Beer’s Law
Equipment
Plastic well plate with six wells, dropper, small glass stir rod, overhead projector.
Reagents
KMnO4 solution of sufficient concentration to be completely opaque.
Presentation
- Place 5 mL of DI water in wells 1-3, place 10 mL of DI water in wells 4-6. Place well tray on overhead projector.
- Using the KMnO4 solution, add 1 drop to well 1, 2 drops to well 2 and 4 drops to well 3. Stir the wells.
- Using the KMnO4 solution, add 2 drops to well 4, 4 drops to well 5 and 8 drops to well 6. Stir the wells.
Hazards
Potassium permanganate is a strong oxidizer. Dust causes severe respiratory irritation. Either solid or solutions cause burns of skin.
Discussion
It should be noted that the KMnO4 concentrations in wells 1, 2, and 3 are equal to the KMnO4 concentrations in wells 4, 5, and 6 respectively. This is not a rigorous experimental or mathematical proof, but does convey the essence of Beer’s law.
In wells 1-3, the only parameter that is changing is the concentration of the KMnO4. The concentration in well 3 is twice that of well 2 which is twice that of well 1. It may be reasonably said that the amount of light absorbed or the absorbance A of the solution is proportional to the concentration C of the absorbing species.
A a C
If the wells on the two different rows are compared, it appears that the absorbance of well 4 matches that of well 2 and that 5 matches 6. The KMnO4 concentration in well 4 is half the KMnO4 concentration in well 2. The only difference is that the pathlength b of well 4 is double that of well 2. It may be reasonably said that the absorbance of the solution is proportional to the pathlength.
A a b
Mathematically, something that is proportional to two independent quantities is proportional to their product.
A a bC
We can change the proportionality to an equality by the addition of a proportionality constant. In the case of Beer’s law that proportionality constant is called the molar absorbtivity e.
A = ebC
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This page was last modified 3/26/2000.
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