Materials Included In Kit

Additional Materials Required

Safety Precautions

n-Propyl alcohol is a flammable solvent and a dangerous fire risk—keep away from flames, sparks and other sources of ignition. Cetyl alcohol is slightly toxic by ingestion and is a body tissue irritant. Glycerine may irritate skin and eyes, and is an allergen to some. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Remind students to wash their hands thoroughly with soap and water before leaving the laboratory. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding.The excess lotion samples may be disposed of according to Flinn Suggested Disposal Method #26a. The excess n-propyl alcohol may be disposed of according to Flinn Suggested Disposal Method #18b.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.

Answers to Prelab Questions

Three sunscreen formulas are being tested for comparative effectiveness. A standard absorbance curve is made using known lotions of sun protection factors (SPF) of 4, 8 and 15. The spectrophotometer is set at 350 nm, and formulation and standard lotions samples were prepared by diluting 0.030 g of the lotion in 30 mL of n-propyl alcohol.

The graph of absorbance, A, versus SPF is shown in Figure 2.

1. Use the graph to determine the SPF of each of the sample formulations.

   Formula A has an SPF value of 10.4
   Formula B has an SPF value of 9
   Formula 2 has an SPF value of 2.4

   {12242_PreLabAnswers_Figure_3}
   Sample—Abs.
   SPF 4—0.101
   SPF 8—0.387
   SPF 15—1.409
   
   Formula A: 0.673
   Formula B: 0.493
   Formula C: 0.023

Sample Data

Answers to Questions

You will need class data to answer some of these questions.

1. Was your laboratory prepared suntan lotion effective in blocking UV radiation? Explain.

   All three formulations effectively block the transmittance of UV radiation in the range of 320 nm to 340 nm, but falls off dramatically from 350 nm to 400 nm.

2. How does the effectiveness of the sunscreen lotion change with the different concentrations of oxybenzone? Explain.

   As the concentration of oxybenzone increase, the transmittance of UV radiation decreases dramatically.

3. Using the class data, which suntan lotion formula was the most effective in blocking UV radiation? Explain.

   The formula using 0.60 grams of oxybenzone clearly blocked UV radiation most effectively, virually eliminating it below 340 nm.

References

Special thanks to David A. Katz, retired, Wilmington, DE, who provided Flinn Scientific with the instructions for this activity.

Introduction

Had a great day at the beach? Boating? Hiking? Working in the garden? Skiing? Did you get a sunburn? Did you use a sunscreen? In this lab, you will prepare a sunscreen lotion, then determine just how effective it is at screening the UV rays of the sun.

Concepts

• UV radiation
• Molecular absorbance

Background

The Sun, our nuclear furnace at the center of the solar system, produces a wide range of electromagnetic radiation. Although we are most aware of the visible light from the Sun, there are a number of types of radiation we cannot see. One of these is infrared light, a longer wavelength radiation which is responsible for heating our planet. Another is the shorter wavelength radiation called ultraviolet light. It is this ultraviolet light which causes suntans and sunburns and which increase the risk of developing basal cell carcinoma and malignant melanoma.

Ultraviolet light is divided into three wavelength ranges:

UVA is radiation in the wavelength range of 320–400 nm
UVB is radiation in the wavelength range of 290–320 nm
UVC is radiation in the wavelength range of 100–290 nm

UVC is totally blocked by the ozone layer in the earth's upper atmosphere. This ozone layer blocks some of the UVB, but all UVA radiation passes through this layer. While UVB is most often indentified as the cause of sunburn, some studies indicate that skin damage may also be attributed to UVA radiation.

Mild sunburn is a first-degree radiation burn which produces redening of the skin with accompanying pain. Generally, as the skin heals, redness may persist and the outer epidermis will peel within a week. Prolonged exposure can result in second-degree burns which are charactorized by blistering of the skin and severe pain.

One of the body’s defenses against UV radiation is the production of melanin, a pigment that results in the darkening of the skin. An individuals response to UV radiation and melanin production is dependent on skin color and other genetic factors. Individuals with dark skin, or whose skin readily produces melanin when exposed may still experience sunburn as a result of the high intensity of UV radiation and prolonged exposure.

Sunscreens are cosmetic formulations that block UV rays. Sunscreens are assigned sun protection factors, or SPF ratings that indicate a level of protection from UV radiation. The SPF rating is a multiplier that indicates the length one can safely remain in the sun. For example, if an individual whose unprotected skin becomes sunburn in 5 minutes, then an SPF-15 sunscreen should allow that person to stay in the sun for 15 x 5 minutes or 75 minutes, without burning.

Some active ingredients in sunscreens are:

• Benzyl salicylate and salicylate derivitaves. One of the first sunscreen agents. It provides UVB protection, but not UVA. It is not soluble in water and can be used in waterproof formulations. It is often used in combination with other ingredients. One of the derivative compounds is known as homosalate.
• Benzyl cinnamate and cinnamate derivatives. Another early sunscreen agent. It is an effective UVB blocker, but is not water-proof. Often found in combination with other ingredients.
• PABA (p-aminobenzoic acid). This compound was extensively used in many formulations, however, it was not water soluble and needed to be used in alcohol-based solutions, it would discolor fabrics, and many individuals experienced or developed allergic reactions to it. Most sunscreen lotions are now PABA free.
• Butyl methoxydibenzoylmethane and related compounds. Also known as Parsol 1789 and Parsol A, these compounds are effective UVA blockers. Oxybenzone is a related compound. 
• Zinc oxide and titanium dioxide are two inorganic compounds that are insoluble in most liquids. These block the UV radiation because their preparations are opaque to light. Sunscreen lotions containing these are normally white opaque ointments on the skin.

For this lab, we will study one of these molecules; 2-Hydroxy-4-methoxybenzophenone (also known as oxybenzone). The structure for oxybenzone is shown below in Figure 1.

Experiment Overview

In Part I of this experiment, you will prepare a sunscreen lotion varying the concentration of the active ingredient in the formulations. In Part II, we will evaluate the effectiveness of the various sunscreen formulas using a spectrophotometer in the 320 to 400 nm range and determine the relative ratings of the laboratory prepared sunscreen lotions. Although this UV radiation is in the UVA range, general trends in UV absorption can be observed allowing the sunscreen lotions to be qualitatively compared.

Materials

Prelab Questions

Three sunscreen formulas are being tested for comparative effectiveness. A standard absorbance curve is made using known lotions of sun protection factors (SPF) of 4, 8 and 15. The spectrophotometer is set at 350 nm, and formulation and standard lotions samples were prepared by diluting 0.030 g of the lotion in 30 mL of n-propyl alcohol.

The graph of absorbance, A, versus SPF is shown in Figure 2.

1. Use the graph to determine the SPF of each of the sample formulations. 
   {12242_PreLab_Figure_2}
   Sample—Abs.
   SPF 4—0.101
   SPF 8—0.387
   SPF 15—1.409
   
   Formula A: 0.673
   Formula B: 0.493
   Formula C: 0.023

Safety Precautions

n-Propyl alcohol is a flammable solvent and a dangerous fire risk—keep away from flames, sparks and other sources of ignition. Cetyl alcohol is slightly toxic by ingestion and is a body tissue irritant. Glycerine may irritate skin and eyes and is an allergen to some. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

This experiment is performed as a class project. Student groups will prepare a single formulation of a sunscreen lotion assigned from Table 1. Data and results from all individuals and/or lab groups will be combined for comparison.

Part I. Preparation of Sunscreen Lotion

1. Using a tared weighing boat, mass the quantities of cetyl alcohol, oxybenzone, stearic acid, and glycerin called for in your assigned formulation from Table 1. Add each to a clean 100-mL beaker.
2. Heat the beaker with the organic mixture in a water bath until all the ingredients have melted. Note: Cosmetic ingredients should not be melted over a direct flame or high heat because they may scorch or decompose if they are heated much above the boiling point of water.
3. Mass 10.4 g of water into a tared 100-mL beaker.
4. Using a tared weighing boat, mass 0.13 g of triethanolamine, then add this to the water. Stir. Heat the water solution to a temperature of 80 to 85 °C.
5. After the water solution has reached a temperature between 80 and 85 °C, remove it from the heat and slowly pour the melted cetyl alcohol, oxybenzone, stearic acid and glycerin mixture into the water a little at a time, stirring constantly. It may be helpful to hold the 100-mL beaker using a pair of beaker tongs. Note: If the “organic mixture” has solidified, heat briefly in the water bath to remelt it. If you pour too fast or if you do not stir, the emulsion will be lumpy or the mixture may not form an emulsion.
6. Continue stirring until you have a smooth, uniform paste. Label the beaker and set the sunscreen cream aside to cool. You will need this sunscreen mixture for Part II of this experiment.

Table 1. Sunscreen Lab Formulations
All sunscreen formulations will include cetyl alcohol, stearic acid, glycerin, triethylamine and water in a specific mass combination to provide the lotion part of the sunscreen Part II. Evaluating the Lotions
You will be evaluating the sunscreen you prepared in Part I of this experiment and your instructor will assign you or your lab group at least one other prepared sunscreen to evaluate.

The spectrophotometer should be turned on, set to 400 nm, and allowed to warm up for 15 minutes.

1. Prepare a 0.10% solution (w/v) of your assigned sunscreen lotion formula in n-propyl alcohol. Weigh 0.030 g of sunscreen lotion into a 100-mL beaker.
2. Add 30.0 mL of n-propyl alcohol to the beaker and stir to dissolve the lotion. Label the solution with the Formula number. Following the same procedure, prepare a second 0.10% solution of the sunscreen lotion preparation you were assigned in n-proply alcohol.
3. Fill a cuvet approximately ¾ full with the 0.10% solution of the sunscreen. Record the formula number. You may label the cuvet near the top using a waterproof marker or small piece of laboratory tape.
4. Fill a second cuvet ¾ full with the 0.10% solution of the assigned sunscreen lotion. Record the formula number. You may label the cuvet near the top using a waterproof marker or small piece of laboratory tape.
5. Fill a third cuvet full with n-propyl alcohol. This is your blank solution. You may label the cuvet near the top using a waterproofmarker or small piece of laboratory tape.
6. Place the cuvet with the n-propyl alcohol in the cuvet holder of the spectrophotometer and close the cell compartment cover. Adjust the instrument to read 100% transmittance.
7. Remove the blank and insert the cuvet containing the first sunscreen solution sample. Record the absorbance at 400 run.
8. Remove the sunscreen solution sample, and insert the cuvet containing second laboratory prepared sunscreen lotion. Record the transmittance at 400 nm.
9. Remove the sunscreen solution sample. Change the wavelength reading of the spectrophotometer to 390 nm. Place the blank in the sample holder, close the cover, and readjust the instrument to 100% transmittance.
10. Remove the blank and insert each of the cuvets containing the sunscreen solution samples. Record the transmittances at 390 nm.
11. Continue to take readings at 10 nm intervals to 320 nm. Remember to readjust to 100% transmittance the instrument using the blank solution at every wavelength before reading the sunscreen transmittance. Note: Some spectrophotometers may not give readings to 320 nm, if this occurs, stop at 330 nm.
12. After collecting all the data, graph the transmittance (y-axis) vs. the wavelength (x-axis) for each sunscreen sample using graph paper or a graphing program like Excel on one of the laboratory computers. If using Excel, save your data and graph files on the computer, as instructed by your instructor, and print out a copy of the graph.
13. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12242_Student1.pdf