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)

10-11 103

1020104

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

10-11 103

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