ch 18 – let there be light ch 19 – spectrophotometry...
TRANSCRIPT
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Ch 18 – Let There Be Light
Ch 19 – Spectrophotometry: Instruments and Applications
Properties of Light - Sec 18-1
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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?
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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:
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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 ?
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Absorption of Light – Sec 18-2
Spectrophotometer:
Radiant power:
Monochromator:
Transmittance and Absorbance
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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 -
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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
R
O
Y
G
B
V
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.
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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
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Analysis based on the absorbance of the colored product of this reaction-
Absorbance Spectrum of the Colored Product
Absorbance max = __________ nm
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Construction of a Calibration Curve (Standard Curve)
Table 18-2
Sample Absorbance at 543 nm in a
1.0 cm cuvette
Corrected Absorbance
(blank subtracted)
Blank 0.003
Standards
0.4575 ppm0.9150 ppm1.830 ppm
0.0850.1670.328
Unknown 0.281
Unknown 0.277
Calibration Curve for Nitrite Analysis(blank subtracted)
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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
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Double-Beam Spectrophotometer Hitachi UV-Vis – U2000
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Sample Cuvettes Light Sources
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Monochromator Light Detectors
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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
Spectrophotometric Analysis of a Mixture – Sec 19-2 A = AX + AY because Beer’s Law is additive
= X b [X] + Y b [Y]
A″ = A″X + A″Y
= ″X b [X] + ″Y b [Y]
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