In the Laboratory
Cost-Effective Teacher
An Inexpensive Water Jacket for a Polarimeter Tube F. A. Kundell* and W. A. Adkins Henson School of Science and Technology, Salisbury State University, Salisbury, MD 21801; *
[email protected] The acid-catalyzed inversion of sucrose is a traditional physical chemistry experiment. The progress of the reaction is normally followed for between one and two hours by the change in optical rotation using a polarimeter. We use a Polyscience half-circle model SR-6 with a 200-mm sample tube and a halogen lamp. If the sample tube is left in the polarimeter with the lamp on, the temperature of the sample solution will increase by as much as 10 °C in an hour. To maintain a “somewhat” constant temperature we remove the tube between readings and/or turn off the lamp. Ideally, we should use a water-jacketed sample tube but they are prohibitively expensive.
epoxy
water jacket
epoxy washer
Temperature Study Using the Water Jacket A temperature study showed that the tube functions quite well at temperatures at or above room temperature. The results of the temperature study can be found in Table 1. In
1516
ports
Figure 1. Polarimeter tube with water jacket.
Table 1. Temperature Study
Design and Construction of the Water Jacket In attempting to build a water-jacket for a conventional 200-mm sample tube, we found that several, but not all, brands of 3⁄4 -inch (190-mm) chlorinated polyvinyl chloride (CPVC) pipe fit snugly over the smaller end (toward the lamp) of the sampling tube (Fig. 1). We cut a 92-mm length of pipe and drilled two holes, which snugly fit two 25-mm lengths of stainless steel tubing used as water ports. We used stainless steel because it was available in the laboratory, but glass or clean copper tubing should also work. One end of the CPVC pipe was bonded to the center support using Dexter Epoxi-Patch. The other end of the CPVC pipe was on the metal shoulder of the tube and was also sealed with epoxy. Since CPVC pipe is relatively thin, an epoxy mound was placed around each port and topped with a steel washer, and the excess was wiped off. After the epoxy was set, the water jacket was sanded and painted to give a more professional appearance. The jacketed tube should be tested before use in the polarimeter. We used a constant-temperature bath with a SingleSyringe Pump (GRI – Model 14251-001) pumping at a rate of about 500 mL per minute. Water flows two- to three-times this rate would also be appropriate. It is important to have the water flow from the bath through the polarimeter tube to the pump and then back to the bath. In that way the pump is not changing the water temperature before it arrives at the polarimeter tube. Since only the lamp end of the polarimeter tube is jacketed, we were apprehensive about using this halfjacketed tube at temperatures markedly different from room temperature, even though studies at several temperatures would allow for the determination of the activation energy.
epoxy
Temperature/°C Bath
Jacket End
Non-Jacket End
Angle
46
45
43
45° eyepiece up
36
35
34
45° eyepiece up
26
26
25
45° eyepiece up
15
16
21
10° eyepiece down
0
2
20
10° eyepiece down
Note: Room temperature was 21 °C.
doing the temperature study we had the polarimeter tilted at about a 45° angle from the horizontal (eyepiece up). This facilitated the convective flow of the sample fluid in the tube while not significantly interfering with the operation of the polarimeter. During the determinations the tube holder and the partially exposed tube were covered with an insulating cloth (comparable to a washcloth folded in half ). For temperatures below room temperature convection was impaired by the inability of the polarimeter to adjust to a 45° angle with the eyepiece down. In addition, a polarimeter in such a position would be very difficult to read. The temperatures were determined by removing the polarizing plate and other parts from the end of an older (broken) polarimeter and inserting a thermometer. The readings for the jacket end of the polarimeter tube should be accurate. However, for temperature readings from the nonjacketed end, the presence of the thermometer at the end of the tube inhibited convection and thus probably resulted in a somewhat low reading, particularly at 45 °C. If the jacket is properly constructed, leakage can only occur with the degradation of the epoxy. In our use we have not seen such degradation. However, we would be cautious for applications where the water jacket is in constant use. Within these limitations, we have found that the jacketed polarimeter tube works very well even when the lamp remains on, and the price is right.
Journal of Chemical Education • Vol. 78 No. 11 November 2001 • JChemEd.chem.wisc.edu
