Major Chemical Concept
Level
Expected Student Background
Time
Safety
Materials
Background Notes
Alternative Activity
Relevant National Science Education Standards

Major Chemical Concept

Various sunscreen products absorb ultraviolet radiation in the range of 250–320 nm (2500–3200 Angstroms) as does a reference compound, benzophenone.

Level

This activity is an optional lab that could be done with honors students or as a research product to produce quantitative results compared with the qualitative ultraviolet lab utilizing sun-sensitive paper as the reacting medium.

Expected Student Background

This lab can be used with students with varying backgrounds including some understanding of organic structural formulas, redox, and molecular transformation. Terminology such as electromagnetic radiation, wavelength, and units of Angstroms should be familiar to students.

Time

A minimum of 45 minutes for set-up and another 45 minutes for follow-up to mass products. The experiment itself should run for a minimum of 4 hours.

Safety

1. Wear safety goggles or sunglasses if in sunlight or UV light.
   
2. Caution students not inhale any materials used.
   
3. No open flame should be in lab while working with the isopropyl alcohol.
   
4. Students should wear latex gloves when handling benzophenone and glacial acetic acid.

General Safety Guidelines

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 product)
• Zinc oxide (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)
• 24 stirring rods
• 12 spatulas
• Tare paper or 50 ml beakers
• Balances

Background Notes for the Teacher

1. Ultraviolet (UV) radiation entering the upper atmosphere is in the range 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 in the range of 290-320 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.

2. The chemistry of this experiment involves the reaction of benzophenone with isopropyl alcohol 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 isopropyl alcohol. The benzopinacol is the insoluble product that is removed when the solution is filtered. (While The Merck Index says that benzopinacol is soluble (39 parts) in boiling isopropanol, it is insoluble in isopropanol at room temperature.) The appearance of benzopinacol is that of a white precipitate—partially translucent like napthalene crystals. The less light transmitted through the test tube, the less there is of product formed.
   

3. For 3 grams of benzophenone used in this experiment, the maximum product is 3.02 grams. To make comparisons between the effectiveness of various sunscreen products, it is best not to allow the uncovered control tube to run to 100% completion (3.02 g). To determine this maximum time, a pre-class run should be made of 5 control tubes that are terminated at various intervals up to 24 hours to establish time parameters for the students. Otherwise, allow students to do this preliminary determination as part of an investigative aspect to the exercise.

4. Crystalline solid benzophenone can be ordered for a reasonable price from Aldrich Chemical (500 g/$25; tel. 1-800-558-9160) or Flinn Scientific (100 g/$18.30; tel. 1-800-452 1261). The benzophenone may be used as is without additional purification.

5. Culture tubes that have a screw top could be substituted for regular test tubes with stopper. A small piece of plastic wrap over the tube is then closed with the screw top for a leak-proof "stopper". Culture tubes with screw top are available from Flinn (tel.# 800 828 7777).

6. The purpose of the glacial acetic acid is to prevent any alkali derived from the glass (tube) from destroying the reaction product. Unchecked, alkali will cleave into benzopincol into benzhydrol and benzophenone.

7. If using sunlight as a UV source it is best to carry out the activity outdoors. Reaction rates will vary depending on outdoor temperature (a good opportunity to do some original student research). Also, you can try this experiment indoors with sunlight coming through the window. However, you need to check to see what wavelengths of UV are blocked by glass. Alternately, you can also use a UV light source other than sunlight. Again, you need to know what wavelengths you are working with—another good activity for student investigation. Benzophenone absorbs UV in the 3200 Ĺ range.

   Many high school science departments have UV lamps which operate within several narrow bands of the UV spectrum. Should you not have access to some science equipment at your school, you can purchase a UV bulb in the lighting section of hardware store or home improvement center. These are 18 inches in size for which you will need a proper bulb holder or fixture. Alternatively you can order an entire UV lamp assembly from Eductional Innovations (1-888-912-7474, www.teachersource.com). The flourescent UV (or black light) part number is: #UV-600 ($24.95) which includes bulb plus fixture. Also, you can buy complete UV lamp systems at record shops and other places that sell posters, gizmos, and minerals that fluoresce under UV light.

Alternative Activity

Instead of using a test tube with a stopper 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 3.0 grams of benzophenone in the test tube, 18 × 150 mm. Add the 20 ml of isopropyl alcohol and place in warm water (40°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 chemical 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 cardboard), 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

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 understandings about scientific inquiry.

Physical Science — The activity includes 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 understandings about 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 delves into science as a human endeavor and the nature of scientific knowledge.

USDA UV-B Monitoring Program — contains information on UV radiation, especially UVB, from the United States Department of Agriculture.

References

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.

This activity was created by Ron Tempest, Germantown Academy, Fort Washington, Pennsylvania.