MSDS Sheets for Reagents Used

The following MSDS sheets are available on-line from Cornell University:

isopropanol
acetic acid
benzophenone

Materials

(For 24 students working in pairs)

• 12 test tube racks
• 72 test tubes, 18 × 150 mm
• 72 stoppers to fit test tubes after the stoppers have been covered with aluminum foil (can also use Saran wrap with rubber band; tube can remain upright)
• Isopropyl alcohol, 500 ml
• Benzophenone, 250 g
• Glacial acetic acid, 12 dropper bottles
• Various sunscreen products with good range of SPF values (students provide their favorite products)
• 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 as a backup for a cloudy day)
• 24 stirring rods
• 12 spatulas
• Tare paper or 50 ml beakers
• Balances

Background Notes

Ultraviolet (UV) radiation entering the upper atmosphere has frequencies of 200–400 nm. The upper atmosphere absorbs out UV in the range of 200–290 nm which is the most energetic of the UV radiation. Oxygen in the outer part of the earth's atmosphere absorbs the shorter wavelengths of electromagnetic radiation from X-rays through UV. In turn, some of the oxygen is converted into ozone (O₃) that will also absorb UV in the 200–250 nm range. Of the UV reaching the earth's surface, 10% is between 290 and 320 nm that will interact with the human skin causing either sunburn or tanning. The remaining 90% of the UV reaching earth is in the range of 320-400 nm and is the lowest energy UV. In this particular experiment, benzophenone has maximum absorption of UV at 284 and 340 nm.

The chemistry of this experiment involves the reaction of benzophenone with isopropanol that depends on the absorption of UV light. The conversion products include benzopinacol produced by the photochemical reduction of 2 molecules of benzophenone. A second product is acetone, the oxidation product of 1 molecule of isopropanol. The benzopinacol is the insoluble product that is removed when the solution is filtered. The less light transmitted through the test tube, the less there is of product formed.

Reaction of benzophenone and isopropanol in the presence of UV radiation.

Alternative Activity

Instead of using a stoppered test tube to hold the benzophenone/isopropyl solution mix, you can substitute glass Petri dishes (100 × 15 mm). The dish can be sealed with silicone grease. The Petri dish will hold 30 ml of solution. Place the 3.00 grams of benzophenone in the test tube, 18 × 150 mm. Add the 20 ml of isopropanol and place in warm water (400°C). Stir carefully with a NON-plastic rod (glass, wood, etc.) until all the benzophenone has dissolved. The bottoms of the Petri dishes need to be marked to identify what treatment is being used. The lids, including the overhang, should be evenly coated with the sunblock material or other substance being used (baby oil, zinc oxide, etc.). When the dishes are in place outside (on some kind of solid but movable surface such as a sheet of corrugated carboard), the dissolved benzophenone can be poured into the dishes, then covered. Results should occur in 6 hours or less in full sunshine.

Typical data for a 4–5-hour exposure in full afternoon sun include the following:

Test tube, control = 1.07 g of product
Petri dish, control = 1.83 g of product
Test tube, sunblock, SPF 8 = 0.46 g of product
Test tube, sunblock, SPF 30 = 0.27 g of product

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

Relevant National Science Education Standards

Unifying Concepts and Processes — The activity involves evidence, explanations, change, constancy, and measurement.

Science as Inquiry — The activity teaches and reinforces the abilities necessary to do scientific inquiry and reinforces understanding of scientific inquiry.

Physical Science — The activity involves chemical reactions and the interactions of matter and energy.

Earth and Space Science — The activity involves energy in the earth system.

Science and Technology — The activity involves understanding of science and technology.

Science in Personal and Social Perspectives — The activity explores personal and community health, environmental quality, and natural and human-induced hazards.

History and Nature of Science — The activity explores science as a human endeavor and the nature of scientific knowledge.

Relevant New Jersey State Science Education Standards

5.1	The activity is centered on conducting systematic observations, interpreting and analyzing data, drawing conclusions, and communicating results.
5.3	The experiment is quantitative, requiring students to calculate the extent of the photochemical reaction of benzophenone and acetic acid in each test by measuring the amounts of products. Stoichiometric calculations are essential to this task.
5.4	Students should understand how the science of photochemistry is applied to the practical technology of sunscreens and sunblocks.
5.6	The atomic and molecular nature of matter are central to this investigation of a photochemical reaction, as is the behavior of matter at the molecular level during the reaction.

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.