Sand Bath Technique—Hot Plate Calibration

A most convenient piece of equipment for heating or stirring or for performing both operations simultaneously on a microscale level is the hot-plate-magnetic stirrer. Heat transfer from the hot surface to the reaction flask is generally accomplished with a crystallizing dish containing a shallow layer of sand that can conform to the size and shape of the particular vessel employed. The temperature (external) of the system is monitored by a thermometer embedded in the sand near the reaction vessel. 

The setting is determined from your control setting-temperature calibration curve.This procedure will allow the heated bath to reach a relatively constant temperature by the time it is required. You wiU then be able to make smaU final adjustments more quickly, if necessary. 

Heavy layers of sand act as an insulator on the hot-plate surface, which can damage the heating element at high temperature settings. When temperatures over 150 C are required, it is especially important to use the minimum amount of sand. 

Recording the weight of sand used and the size of the crystaUizing dish will help to make the graph values more reproducible. 

The high sides of the crystallizing dish protect the apparatus from air drafts, and so the dish also operates somewhat as a hot-air bath. Heating can be made even more uniform by covering the crystallizing dish with aluminum [www]H foil (see Fig. 3.12 and Chapter 3W, Fig. 3.1W).This procedure works well, but is a bit awkward and is required in only a few instances. 

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