1. Beers Law and Spectrophotometry Tristan Ursell APh 162 California Institute of Technology

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

3. Conceptual Basis of Beers 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: Beers 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 Beers Law? Only at low concentrations, where the absorbance is linear (single scattering event): Rule of Thumb: A<1 for accurate results