ANALYTICAL CHEMISTRY CHEM 3811

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ANALYTICAL CHEMISTRY
CHEM 3811


CHAPTER 18




DR. AUGUSTINE OFORI AGYEMAN

Assistant professor of chemistry

Department 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


Examples

heat energy in microwaves

light from the sun

X
-
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 10
8

m/s)

ELECTROMAGNETIC RADIATION

one second

λ
1

λ
3

λ
2

ν
1

= 4 cycles/second

ν
2

= 8 cycles/second

ν
3

= 16 cycles/second

amplitude

peak

trough

ELECTROMAGNETIC RADIATION

node

Gamma

rays

X rays

Ultr
-

violet

Infrared

Microwaves


Radio frequency


FM Shortwave AM

Visible

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

Wavelength (m)

Frequency (s
-
1
)

10
-
11

10
3

10
20

10
4

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 10
8

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 10
6

Hz = 9.01 x 10
7

Hz


c = 3.00 x 10
8

m/s


λ

= c/
ν

= [3.00 x 10
8

m/s]/[9.01 x 10
7

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
~
hc
λ
hc
h
ν
E
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

Energy

Absorption

Emission

Excited

state

Ground

state

Gamma

rays

X rays

Ultr
-

violet

Infrared

Microwaves


Radio frequency


FM Shortwave AM

Visible

10
-
11

10
3

10
20

10
4

ELECTROMAGNETIC RADIATION

Bond breaking

and ionization

Electronic

excitation

vibration

rotation

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

P
o

P

o
P
P
T

0 < T < 1


P
o

= radiant power of light striking a sample

P = radiant power of light emerging from sample

ABSORPTION OF LIGHT

Transmittance (T)


-

No light absorbed: P = P
o

and T = 1


-

All light absorbed: P = 0 and T = 0


Percent Transmitance (%T)

0% < %T < 100%

100
x
P
P
%T
o

ABSORPTION OF LIGHT

Absorbance (A)

-

No light absorbed: P = P
o

and A = 0


-

1% light absorbed implies 99% light transmitted


-

Higher absorbance implies less light transmitted

logT
P
P
log
P
P
log
A
o
o



















ABSORPTION OF LIGHT

Beers Law


A =
ε
bc


A = absorbance (dimensionless)


ε

= molar absorptivity (M
-
1
cm
-
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)
3
2+

λ
max

= 452 nm

ABSORPTION OF LIGHT

λ
max

= 540 nm

Absorption Spectrum of 3.0 mM Cr
3+

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)
Absorbance
ABSORPTION OF LIGHT

Maximum Response (
λ
max
)


-

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
Absorbance
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

ABSORPTION OF LIGHT

Complementary Colors

λ
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

Purple
-
red

Violet

Violet
-
blue

Blue

Blue
-
green

Green

Color Absorbed


Violet

Violet
-
blue

Blue

Blue
-
green

Green

Yellow
-
green

Yellow

Orange

Red

Red

ABSORPTION OF LIGHT

Complementary Colors

ABSORPTION OF LIGHT

Complementary Colors


Ru(bpy)
3
2+

λ
max

= 450 nm

Color observed with the eye: orange

Color absorbed: blue


Cr
3+
-
EDTA complex

λ
max

= 540 nm

Color observed with the eye: violet

Color absorbed: yellow
-
green

ABSORPTION OF LIGHT

Cuvet


-

Cell used for spectrophotometry


Fused silica Cells (SiO
2
)

-

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 P
o

P
o

P

Light

source

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)

P
o

P

Light

source

monochromator

(selects

λ
)


sample

computer


detector


reference

b