ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18
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Transcript of ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 18
ANALYTICAL CHEMISTRY CHEM 3811
CHAPTER 18
DR. AUGUSTINE OFORI AGYEMANAssistant professor of chemistryDepartment of natural sciences
Clayton state university
CHAPTER 18
ELECTROMAGNETIC RADIATION
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
Examplesheat energy in microwaves
light from the sunX-ray
radio waves
Three Characteristics of Waves
Wavelength (λ) - Distance for a wave to go through a complete cycle
(distance between two consecutive peaks or troughs in a wave)
Frequency (ν)- 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)
ELECTROMAGNETIC RADIATION
one second
λ1
λ3
λ2
ν1 = 4 cycles/second
ν2 = 8 cycles/second
ν3 = 16 cycles/second
amplitude
peak
trough
ELECTROMAGNETIC RADIATION
node
Gamma rays
X rays Ultr-violet
Infrared Microwaves Radio frequency FM Shortwave AM
Vis
ible
Visible Light: VIBGYORViolet, Indigo, Blue, Green, Yellow, Orange, Red
400 – 750 nm
- White light is a blend of all visible wavelengths
- Can be separated using a prism
Wavelength (m)
Frequency (s-1)
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ELECTROMAGNETIC RADIATION
- Inverse relationship between wavelength and frequency
λ α 1/ν
c = λ ν
λ = wavelength (m)
ν = frequency (cycles/second = 1/s = s-1 = hertz = Hz)
c = speed of light (3.00 x 108 m/s)
ELECTROMAGNETIC RADIATION
An FM radio station broadcasts at 90.1 MHz. Calculate the wavelength of the corresponding radio waves
c = λ ν
λ = ?ν = 90.1 MHz = 90.1 x 106 Hz = 9.01 x 107 Hz
c = 3.00 x 108 m/s
λ = c/ ν = [3.00 x 108 m/s]/[9.01 x 107 Hz]
= 3.33 m
ELECTROMAGNETIC RADIATION
Albert Einstein proposed that
- Electromagnetic radiation is quantized
- Electromagnetic radiation can be viewed as a stream of‘tiny particles’ called photons
h = Planck’s constant (6.626 x 10-34 joule-second, J-s)ν = frequency of the radiation
λ = wavelength of the radiation = 1/ λ = wavenumber (m-1)
THE ENERGY OF PHOTONS
ν~
ν~hcλ
hchνE photon
THE ATOMIC SPECTRUM
Transmission- Electromagnetic radiation (EM) passes through matter
without interaction
Absorption- 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
THE ATOMIC SPECTRUM
Ene
rgy
Absorption Emission
Excitedstate
Groundstate
Gamma rays X rays Ultr-
violetInfrared Microwaves Radio frequency
FM Shortwave AMV
isib
le
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ELECTROMAGNETIC RADIATION
Bon
d br
eaki
ngan
d io
niza
tion
Ele
ctro
nic
exci
tati
on
vibr
atio
n
rota
tion
Molecular Processes Occurring in Each Region
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)
ABSORPTION OF LIGHT
Transmittance (T)
- The fraction of incident light that passes through a sample
Po P
oP
PT
0 < T < 1
Po = radiant power of light striking a sampleP = radiant power of light emerging from sample
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% < %T < 100%
100xP
P%T
o
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
logTP
Plog
P
PlogA
o
o
ABSORPTION OF LIGHT
Beers Law
A = εbc
A = absorbance (dimensionless)
ε = molar absorptivity (M-1cm-1)
b = pathlength (cm)
c = concentration (M)
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
ABSORPTION OF LIGHT
Absorption Spectrum of 0.10 mM Ru(bpy)32+
λmax = 452 nm
ABSORPTION OF LIGHT
λmax = 540 nm
Absorption Spectrum of 3.0 mM Cr3+ complex
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
350 400 450 500 550 600
Wavelength (nm)
Abs
orba
nce
ABSORPTION OF LIGHT
Maximum Response (λmax)
- Wavelength at which the highest absorbance is observed for a given concentration
- Gives the greatest sensitivity
ABSORPTION OF LIGHT
Calibration Curve
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
0 0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016 0.0018
Concentration, moles/L
Abs
orba
nce
ABSORPTION OF LIGHT
Complementary Colors
- White light contains seven colors of the rainbow (ROYGBIV)
- Sample absorbs certain wavelengths of light and reflects ortransmits some
- The eye detects wavelengths not absorbed
ABSORPTION OF LIGHT
Complementary Colors
λmax
380-420420-440440-470470-500500-520520-550550-580580-620620-680680-780
Color Observed
Green-yellow YellowOrange
RedPurple-red
VioletViolet-blue
BlueBlue-green
Green
Color Absorbed
VioletViolet-blue
BlueBlue-green
GreenYellow-green
YellowOrange
RedRed
ABSORPTION OF LIGHT
Complementary Colors
ABSORPTION OF LIGHT
Complementary Colors
Ru(bpy)32+
λmax = 450 nmColor observed with the eye: orange
Color absorbed: blue
Cr3+-EDTA complexλmax = 540 nm
Color observed with the eye: violetColor absorbed: yellow-green
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
ABSORPTION OF LIGHT
Single-Beam Spectrophotometer
- Only one beam of light
- First measure reference or blank (only solvent) as Po
Po PLightsource
monochromator(selects λ) sample computer detector
b
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)
Po
PLightsource
monochromator(selects λ) sample computer detector
reference
b