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					                        Beer’s Law: Using a Colorimeter

Introduction:

In this experiment, the concentration of an unknown cobalt (II) nitrate solution will be
determined using the colorimeter. A green light from the LED light source will pass
through the solution and strike a photocell. The Co(NO3)2 solution used in this
experiment has a pink color; higher concentrations absorb more light than solutions of
lower concentrations.

Four Co(NO3)2 solutions will be tested; the resulting data will be used to determine the
concentration of 2 unknown solutions. When a graph of absorbance vs. concentration is
plotted, a straight line should be generated, indicating a direct relationship. This
relationship is known as Beer’s Law. The concentration of an unknown solution is
determined by measuring its absorbance and finding the corresponding concentration
from the graph.

Purpose:

The purpose of this experiment is to use the colorimeter and Beer’s Law to determine the
concentrations of unknown solutions.

Equipment/Materials:

laptop                                           plastic rectangular cuvettes w/ lids
serial box interface                             Kimwipes
LoggerPro software                               standard Co(NO3)2 solutions
Vernier Colorimeter                              unknown Co(NO3)2 solutions
printer                                          droppers


Safety:

      Always wear goggles in the lab.
      Do not spill any solutions on your skin; inform the instructor of any accidents.


Procedure:

1. Double click on the LoggerPro icon. Go to the File menu and select Open. Open
   Chemistry with Computers, then open Experiment 11. Choose Exp 11 Colorimeter.
2. Plug the colorimeter into Port 1 on the Serial Box.

3. Go to the Experiment menu and choose Calibrate. Click Perform Now. With the
   sample compartment empty, close the lid. Turn the wavelength knob of the
   colorimeter to the 0% T position. (The light source will be turned off, so no light is
   received by the photocell.) The number 0 should appear in the % edit box. When the
   voltage reading stabilizes, click Keep.

4. Prepare a blank by rinsing a cuvette with distilled water. Fill the cuvette ¾ full of
   distilled water.

Remember: Cuvettes should always be wiped on the outside with a Kimwipe.
          Hold the cuvettes by the top edge of the ribbed sides.
          Solutions should be free of bubbles.
          Position the cuvette with the clear side in the light path.

5. For reading 2, turn the wavelength knob of the colorimeter to the Green LED position
   (565 nm) and place the blank in the colorimeter. The colorimeter will be calibrated to
   show 100% of the green light being transmitted through the blank cuvette. The
   number 100 should appear in the % edit box. When the voltage has stabilized, click
   Keep, then click OK.

6. Remove the blank cuvette. Rinse the cuvette twice with ~1 mL portions of the lease
   concentrated standard solution to be tested. Fill it ¾ full with the solution, and wipe
   with a Kimwipe. Place the cuvette in the colorimeter and close the lid.

7. Click Collect. Wait for the absorbance value to stabilize, then click Keep. Type the
   concentration in the edit box and press Enter. The data point should now be plotted
   on the graph.

8. Remove the cuvette and rinse twice with ~1 mL portions of the next standard solution.
   Wipe the cuvette, place it in the colorimeter, and close the lid. When the absorbance
   stabilizes, click Keep, type the concentration in the edit box, and press Enter.

9. Repeat step 7 for all standard solutions. When finished, click Stop.

10. Record the absorbance and concentration data pairs in your data table.

11. Rinse and fill a cuvette with an unknown solution. Wipe the cuvette, place it in the
    colorimeter, and close the lid. Record the absorbance value in your data table. Do
    not click collect!

12. Repeat step 10 for all unknowns.

13. Click on the graph of absorbance versus concentration. To see if the curve represents
    a direct relationship between these two variables, click the Linear Regression button,
   which is the third button from the right. A best fit or linear regression line will be
   shown for your data points.

14. If instructed to print the graph or data table, click on the appropriate window. Go to
   the File Menu and select Print. Enter your names and print the graph.

Data:

                Concentration (M)                                 Absorbance




        Unknown Number                    Absorbance                   Concentration




Calculations:

1. Use the following method to determine the unknown concentration. With the linear
   regression curve still displayed on your graph, choose Interpolate from the Analyze
   menu. A vertical cursor will appear on the graph. The cursor’s x and y coordinates
   are displayed at the bottom of the floating box. Move the cursor along the regression
   line until the absorbance (y) value is approximately the same as the absorbance value
   recorded for the unknown. The corresponding x value is the concentration of the
   unknown in Molarity. Record this value in the data table.

2. If instructed, print this screen following the procedure from step 13.

3. Optional second method of determining the concentration:
   Remove the regression line by clicking on the corner of the floating box so that the
   box and regression line disappear. Go to the Analyze menu and choose Curve Fit.
   Select Proportional (y=Ax). Click on Try Fit and then OK. Go to the Analyze menu
   and select Interpolate. Determine the concentration as you did in step 1. Record the
   concentration in the Data Table. Place this value in parentheses beneath that recorded
   in step 1. Compare the values.

4. If instructed, print this screen as in step 13 of the procedure.

5. Repeat the calculations for all unknowns.
Questions:

1. What does absorbance mean? How does it compare to transmittance?




2. Should the line in the standard curve pass through the origin? Why or why not?




Source:
Chemistry with Computers, Holmquist, D. and Volz, D., Vernier Software, 1997, pages
      11-1  11-2T (with slight adaptations for Advanced Instrumentation Workshop
      2000).

				
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posted:2/1/2013
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