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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1020104
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