A PRECAUTION IN THE USE OF MAGNETIC STIRRERS MOSES PASSER University of Minnesota, Duluth, Minnesota
STIRRING hars for magnetic stirrers are available unsealed, or sealed in one of a variety of coatings: glass, Teflon, Kel-F, or a hard thermosetting plastic. Some time ago we carried out a RastSoltys camphor molecular-weight determination using vegetahle oil in a tallform beaker as the bath, which was internally heated by a coiled resistance wire and stirred by a magnetic stirrer with a hard-plastic-coated stirring bar. The sample was enclosed in a sealed, thin-walled, melting-point capillary attached to the thermometer. The heating proceeded normally and no unusual phenomena were observed until the temperature reached about 170°C. Then suddenly a violent explosion took place, spattering hot oil onto the face and neck of the operator. (Fortunately, the wearing of safety glasses was a routine requirement in the laboratory, and since the liquid was vegetable oil rather than the sulfuric acid sometimes used, the resulting burns were not permanent.)
The following observations indicate strongly that it was the plastic housing of the stirring bar, rather than the sealed capillary, which exploded: (1) Only the bottom of the beaker was shattered, and it was sheared off neatly, leaving the cylindrical beaker wall intact. (2) The plastic housing of the stirring bar was shattered. (3) The thermometer was not damaged. (4) Later experiments with the same compound in which Teflon-coated and Kel-F-coated stirring hars were used did not result in explosions. We would therefore suggest that for bigh-temperature work in inert solvents only uncoated magnetic stirring hars be used, and where a protective coating is necessary that it be a soft material which will tear rather than shatter when subjected to the high pressures caused by trapped water vapor, air, or othergases.
