In the Laboratory
Experimental Determination of Ultraviolet Radiation Protection of Common Materials
W
Susana C. A. Tavares and Joaquim C. G. Esteves da Silva CIQ(UP), Department of Chemistry, Faculdade de Ciências da Universidade do Porto, 4169-007 Porto, Portugal João Paiva* CFC(UP), Department of Chemistry, Faculdade de Ciências da Universidade do Porto, 4169-007 Porto, Portugal; *
[email protected]
The problems related to the ozone layer are increasingly becoming a social concern and are widely discussed in the media. The ecological and social impact of the growing depletion of the ozone layer has assumed enormous proportions. Chemistry teaching, increasingly integrating the four-sided relationship of science–technology–society–environment, needs to reflect this relationship and use creative laboratory activities that motivate the student and enable meaningful learning. Many proposals have been developed under this theme, mainly concerned with the protective effect of sunscreens (1–4), but also with general environmental issues (5). This article proposes four simple experiments to demonstrate the protection from UV radiation provided by different systems and constitutes a simple simulation of the protection provided by the ozone layer. The following protection systems were studied: (i) plexiglass plates obtained from a plastic materials store or general scientific supply store (plexiglass absorbs UV radiation and plates of this material were used to simulate the ozone layer); (ii) sunscreens; (iii) several cotton cloths of different colors (simulating clothing that protects skin from sun exposure); and (iv) natural organic matter (a material that strongly absorbs UV radiation) that was extracted from soil or can be obtained from any chemical manufacturer. The study of the sunscreen UV protection effect was carried out by the application of a thin uniform film of cream over a glass plate support. The aqueous solutions of natural organic matter were prepared by weighing solid fulvic acids extracted from soil (6) followed by dissolution in deionized water. Aqueous solutions of natural organic matter can also be obtained by filtering a mixture of different quantities of soil and water. The source of radiation was a 400 W Philips mercury lamp that is commercially available in any specialized lamp store. This lamp has a maximum emission output at 254 nm and is accompanied by several other sharp lines that are distributed over the ultraviolet to green region (313, 365, 405, 436, and 546 nm). Consequently, this lamp shows emission in the dangerous UV-B wavelength range (290 to 320 nm). It is this radiation that reaches the earth’s surface as consequence of the ozone-layer depletion. The UV radiation exposure was monitored by following the oxidation of iodide to triiodide (2I− → I2 + 2e−; I2 + I− → I3−) by UV light (7). Aqueous iodide solutions show an intense absorption in the UV and generate free iodine on illumination with mercury lamps (7). The color change of the iodide solution is from colorless to yellow when irradiated, allowing the effect of different UV protection systems to be evaluated. The students can follow this experiment easily and no specialized lab equipment, sophisticated calculations, or difficult concepts are required. This experiment may be undertaken by secondary and university students using either a qualitative (color) or quantitative (“absorbance”) approach.
Experiment Aqueous solutions (1 mol∙L) of potassium iodide (Merck) were used as the UV irradiation indicator. Typically, 25 mL of the solution is placed in a 100 mL beaker wrapped in aluminum foil. The open top of the beakers was then covered by either plexiglass plates, glass plates used as sunscreen support, cotton cloth of different colors, or a 250 mL beaker containing 100 mL of an aqueous solution of natural organic matter. The color change of the indicator solution (increase of yellow color) was monitored at the end of one hour of exposure time. This exposure time was optimized for both the type and power of UV lamp available in the laboratory and for the iodide concentration of the indicator solutions. The experimental conditions will need to be optimized if other UV lamps or other times of exposure are used. Four different types of experiments were carried out with (i) 3 mm plexiglass plates used on top of each other to increase the protection level (1 to 4 plates); (ii) four sunscreens with different protection levels (SPF of 10, 20, 30, and 60); (iii) cotton cloth of different colors (blue, pink, white, green, red, and black); and (iv) aqueous solutions of different concentrations of natural organic matter (10, 25, 50, 80, and 100 mg∙L). Samples were irradiated for one hour (which fits within the normal class time) with a 400 W Philips mercury lamp that was placed 20 cm above the indicator solution. The irradiation was carried out inside a fume hood whose windows had been darkened. The quantity of photochemically generated triodide was monitored at 352 nm with a UV–vis spectrophotometer Unicam, Heλios γ, using 1 cm plastic cells. The statistical analysis of variance (ANOVA) of the UV radiation protection effect of the four factors under analysis was done using Excel (Microsoft). Hazards Extreme caution should be taken with the UV lamp, which cannot be looked at directly by the students. Standard precautions should be taken when handling reagents (glass), especially with the secondary students. Results Some typical experimental results obtained for the four protection factors are shown in Tables 1 to 4. This information can be easily converted into bar charts and graphs. As expected, a global analysis of the results shows that the 352 nm absorbance of the indicator solution decreases (yellow color of the solutions decreases) with increasing thickness of plexiglass plates; increasing sunscreen protection factor; the cotton cloth colors, black ≈ red ≈ green > white > blue ≈ pink; and increasing concentration of the natural organic matter solution. The ANOVA results
www.JCE.DivCHED.org • Vol. 84 No. 12 December 2007 • Journal of Chemical Education 1963
In the Laboratory
(Tables 1 to 4) show that the protection factors have a statistically significant effect on the UV protection (F >> Fcritical). The results observed with the cotton cloth (Table 3) should be interpreted with caution because, besides the absorption of radiation by the coloring dye, there is also a marked scattering of light. These two effects result in the highest protection factor being seen for the cotton cloth when compared with the other three factors. Nevertheless, this result is meaningful and demonstrates to the secondary students the importance of wearing some clothing to protect their skin from sunlight. The results observed for the other three protection systems were as expected, giving higher UV absorption for the thickest plexiglass plate (Table 1); the higher SPF factor of the sunscreen (Table 2); and the highest concentration of the natural organic matter in solution (Table 4). Conclusions A “good laboratory idea” may contain various pedagogic factors that can be applied in different ways. The experiments described here may be used either in a directed or more flexible Table 1. Typical Absorbance Values as a Function of the Plexiglass Plate Width Width of the Plexiglass Plate/mm
Test 1 2 3
0
3
6
9
12
0.952 0.950 0.955
0.912 0.912 0.915
0.872 0.873 0.867
0.849 0.843 0.841
0.816 0.818 0.815
Note: The ANOVA results are F = 1118.5 and Fcritical = 3.5.
Table 2. Typical Absorbance Values as a Function of the Sunscreen Protection Factor Sunscreen Protection Factor
Test 1 2 3
0
10
20
30
60
0.832 0.813 0.819
0.638 0.621 0.620
0.535 0.558 0.551
0.526 0.512 0.523
0.475 0.461 0.474
Note: The ANOVA results are F = 689.9 and Fcritical = 3.5.
Table 3. Typical Absorbance Values as a Function of Different Color Cotton Cloth Test 1 2 3
Color Blue
Pink
White
Green
Red
Black
0.453 0.459 0.452
0.434 0.430 0.432
0.317 0.316 0.318
0.147 0.145 0.144
0.135 0.136 0.137
0.139 0.141 0.136
Table 4. Typical Absorbance Values as a Function of the Concentration of the Natural Organic Matter Solution
1 2 3
Acknowledgements The authors thank the reviewers for important suggestions and corrections. WSupplemental
Note: The ANOVA results are F = 14,064.7 and Fcritical = 3.5.
Test
manner, using the proposal of the problem and a list of material available. For example, the problem might simply be stated as, “How can we simulate the ozone layer depletion and its effects using the iodide to triiodide transformation as an indicator of UV radiation exposure in the presence of plexiglass plates, sunscreens, cotton cloths, and aqueous solutions of natural organic matter as UV radiation protectors?” Here, the students construct their own experimental procedure, with feasible alternatives. These experiments were carried out as a pilot experiment with first-year university general chemistry students and with 13–15 year-olds in secondary school. For the secondary students, some information was omitted or simplified. The use of bar charts and graphs to represent Tables 1 to 4 is recommended. In both cases, the impact of the laboratory experiments on the student’s motivation was very positive. We had some interesting comments from students, “Chemistry in this way is more fun, comprehensive and useful”, said one of the 30 first-year university students who participated in the pilot experiment. A secondary student (25 students in the pilot case) also remarked about the motivation increase, “I did not like chemistry too much, but that activity brought a new and better approach to understand and experiment science”. Those comments from students where collected in an open question during the final evaluation at the end of the activities. We do not have quantitative and operational evidence about motivation and understanding improvements with these experiments, but students stressed qualitatively that this new way of learning motivated them toward the study of the chemistry. We hope to develop more systematic studies of the impact of this type of experiment on students. In particular, it would be interesting to carry out this experiment with a group of secondary students and then use the same experiment with the same group at the first-year university level to see how the students apply their newly acquired calculation skills, laboratory expertise, and conceptual “weapons” to this problem.
Concentration of Natural Organic Matter Solution/(mg/L) 10
25
50
80
100
0.859 0.837 0.846
0.808 0.809 0.807
0.711 0.717 0.718
0.604 0.601 0.607
0.519 0.513 0.515
Note: The ANOVA results are F = 1845.7 and Fcritical = 3.5.
Material
Additional details of the experimental procedure, student protocols, experimental evaluation, and a complementary photo gallery are available in this issue of JCE Online. Literature Cited 1. Kimbrough, Doris R. J. Chem. Educ. 1997, 74, 51. 2. Walters, Christina; Keeney, Allen; Wigal, Carl T.; Johnston, Cybthia R.; Cornelius, Richard D. J. Chem. Educ. 1997, 74, 99. 3. Lawrance, Glen D.; Fishelson, Stuart. J. Chem. Educ. 1999, 76, 1199. 4. Fujishige, Shouei; Takizawa, Sumiko; Tsuzuki, Kaoru. J. Chem. Educ. 2001, 78, 1678. 5. Klemm, Otto. J. Chem. Educ. 2001, 78, 1641. 6. Silva, Joaquim C. G. Esteves da; Machado, Adélio A. S. C.; Oliveira, César J. S. Environm. Tox. Chem. 1998, 17, 1268. 7. Jortner, Joshua; Levine, Raphael; Ottolenghi, Michael; Stein, Gabriel. J. Phys. Chem. 1961, 65, 1232.
1964 Journal of Chemical Education • Vol. 84 No. 12 December 2007 • www.JCE.DivCHED.org
