Properties of Light - Sec 18-1
Ch 18 – Let There Be Light
Ch 19 – Spectrophotometry:
Instruments and Applications
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Relation between frequency and wavelength
 = c
= wavelength (meters, cm, nm, etc)
Example (p. 376) - Relating Wavelength
and Frequency
What is the wavelength of radiation in your
microwave oven, whose frequency is 2.45
GHz?
= frequency (cycles per second, Hertz, s-1)
c = speed of light (2.997 x 108 m/s)
Unit analysis:
2
Light can also be thought of as a particle or
PHOTON
Electromagnetic Spectrum
E = h
E = energy (Joules)
H = Planck’s constant (6.626 x 10-34 J·s)
= frequency (cycles per second, Hertz, s-1)
And combining with  = c -
X-Rays:
UV-Vis:
Infrared:
Microwave:
Radio:
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Example (p. 377) – Photon Energies
By how many joules is the energy of a
molecule increased when it absorbs (a)
visible light with a wavelength of 500 nm or
(b) infrared radiation with a wavenumber of
1,251 cm-1 ?
Ground state:
Excited state:
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Absorption of Light – Sec 18-2
Transmittance and Absorbance
Spectrophotometer:
Radiant power:
Monochromator:
5
Example (p. 379) – Absorbance & Transmittance
What absorbance corresponds to 99% transmittance?
To 0.10% transmittance?
Beer’s Law: absorbance is proportional to
the concentration of light-absorbing
molecules in the sample
A = bc
Beer’s Law for a mixture -
6
Example (p. 381) – Using Beer’s Law
The peak absorbance of 3.16 x 10-3 M KMNO4 at 555
nm in a 1.000 cm pathlength cell in Fig 18-5 is 6.54. (a)
Find the molar absorptivity and percent transmittance
of this solution. (b) What would the absorbance be if
the pathlength was 0.100 cm? (c) What would the
absorbance be in a 1.000 cm cell if the concentration
was decreased by a factor of 4?
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Absorption Spectrum – CoCl2
Example (p. 382) – Finding Concentration from the
Absorbance
Gaseous ozone has a molar absorptivity of 2700 M-1cm-1
at the absorption peak near 260 nm in the spectrum
below. Find the concentration of ozone (mol/L) in air if a
sample has an absorbance of 0.23 in a 10.0 cm cell. Air
has negligible absorbance at 260 nm.
R
O
V
B
Y
G
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Example (p. 383) – How Effective is Sunscreen?
What fraction of ultraviolet radiation is transmitted through
the sunscreen in the spectrum below at 300 nm?
Using Beer’s Law – Sec 18-4
e.g. measuring NO2¯ in aquarium water
9
Analysis based on the absorbance of the colored
product of this reaction-
Absorbance Spectrum of the Colored Product
Absorbance max = __________ nm
10
Construction of a Calibration
Curve (Standard Curve)
Calibration Curve for Nitrite Analysis
(blank subtracted)
Table 18-2
Sample
Absorbance
at 543 nm in a
1.0 cm
cuvette
Blank
0.003
Standards
0.4575 ppm
0.9150 ppm
1.830 ppm
0.085
0.167
0.328
Unknown
0.281
Unknown
0.277
Corrected
Absorbance
(blank
subtracted)
11
Example (p. 389) – Using the Standard
Curve
The Spectrophotometer – Sec 19-1
From the data from Table 18-2, find the
molarity of nitrite in the aquarium.
12
Double-Beam Spectrophotometer
Hitachi UV-Vis – U2000
13
Sample Cuvettes
Light Sources
14
Monochromator
Light Detectors
15
Spectrophotometric Analysis of a Mixture – Sec 19-2
A = AX + AY
because Beer’s Law is additive
= X b [X] + Y b [Y]
A″ = A″X + A″Y
= ″X b [X] + ″Y b [Y]
A = total absorbance at wavelength 1
A = total absorbance at wavelength 2
X = molar absorptivity of molecule X at wavelength 1
X = molar absorptivity of molecule X at wavelength 2
Y = molar absorptivity of molecule Y at wavelength 1
Y = molar absorptivity of molecule Y at wavelength 2
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