In the Classroom
Tested Demonstrations
The Use of an Inexpensive Laser Pointer to Perform Qualitative and Semiquantitative Laser Refractometry submitted by:
Amarílis de Vicente Finageiv Neder,* Edgardo García, and Leonardo N. Viana Instituto de Química, Universidade de Brasília, C.P. 04478, CEP 70919-970 Brasília DF, Brazil; *
[email protected] checked by:
Ruiess Van Fossen Ramsey Department of Chemistry, Indiana University of Pennsylvania, Indiana, PA 15705-1001 Nadine Szczepanski Department of Chemistry, MacMurray College, Jacksonville, IL 62650-2590
Refractometry is an analytical chemistry technique based on the measurement of the refractive index of a material, a property that is useful for both identification of substances and determination of the composition of mixtures. Light travels at a different velocity in condensed phases than in vacuum, and the refractive index of a medium is defined as the ratio of the velocity of a monochromatic light in vacuum to its velocity in the medium being measured (1). The incorporation of laser experiments in the undergraduate curriculum has slowly increased (2). Few articles in the literature explore laser refractometry for beginners, and most of them employ costly He–Ne lasers as the light source (3, 4). We are aware of one paper in an electronic journal that mentions the use of a diode laser instead of a He–Ne laser; however its main concern is demonstrating the phenomenon of polarizability (5). The experimental setup described below is simpler and does not require the use of mirrors, NMR tubes, or other cylindrical pieces of glass with precision bores. This paper describes a simple low-cost laser refractometry experiment designed to be executed by inexperienced students or as a classroom demonstration. The purpose of this experiment is to estimate the refractive index (n) of various liquids using a 25-mL beaker and a regular pocket laser pointer. A laser beam that obliquely strikes the beaker containing a liquid is bent by an angle related to the index of refraction of the liquid, according to Snell’s law (6 ). An excellent overview of measuring refractive indices, which includes a discussion of why any refractions due to the prism walls are of no consequence, is available in the text by Edmiston (7). To increase the interest of the students, a problem is posed: they must identify a pair of organic compounds with similar chemical properties according to their relative refractive indices (n). To reach this goal, they choose a pair of chemically related substances for which the n values are found in a handbook (8) and then compare the relative locations where the laser beam hits a paper screen after being refracted by the two chosen substances and two reference substances. It is recommended that substances with very different n values be used as references—for example, water (1.332) and toluene (1.496). In this way any other substance with n within this range can be identified according to the proximity of the refracted beam to one extreme or another. This procedure enables one to distinguish between ethanol (1.361) and 1propanol (1.385), acetone (1.359) and butanone (1.379),
Screen
15 cm
Tangent Line Beaker
Normal Line
Laser Pointer
Figure 1. Diagram of experimental setup.
Figure 2. Complete experimental setup.
JChemEd.chem.wisc.edu • Vol. 78 No. 11 November 2001 • Journal of Chemical Education
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In the Classroom
cyclohexane (1.427) and hexane (1.375), and ethyl ether (1.353) and tetrahydrofuran (1.405), among many possibilities. The experimental setup is shown in Figures 1 and 2. We feel that this experiment provides a useful alternative to conventional methods that require the use of expensive equipment to perform laser refractometry experiments. Materials Common laser pointer Commercial-grade solvents Beaker (25 mL; any diameter will work) Watch glass Flat screen (glass or metal support), at least 15 cm long and 5 cm high Strip of white paper or masking tape (minimum length 15 cm) attached to the screen Small white label (ca. 1 × 1 cm) Copy of Figure 1 Scissors and tape
Experimental Procedure 1. Make an approximately 0.5-mm hole with the sharp point of a pencil in the middle of a white label (ca. 1 × 1 cm) and attach the label on the outside wall of the beaker, near its bottom. The laser beam will be collimated through this hole. 2. Attach a copy of the drawing shown in Figure 1 to the lab bench (see Fig. 2). 3. Place the beaker in such a way that the normal line passes through its center and its surface touches the tangent line, as shown in Figure 1. 4. Make three small marks with a permanent-ink pen on the outside wall of the beaker from its bottom (e.g., one near the intersection of the two straight lines and the two others coincident with some mark you draw in Fig. 1). This facilitates the replacement of the beaker in the original position after each liquid exchange. It is advisable to moisten the paper with a small quantity of water, so the beaker adheres more strongly to the paper and is not dislodged during the following steps. 5. Attach the strip of paper or masking tape (~15 cm long) to the base of a screen made of glass, metal, or any other material that allows easy removal of the paper or tape at the end of the experiment. 6. Place the screen on the lab bench top about 15 cm away from the beaker. 7. Using strips of tape, fix the laser pointer on the position specified on the drawing, as near to the hole in the label as possible, making approximately a 45° angle with the tangent and normal lines. It will remain in the same position during the whole procedure.
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8. Place the empty beaker on the bench following the directions given in steps 3 and 4. Turn on the laser and mark with a pencil the position where the beam strikes the screen. Make sure the hole in the label is positioned in the center of the laser’s light beam. If the image on the screen is a dash instead of a point, place a pencil mark in the middle of the dash. This mark will be used as a reference for adjusting the position of the beaker before each measurement. 9. Fill the beaker with enough water to cover the hole in the label, turn on the laser, and mark the location where the beam strikes the screen. Remember to keep the hole positioned in the center of the beam during this procedure. 10. Repeat steps 8 and 9 for toluene, covering the beaker with a watch glass to avoid inhaling solvent vapors. 11. Choose a pair of liquids to be identified and repeat steps 8 and 9 for each one (it is not necessary to wash the beaker after each measurement). Do not forget to cover the beaker with a watch glass to avoid inhaling solvent vapors during the measurement.
To obtain good results it is important to always replace the beaker in the same position and to keep the laser pointer in place—which gets easier with practice. A support for the laser pointer can easily be made from stiff paper or cardboard to which the laser is fixed with strips of tape. After the laser beam is adjusted, the base of the support is taped to the position shown on the drawing. The position of the beaker can also be fixed by attaching it to the paper using double-sided tape or rolled masking tape on its bottom. A Pasteur pipet is then used to remove the liquids after each measurement. A piece of paper towel may be used to remove any lingering solvent by capillary action. Hazards Avoid looking directly at the laser beam. Owing to the high vapor pressure and flammability of some liquids it is advisable to carry out the experiment in a fume hood or at least to cover the beaker with a watch glass and keep stock bottles well closed during execution. Make sure there is no open flame and that no one is working with matches in your vicinity. Do not use the bench sinks to dispose of flammable solvents. All the liquids tested can be stored in separate small bottles and reused, since low level contamination (up to 10%) does not interfere with the results. Discussion The refracted beam reaches the screen at a different point for each substance, enabling one to identify the liquids according to the proximity of the points to those of water or toluene. With the experimental setup shown in Figure 2 we were able to perform qualitative and semiquantitative refractive index measurements. The quantitative predictions for the refractive index values are significant only to the second deci-
Journal of Chemical Education • Vol. 78 No. 11 November 2001 • JChemEd.chem.wisc.edu
In the Classroom 1.50
Refractive Index
1.45
1.40
1.35
1.30 0
5
10
15
20
25
30
Distance / mm Figure 3. Linear correlation between literature refractive index (8) and measured refracted laser beam distance on the screen for the substances shown in Table 1.
Table 1. Refractive Index of Various Liquids Liquid
D/mma
Refractive Index, n Exptl ± 0.004b Lit. (8), 20–25 ºC
Water
0
1.3325
1.3325
Ethyl ether
4.8
1.358
1.3526
Ethanol
5.0
1.359
1.3611
Acetone
6.0
1.365
1.3588
Hexane
7.5
1.373
1.37506
2-Propanol
8.5
1.378
1.3776
Butanone
8.8
1.380
1.3788
tert-Butanol
10.0
1.387
1.3878
1-Propanol
11.0
1.392
1.3850
Isobutanol
11.5
1.395
1.394
1-Butanol
12.5
1.400
1.3993
Tetrahydrofuran
14.8
1.413
1.4050
Cyclohexane
17.0
1.425
1.4266
Cyclohexene
20.0
1.441
1.4465
Cyclohexanone
22.5
1.455
1.4507
Toluene
30
1.4961
1.4961
aRefracted laser beam distance relative to water reference point on the screen. bPredicted refractive index using the equation n = 0.0054D + 1.3325, fitted with data for water and toluene only.
mal place. Nevertheless, the experiment has the precision and simplicity required for the students to grasp the basic concepts of refractometry. The results in Figure 3 show an excellent linear correlation between the literature refractive index (n) (8) and the laser beam position on the screen (D) for all of the data points in Table 1. A correlation coefficient r = .9960 was obtained for the fitted linear function n = 0.0055D + 1.3300. It is important to stress that virtually the same equation (n = 0.0054D + 1.3325) is obtained upon fitting just two data points at the extremes of the range examined, those for water and toluene. Therefore only two measurements are necessary to find the linear equation that can be used to predict the refractive index for other materials. Comparison of the predicted values with literature values using this procedure (Table 1) indicates that the quality of the predicted values can be trusted to the second decimal place. This refractometry experiment is inexpensive and simple enough to be implemented in high school or undergraduate general chemistry laboratory classes. Even elementary school children can see how light bends. As a classroom demonstration, it works very well for a small audience (people have to be close to the experimental assembly to see the phenomenon in enough detail). Placing the screen farther from the beaker does not lead to better visualization because the spot from the laser light becomes very diffuse and the resolution decreases with increasing distance between the light and the screen. For larger groups, the use of a Web camera, a computer, or an LCD projector might be considered. Acknowledgments We are very grateful to Peter Bakuzis for his helpful comments during the preparation of this manuscript. We are also thankful for relevant comments by the referees. Literature Cited 1. Alberty, R. A.; Silbey, R. J. Physical Chemistry, 2nd ed.; Wiley: New York, 1997. 2. Steehler, J. K. J. Chem. Educ. 1990, 67, A65–A71. 3. Spencer, B.; Zare, R. N. J. Chem. Educ. 1988, 65, 835–836. 4. Hugues, E. Jr.; Jelks, V.; Hugues, D. L. J. Chem. Educ. 1988, 65, 1007–1008. 5. Van Hecke, G. R.; Karukstis, K. K.; Underhill, J. M. Chem. Educator 1997, 2 (5). We thank a referee for calling this publication to our attention. 6. Halliday, D.; Resnick, R. Physics, Part 2, 3rd ed.; Wiley: New York, 1978. 7. Edmiston, M. D. Phys. Teach. 1988, 24, 160. 8. Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1992–1993.
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