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# Spectrophotometry

## Beer's Law: The Effect of Cell Path Length

What are the factors that affect the amount of light a sample absorbs?

In this exercise, the role of the cell path length is examined. The cell path length is the length of sample that the light passes through. The cell path length is represented by the symbol l and is typically measured in centimeters.

In the experiment presented below, you will perform a series of simulations in which all experimental parameters are the same except for the cell path length. Each simulation will be performed using a different cell path length to isolate the effect of the cell path length on the absorption of light.

1. How does the cell path length affect the intensity of light reaching the detector and why is this behavior observed?
2. How does the transmittance vary with the cell path length? Is this plot linear?
3. How does the absorbance vary with the cell path length? Is this plot linear?

### Experiment

For each simulation:

1. Select a cell path length.
2. For each cell path length, perform the simulation to determine the intensity of light that reaches the detector.
3. Calculate the transmittance.
4. Calculate the absorbance.
5. For each simulation, plot the l,T and l,A points on the graphs.
6. After all the points are plotted, carefully examine the plots to see how T and A vary with l. Is either plot linear?

Notes:

• The intensity of light from the light source is 300.0 photons/sec. (Only 10% of the photons are actually shown on the screen.)
• The transmittance is a fraction and is often expressed as a percentage. In plotting the results, enter T as percent transmittance.
• If possible, run the simulation sufficiently long to detect at least 1000 photons.
 Cell Path Length: 1.00 cm           2.00 cm           3.00 cm 4.00 cm           5.00 cm Number of Photons Detected: Elapsed Time: sec Intensity of Detected Light: photons/sec LightSource Cell Detector Plot Point:       l = cm     T = %     A =

Absorbance Spectrum                     Effect of Concentration