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Experiment 3 - Atoms, Molecules and Light (2/4 pages)

When you lather on sunscreen, you're spreading a chemical on your skin that absorbs in the ultraviolet (UV) region of the electromagnetic spectrum. Ultraviolet radiation can be harmful to the skin. It can damage genetic material near the skin's surface and lead to skin cancer. Because molecules can act as filters, the right molecules can filter out the harmful radiation from the sun. That is basically how sunscreen works. It contains molecules that absorb strongly in the UV region. Your body's natural defense to UV light is also to produce filter molecules. The compound melanin is produced in skin cells and absorbs strongly in the UV region. Melanin pigments your skin, resulting in darker skin after extended exposure to sunlight. The sunscreen industry markets its products with SPF (Sun Protection Factor) ratings. We can test the various SPF ratings by measuring their absorbance and use UV sensitive materials to see the effectiveness of different sunscreens. 

Investigation1: Molecular Spectroscopy

Determine the effectiveness of a sunscreen product by measuring the extinction coefficient of its active ingredients. 

Work in groups of four. 
Sunscreens are mixtures of many compounds; you will measure the absorbance of the whole mixture rather than separate the components. First you will dissolve the sunscreens in ethanol. Then make a series of the ethanol solutions with different concentrations by diluting your original solution. You can then measure the absorbance of the differentdilutions and plot the absorbance of the sunscreen vs. concentration to calculate the extinction coefficient. We'll compare extinction coefficients of various SPF factors to determine their relative effectiveness at absorbing UV light.

Note: There are two investigations in this lab. You will have to share spectrometers. Get the solutions for Investigation 1 made right away. The slowest part will be waiting for your turn on the spectrometer. Investigation 2 can be done while you wait.

Prepare a stock solution of your sunscreen:
1. Your lab instructor will provide a sunscreen product to analyze. Record the SPF factor in your notebook.

2. Obtain a 50.00 mL volumetric flask from the equipment shelf. Rinse the flask twice using a few milliliters of 95% ethanol.

3. Clean and dry a 50 mL beaker and tare it on a digital balance. The stockroom or your GSI will provide mass ranges to be used for each sunscreen. Carefully add the suggested amount of your sunscreen product (a tiny blob) to the beaker and record the mass.

4. Add about 30 mL of 95% ethanol to this beaker and stir the mixture until all of the sunscreen dissolves. 

5. If you are unable to completely dissolve the sunscreen after several minutes of stirring you may want to heat the solution gently on one of the hot plates in the room. Keep stirring as you heat the solution-you will not need to heat it very much to finish dissolving the sunscreen. Be careful, not to overheat the sample. Ethanol is flammable.

6. Once all of the sunscreen is dissolved transfer the solution to the 50.00 mL volumetric flask. Bring the volume of ethanol in the flask to exactly the 50.00 mL line and then mix the solution. Your lab instructor will demonstrate good volumetric technique.

7. Use your mass from step 3 to calculate the mass of sunscreen per mL ethanol. Record your result, keeping careful track of units.

8. Clean a 250 mL beaker by rinsing it twice with a few milliliters of 95% ethanol followed by a few milliliters of the sunscreen solution in your volumetric flask. Discard the rinsate. Pour the remainder of the sunscreen solution into the beaker. Label this "original sunscreen solution".

9. You need to use the 50.00 mL volumetric again, so clean it by rinsing twice using a few milliliters of 95% ethanol. Discard the rinsate. Using your 1.00 mL graduated pipet, transfer exactly 1.00 mL of the original sunscreen solution you just made back into the flask. Again bring the volume of ethanol in the flask to exactly the 50.00 mL line and mix the solution. You have diluted the original sunscreen solution by a factor of 50. This is your "stock solution"

10. Calculate and record the mass of sunscreen per mL in this stock solution.
Note: If you used within the suggested grams of sunscreen, you can skip to step 13. Otherwise, record the absorbance of your stock solution and continue on to step 11 and 12: 

11. Record the absorption spectrum of the stock solution between 250 and 410 nm using the HP 8453 UV-Visible Spectrophotometer. Step-by-step instructions for running the HP spectrometer are included in Appendix I, and your lab instructor will help you run the first sample. (The "noise" below 290nm is caused by the absorbance of the plastic in the cuvette).

12. Read the maximum absorbance of the peak at near 310nm in your spectrum. To continue, it has to be between 0.6 and 1.0 absorbance units. If it is significantly out of this range, repeat step 9 above and make a new stock solution. This time, adjusting the volume of original solution you use as follows: If your absorbance reading at 310 nm is too high, use less, if too low use more. Remember absorbance and concentration are linearly proportional. You should be able to guess what volume to use to bring the absorbance into the right range. Recalculate the sunscreen concentration in the new stock solution.

Make accurate dilutions of your stock solution:

Part 1: Prepare the stock solution and ethanol in burets:

13. When your stock solution is within the correct absorbance range, rinse a buret twice with a few milliliters of 95% ethanol followed by a rinsing with a few milliliters of your stock solution. The stock solution rinse is to make sure that any remaining liquid in the buret is your stock solution and won't effect the concentration of the stock solution you're about to add. Fill the buret with your stock solution and clear any air bubbles from the tip.

14. Rinse a second buret twice with a few milliliters of 95% ethanol and then fill it with pure 95% ethanol solution, clearing any air bubbles from the tip

Part 2. Make the dilutions:

15. Clean six small test tubes by rinsing each of them with a few milliliters of 95% ethanol and drying them with a Kim-Wipe. Label these tubes 1-6. Make a table like the one below in your lab book. Fill each of the tubes according to the table below and complete the missing entries.

Record the absorbance and make a Beer's Law plot: 

16. Run a new blank spectrum and then measure the absorption of each of these six samples at 310nm (be sure to clear all previous spectra before taking your data). Print out the results. p

17. Make a Beer's law plot of your absorption data at 310nm vs. the concentration in grams per milliliter of your six solutions. You may use the origin (0,0) as a data point. This is equivalent to 'blanking' the spectrometer. 

18. Determine the slope of the line using a linear regression routine on a calculator. Be careful with units. Record your value for e (the slope) on the blackboard and copy the results from all the other groups in your lab room before leaving for the day. 

19. If you finish early help other groups with their calculations and regression analysis.

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