In the Laboratory
An Alternative to Mercury Thermometers Alexander Nezlin Research and Development, Aminolab Ltd., P.O. Box 2407, Weizmann Science Park, Rehovot 76123, Israel;
[email protected] Mercury thermometers are still widely used in chemical laboratories. Nearly every chemist has had at some time to dispose of mercury spilled from a broken thermometer. Substitutes are available, alcohol and digital thermometers. Unfortunately their application is limited by the low boiling point of alcohol and by the susceptibility of metallic thermocouples to corrosion. A safe and simple technique is suggested. A thermocouple is inserted into a glass tube, which is closed at one end and filled with silicone oil. The glass tube should be at least as long as the thermocouple and of only slightly larger diameter. The glass tube containing the thermocouple is inserted into the reaction flask via a suitable adaptor (e.g., Aldrich thermometer adaptor). This setup is also adequate for experiments conducted under reduced pressure (e.g., vacuum distillation). In order to work under high vacuum, a male ground-glass joint of an appropriate size could be fused to the tube containing the thermocouple. The thermocouple is protected by glass from the corrosive
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action of the reaction medium. A small quantity of oil (1–2 mL) ensures even heat transfer. This setup has been employed successfully for several years. Variation in temperature readings between conventional thermometers and digital thermometers with a protected thermocouple has never exceeded 0.5 °C, even when the thermocouple was submerged to less that 1⁄4 of its total length. In cases that involve monitoring rapid temperature changes it is advisable to check the response time of the setup. However, for most common laboratory procedures an increase in response time is unimportant. For storage and handling convenience it is recommended that a female standard ground-glass joint be fused to the upper open end of the glass tube. The tube then can be closed with a standard stopper when not in use. In case of breakage one still has the thermocouple, which can be immediately put to further use by simply inserting it into another glass tube. Besides, disposing of 1 mL of silicone oil is much easier than disposing of an equivalent amount of mercury!
Journal of Chemical Education • Vol. 78 No. 3 March 2001 • JChemEd.chem.wisc.edu
