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Spectrophotometry

Beer's Law: The Effect of Concentration

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

In this exercise, the role of the analyte concentration is examined. The analyte is the substance that absorbs the light. (Literally, it is the substance that is being analyzed in the experiment.) The concentration is represented by the symbol c and is typically measured in mole L-1.

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

Your goal is to answer the following questions:

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

Experiment

For each simulation:

  1. Select a concentration.
  2. For each concentration, 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 c,T and c,A points on the graphs.
  6. After all the points are plotted, carefully examine the plots to see how T and A vary with c. Is either plot linear?

Notes:

Concentration:
0.00100 mole L-1           0.00200 mole L-1
0.00300 mole L-1           0.00400 mole L-1
0.00500 mole L-1

Number of Photons Detected:

Elapsed Time: sec

Intensity of Detected Light: photons/sec


       
Light
Source
Cell Detector
Plot Point:       c = mole L-1     T = %     A =        


Effect of Cell Path Length                     Beer's Law

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