationship between wavelength and
frequency ? a 1/? c ? ? ? wavelength
(m) ? frequency (cycles/second 1/s s-1
hertz Hz) c speed of light (3.00 x 108 m/s)
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ELECTROMAGNETIC RADIATION
An FM radio station broadcasts at 90.1 MHz.
Calculate the wavelength of the corresponding
radio waves c ? ? ? ? ? 90.1 MHz 90.1 x
106 Hz 9.01 x 107 Hz c 3.00 x 108 m/s ?
c/ ? 3.00 x 108 m/s/9.01 x 107 Hz 3.33 m
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THE ENERGY OF PHOTONS
Albert Einstein proposed that - Electromagnetic
radiation is quantized - Electromagnetic
radiation can be viewed as a stream of tiny
particles called photons h Plancks
constant (6.626 x 10-34 joule-second, J-s) ?
frequency of the radiation ? wavelength of the
radiation 1/ ? wavenumber (m-1)
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THE ATOMIC SPECTRUM
Transmission - Electromagnetic radiation (EM)
passes through matter without interaction Absorpt
ion - An atom (or ion or molecule) absorbs EM and
moves to a higher energy state
(excited) Emission - An atom (or ion or
molecule) releases energy and moves to a lower
energy state
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THE ATOMIC SPECTRUM
Excited state
Energy
Ground state
Absorption
Emission
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ELECTROMAGNETIC RADIATION
Molecular Processes Occurring in Each Region
10-11
103
Gamma rays
Ultr- violet
Radio frequency FM Shortwave AM
X rays
Infrared
Microwaves
Visible
1020
104
Electronic excitation
rotation
vibration
Bond breaking and ionization
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ABSORPTION OF LIGHT
Spectrophotometry - The use of EM to measure
chemical concentrations Spectrophotometer -
Used to measure light transmission Radiant Power
(P) - Energy per second per unit area of a beam
of light - Decreases when light transmits through
a sample (due to absorption of light by the
sample)
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ABSORPTION OF LIGHT
Transmittance (T) - The fraction of incident
light that passes through a sample
0 lt T lt 1 Po radiant power of light striking a
sample P radiant power of light emerging from
sample
Po
P
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ABSORPTION OF LIGHT
Transmittance (T) - No light absorbed P Po
and T 1 - All light absorbed P 0 and T
0 Percent Transmitance (T)
0 lt T lt 100
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ABSORPTION OF LIGHT
Absorbance (A)
- No light absorbed P Po and A 0 - 1 light
absorbed implies 99 light transmitted - Higher
absorbance implies less light transmitted
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ABSORPTION OF LIGHT
Beers Law A ebc A absorbance
(dimensionless) e molar absorptivity
(M-1cm-1) b pathlength (cm) c concentration
(M)
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ABSORPTION OF LIGHT
Beers Law - Absorbance is proportional to the
concentration of light absorbing molecules in the
sample - Absorbance is proportional to the
pathlength of the sample through which light
travels - More intense color implies greater
absorbance
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ABSORPTION OF LIGHT
Absorption Spectrum of 0.10 mM Ru(bpy)32
?max 452 nm
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ABSORPTION OF LIGHT
Absorption Spectrum of 3.0 mM Cr3 complex
?max 540 nm
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ABSORPTION OF LIGHT
Maximum Response (?max) - Wavelength at which
the highest absorbance is observed for a given
concentration - Gives the greatest sensitivity
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ABSORPTION OF LIGHT
Calibration Curve
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ABSORPTION OF LIGHT
Complementary Colors - White light contains
seven colors of the rainbow (ROYGBIV) - Sample
absorbs certain wavelengths of light and reflects
or transmits some - The eye detects wavelengths
not absorbed
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ABSORPTION OF LIGHT
Complementary Colors
?max 380-420 420-440 440-470 470-500 500-520 520-
550 550-580 580-620 620-680 680-780
Color Observed Green-yellow Yellow Orange Red Pu
rple-red Violet Violet-blue Blue Blue-green Green
Color Absorbed Violet Violet-blue Blue Blue-green
Green Yellow-green Yellow Orange Red Red
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ABSORPTION OF LIGHT
Complementary Colors
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ABSORPTION OF LIGHT
Complementary Colors Ru(bpy)32 ?max 450
nm Color observed with the eye orange Color
absorbed blue Cr3-EDTA complex ?max 540
nm Color observed with the eye violet Color
absorbed yellow-green
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ABSORPTION OF LIGHT
Cuvet - Cell used for spectrophotometry Fused
silica Cells (SiO2) - Transmits visible and UV
radiation Plastic and Glass Cells - Only good
for visible wavelengths NaCl and KBr Crystals -
IR wavelengths
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ABSORPTION OF LIGHT
Single-Beam Spectrophotometer - Only one beam of
light - First measure reference or blank (only
solvent) as Po
b
Po
P
Light source
monochromator (selects ?)
sample
computer
detector
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ABSORPTION OF LIGHT
Double-Beam Spectrophotometer - Houses both
sample cuvet and reference cuvet - Incident beam
alternates between sample and reference with the
aid of mirrors (rotating beam chopper)
b
P
Light source
monochromator (selects ?)
sample
computer
detector
Po
reference