Quant. Anal. Chp 18 Spectromet

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 [Dual Nature of Light]
Wave:
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-Explains physical properties of light itself.
-Physicist
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[Dual Nature of Light]
Particle (Photon):
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-Explains how light interacts with matter.
-Biologist/Chemist
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1. Relation between frequency and wavelength:

 

2. Speed of light in other medium?

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1. c= νλ

 

 2. c/n

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Relation between energy and frequency (or wavelength):

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  • E=h ν = h(c/ λ)
  • Smaller Wavelength →Higher Frequency → More Energy

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What are the ways light interacts with matter?
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Light can be:

  • Reflected
  • Refracted
  • Scattered
  • Emitted
  • Absorbed

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Absorption-
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Transfer of energy from photon to atom or molecule which produces transition from a lower energy to a higher energy level.
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Emission-
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Production of photon of energy from atom or molecule which is originally in a higher energy level and which returns atom or molecule to a lower energy level.
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What are the 5 main components of a spectrophotometric experiment?
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[image]
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Transmittance:
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  • The fraction of incident light that passes through a sample.
  • T = P/P0
  • P0 is the irradiance (radiant power) striking the sample and P is irradiance emerging from other side of sample.
  • P P0
  • As %: %T = (P/P0) * 100%

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Absorbance:
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  • Most useful quantity for chemical analysis.
  • A = log(P0/P) = -log(T)

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Beer-Lambert Law:
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Aλλbc

 

  • The reason why absorbance is so important is that absorbance is proportional to the concentration of light-absorbing molecules in the sample.

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Limitations of Beer-Lambert Law:
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  • Works well for dilute solutions (≤ 0.01 M) of most absorbing substances.
  • Light must be monochromatic
  • Pathlength must be constant (square cuvette) (typically is 1 cm)
  • Sample should not:
    • Fluoresce or phosphoresce
    • Scatter light (heterogeneous)
    • Change its chemical composition

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Common Cuvets for Ultraviolet and Visible Measurements:
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  • Standard 1-cm path
  • Cylindrical
  • Microcells

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