Ch 18 – Let There Be Light
Ch 19 – Spectrophotometry:
Instruments and Applications
Properties of Light - Sec 18-1
Relation between frequency and wavelength
 = c
= wavelength (meters, cm, nm, etc)
= frequency (cycles per second, Hertz, s-1)
c = speed of light (2.997 x 108 m/s)
Unit analysis:
Example (p. 376) - Relating Wavelength
and Frequency
What is the wavelength of radiation in your
microwave oven, whose frequency is 2.45
GHz?
Light can also be thought of as a particle or
PHOTON
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 -
Electromagnetic Spectrum
X-Rays:
UV-Vis:
Infrared:
Microwave:
Radio:
Ground state:
Excited state:
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 ?
Absorption of Light – Sec 18-2
Spectrophotometer:
Radiant power:
Monochromator:
Transmittance and Absorbance
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 -
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?
Absorption Spectrum – CoCl2
R
V
O
B
Y
G
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.
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
Analysis based on the absorbance of the colored
product of this reaction-
Absorbance Spectrum of the Colored Product
Absorbance max = __________ nm
Construction of a Calibration
Curve (Standard Curve)
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)
Calibration Curve for Nitrite Analysis
(blank subtracted)
Example (p. 389) – Using the Standard
Curve
From the data from Table 18-2, find the
molarity of nitrite in the aquarium.
The Spectrophotometer – Sec 19-1
Double-Beam Spectrophotometer
Hitachi UV-Vis – U2000
Sample Cuvettes
Light Sources
Monochromator
Light Detectors
Spectrophotometric Analysis of a Mixture – Sec 19-2
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
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]