Download Quantitative Chemical Analysis 7e

Daniel C. Harris
Quantitative Chemical
Analysis
Seventh Edition
Chapter 18
Fundamentals of
Spectrophotometry
Copyright © 2007 by W. H. Freeman and Company
Light has:
• Intensity
• Color (wavelength)
• Polarization
• Intensity
Transmittance
Absorbance
Transmittance
Absorbance
Beer’s Law
ε = 313 M-1 cm-1
b = 2 cm
T = 3.16% = 0.0316
A = -logT = 1.50
c = 1.5/(313 M-1) (2 cm-1) = 0.0024 M
dP(x)/dx = -κ P(x)
P(x) = exp(-κ x)
Beer’s law
I  
N
N
x  I
A  x
N
 
x  I
V
 x  I
I
dI


  I
x
dx
 
Dx
hn
A
 I ( x)  I 0e  x
It
 e  x
I0
 1 
NB:  cm2    3  x  cm
 cm 
A
I
It
 e  x  10 cx
I0
L

  mole 
NB:  
c
x  cm


 mole  cm   L 
 c  optical density

N avogadro

Molar extinction
Coefficient
~250 L.mol-1cm-1
   absorptivity

2.303
N avogadro
Cross section
~10-18 cm2
Absorption of a mixture
Isosbestic point
HIn
H  I
(wavelength) x (frequency) = speed [m/s]
λν = c [108 m/s]
436 nm  what frequency?
Visible spectrum
Your Eyes See the Color that is NOT Absobed
Hexaquotitanium (III) Solutions Appear Violet Due to
Absorption of Yellow and Green Light
Subtraction of Light by Absorption
Leads to Color you See
What makes a molecule absorb light?
What makes a molecule absorb light?
What makes a molecule absorb light?
Electron motion (fs) 
resonant frequency
~ 10-15 s  ν ≈ 1015 s-1
~ 300 nm
UV-VIS
What makes a molecule absorb light?
Electron
motion
UV-VIS excites electrons!
E=hν
E=hν
E=hν
E=hν
Electron diffraction pattern from a beryllium atom
What makes a molecule absorb light?
Nuclear
motion
~ 3000 nm
infrared
What makes a molecule absorb light?
Nuclear
motion
~ 300,000 nm
microwave
What makes a molecule absorb light?
Oscillating DIPOLE moment
What makes a molecule absorb light?
What makes a molecule absorb light?
What makes a molecule absorb light?
green
house
gas
http://science.widener.edu/svb/ftir/ir_co2.html
What makes a molecule absorb light?
[cm-1] = 1/λ = ν/c =E/hc
What makes a molecule absorb light?
3200 cm−1 broad, strong O-H stretch (alcohols)
3000 cm−1 broad, medium O-H stretch (carboxylic acids)
1200 cm−1 strong, O-H bending
2800 cm−1 strong, C-H stretch
1400 cm−1 variable, C-H bending
1700 cm−1 strong, C=O stretch
1200 cm−1 strong, C-O stretch
What makes a molecule absorb light?
What makes a molecule absorb light?
E=hν
E=hν