In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
Build a Simple Polarimeter
Frank E. Stary* and Norman Woldow Department of Chemistry, Maryville University, St. Louis, MO 63141-7299; *
[email protected] Polarimeters are useful in studying samples with optical activity such as sugars and amino acids. Low-cost polarimeters (1–3) could be introduced early in high school. Instruments built according to the following design cost about $10 each and may be constructed in a day in a home workshop by a high-school student with woodworking experience and modest soldering skills. All parts are readily visible to the student, so the unit is not treated as a “black box”. The unit was tested by measuring the rate of mutarotation of glucose (4 ). It is important for students to understand that good results can be obtained from homemade instruments. For an assembly schematic, see Figure 1.
1 2
"
2 3 4 5
Parts List The numbers in the following list refer to the corresponding numbers in Figure 1.
6 13" 7
1. Three round-head brass screws 1 in. long (No. 4)
1 2
2. One circular 4 ⁄2-in. diameter 360° protractor photocopied on the bottom side of transparency film used in overhead projectors 1
9 10 11 12 13
4. Circular 4-in.-diameter, 1⁄2-in.-thick piece of plywood
6. One 13-in.-long PVC pipe of 4-in. inside diameter, 1 ⁄4-in. thick
"
8
3. One circular 41⁄2-in.-diameter, 5⁄32-in.-thick piece of plywood cut out with a jigsaw or a hole saw (which would be too small by the width of the saw kerf ) 5. Analyzing filter (polarizing filters from Frey Scientific1 item no. G990226; Nichol prisms are also possible, see Edmund Scientific2 too)
1
Figure 1. Assembly schematic for polarimeter. Numbers refer to numbered parts list.
7. Three flat-head brass screws 1 in. long (No. 8) 8. Polarizing filter (see part #5) 9. Circular 4-in.-diameter, 1⁄2-in.-thick piece of wood 10. One yellow light-emitting diode (LED) 585 nm, spectral line half-width 30 nm. See the specification sheet from Industrial Devices, Inc. Edgewater, NJ 07020. In comparison, the standard sodium D line is about 589 nm. 11. One 680-Ω resistor or one 360-Ω resistor (for higher light output but shorter life of the LED) 12. One 1⁄2-in.-diameter momentary contact switch 13. One 9-V battery (a battery clip would allow the battery to be disconnected for storage)
Construction Connect parts 2, 3, and 4 together with the screws. Glue part 5 to part 4. Glue part 8 to part 9. Insert part 9 into part 6 and attach with screws. Insert parts 10–13 into part 9. Soldering is best. The longer lead from part 10 goes to part 11. Epoxy may be used to attach part 13. A 1⁄2-in. hole near the bottom of part 6 allows mounting of part 12. The cell is a 100-mL graduated cylinder. For advanced polarimeters, see refs 5–7. 644
Notes 1. Frey Scientific, 100 Paragon Parkway, Mansfield, OH 44903; www.freyscientific.com; 1-800/225-3739. 2. Edmund Scientific, 60 Pearce Ave., Towanda, NY 141506711; www.edmundscientific.com; 1-800/728-6999.
Literature Cited 1. 2. 3. 4.
Kapauan, A. F. J. Chem. Educ. 1973, 50, 376. Shavitz, R. J. Chem. Educ. 1978, 55, 682. Vennos, M. S. J. Chem. Educ. 1969, 46, 459. Bettelheim, F. A. Experimental Physical Chemistry; Saunders: Philadelphia, 1971; pp 269–277. 5. Isaacs, N. S. J. Chem. Educ. 1983, 60, 607–610; double-beam polarimeter, 0.06°. 6. Instruments for Research and Industry, Inc.; http://www. isquaredr.com (accessed Mar 2001); student polarimeters, $266 for 1°. 7. Rudolph Research Analytical; http://www.rudolphresearch.com (accessed Mar 2001); research-grade instruments, cost thousands of dollars for 0.02°.
Journal of Chemical Education • Vol. 78 No. 5 May 2001 • JChemEd.chem.wisc.edu
