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Beer’s Law and Spectrophotometry

Tristan Ursell APh 162 California Institute of Technology. Beer’s Law and Spectrophotometry . Two-fold: - Relates concentration to the optical measurement of ‘absorbance’ - combined with spectrophotometry can be used to distinguish and compare different molecules in

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Beer’s Law and Spectrophotometry

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  1. Tristan Ursell APh 162 California Institute of Technology Beer’s Law and Spectrophotometry

  2. Two-fold: - Relates concentration to the optical measurement of ‘absorbance’ - combined with spectrophotometry can be used to distinguish and compare different molecules in solution Uses of Beer’s Law:

  3. Conceptual Basis of Beer’s Law • - Light of a particular wavelength enters the ‘sample’. • Light scatters from particles in solution (no momentum change) reducing light transmission • - Light is absorbed by molecules/particles (momentum • change) and remitted at different wavelengths, reducing light transmission

  4. A little more In-depth: Beer’s Law is stated in a way to make certain quantities easy to compare and interpret. Parameters: l – sample pathlength (usually 1cm) c – concentration (mol/vol) – molar absorption coefficient ( ) I – light intensity (W/m^2)

  5. A little more In-depth:

  6. When can I use Beer’s Law? Rule of Thumb: A<1 for accurate results Only at low concentrations, where the absorbance is linear (single scattering event):

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