Materials

• Test tubes, 18 × 150 mm
• Test tube rack
• Stoppers to fit test tubes after the stoppers have been covered with aluminum foil (can also use plastic wrap with rubber band; tube can remain upright)
• Isopropyl alcohol
• Benzophenone
• Glacial acetic acid
• Dropper bottles
• Various sunscreen products with good range of SPF values (students provide their favorite product)
• Zinc oxide sunblock (commercial product; could try plain and with vitamin E)
• Baby oil (mineral oil base)
• Ultraviolet lamp (if not using the sun or a backup for a cloudy day)
• Stirring rods
• Spatulas
• Tare paper or 50-ml beakers
• Balances

Procedure (Preparation)

1. Wearing latex gloves, accurately weigh 3.00 g of finely divided benzophenone and transfer to a test tube (18 × 150 mm).

2. To the test tube, add 20 ml of isopropanol (2-propanol).

3. Immerse the test tube in a hot (70°C) water bath, carefully stirring the solution to dissolve the benzophenone.

4. To the solution, add 1 drop of glacial acetic acid and stir.

5. Wash the stirring rod in the test tube with more isopropanol until the level of alcohol in the test tube is high enough to meet the bottom of a tight fitting stopper (or plastic wrap) to completely seal the solution from any air.

6. Repeat the above procedure for as many test tubes as you have potential sunscreen compounds to be tested plus two additional tubes.

7. For each test tube filled with solution, add a tightly fitting stopper with an aluminum foil lining.

8. Invert the test tube in a test-tube rack if using a stopper. If using plastic wrap, the tube should remain upright.

9. Set aside one of the prepared tubes to act as a control, having no coating on the outside of the test tube.

10. For another prepared test tube, coat the glass surface uniformly with zinc oxide.

11. For each sunscreen product to be tested, apply the product uniformly to the entire glass surface of a prepared test tube.< p>
12. Move the test tube-rack to your designated source of UV light—sunlight or a UV lamp.

13. After 4 hours, prepare for removing and observing for insoluble (precipitated) product that has formed in the test tubes.

Procedure (Chemical Analysis)

For this part of the activity you will need to print out a copy of the data sheet to record your results.

1. Set up a filter system for each test tube's solution as shown below.
   

   Assembly of the filter system.

2. Weigh a piece of filter paper for each test tube to be processed. Record the mass on your data sheet. Mark each paper with pencil for future reference.

3. Transfer the solution of each test tube to an individual filter and its paper.

4. Wash each test tube of any solid contents remaining behind by using 10 ml of isopropyl alcohol, transferring into the filter.

5. Allow the paper and its contents to dry in the air or in a warm (100°C) drying oven. Do not allow the product to remain in the oven much longer than it takes to completely dry the paper and the product. Otherwise, some of the product may evaporate.

6. Weigh the dried paper and its contents. Record the mass on your data sheet.

7. Clean up materials and equipment according to your teacher's instructions.

Data Analysis

1. Calculate the mass of product collected on the filter paper by simple subtraction of the mass of empty filter paper from the mass of filter paper plus product. Record you result on you data sheet.

2. How does the mass of product in the tube without any covering compare with the sunscreen test tubes and the zinc oxide? What is the function of the uncoated test tube?

3. How does the mass of product in the tube covered with zinc oxide compare with the mass of product from any of the tubes covered with commercial sunscreen product? What is the function of the tube covered with zinc oxide?

4. How do the masses of product formed in the tubes covered with commercial sunscreen product compare with their sun protection factor (SPF), a numerical system?

5. Based on your data and conclusions to questions #2–4, how would you explain in general terms what happens chemically in the test tubes when the solution is exposed to ultraviolet light? How does this relate to the mass of product formed?

Trying to Look SUNsational? Complexity Persists in Using Sunscreens — from the U.S. Food and Drug Administration.

Chemistry Hits the Beach The Alchemist, April 2000. (Free registration is required to view this Web site.)

Wu, Corrina. "Melanoma Madness: The Scientific Flap over Sunscreens and Skin Cancer Chemical Studies," Science News, 1998, 153 (23), 360.