because of acute or chronic toxicity unless special precautions have been taken to ensure continuous venting of the atmosphere inside the oven. (See Vignette 7.2.) Thus, do not dry most organic compounds in a conventional unvented laboratory oven.

To avoid explosion, do not dry glassware that has been rinsed with an organic solvent in an oven until it has been rinsed again with distilled water. Potentially explosive mixtures can be formed from volatile substances and the air inside an oven.

Bimetallic strip thermometers are preferred for monitoring oven temperatures. Do not mount mercury thermometers through holes in the tops of ovens with the bulb hanging into the oven. If a mercury thermometer is broken in an oven of any type, close the oven and turn it off immediately to avoid mercury exposure. Keep it closed until cool. Remove all mercury from the cold oven with the use of appropriate cleaning equipment and procedures (see Chapter 6, section 6.C.10.8). After removal of all visible mercury, monitor the heated oven in a laboratory chemical hood until the mercury vapor concentration drops below the threshold limit value. (For information about reducing the use of mercury in thermometers, see Chapter 5, section 5.B.8.)

VIGNETTE 7.2
Muffle furnace fire

A laboratory specializing in the analysis of paint samples was asked to analyze pigmented polypropylene. The first step of the analytical protocol called for ashing the sample in a muffle furnace. The technician loaded the furnace with four crucibles containing a total of approximately 110 g of polypropylene. The temperature was set to ramp up to 900 °C. At approximately 500 °C a fire erupted from the furnace, which was quickly extinguished.

Two major contributing factors to the fire were identified. First, the technician had no experience with the analysis of polypropylene-containing samples and did not recognize that polypropylene begins to decompose at approximately 500 °C to low-molecular-weight olefins. Second, the amount of organic matter placed in the furnace in the form of the polypropylene samples was significantly more than that in the usual paint samples.

7.C.5.2 Hot Plates

Laboratory hot plates are often used when solutions are to be heated to 100 °C or higher and the inherently safer steam baths cannot be used as the source of heat. As previously noted, use only hot plates that have completely enclosed heating elements in laboratories. Although almost all laboratory hot plates currently sold meet this criterion, many older ones pose an electrical spark hazard arising from either the on/off switch located on the hot plate, the bimetallic thermostat used to regulate the temperature, or both. Normally, these two spark sources are located in the lower part of the hot plate in a region where any heavier-than-air and possibly flammable vapors evolving from a boiling liquid on the hot plate would tend to accumulate. In principle, these spark hazards are alleviated by enclosing all mechanical contacts in a sealed container or by using solid-state circuitry for switching and temperature control. However, in practice, such modifications are difficult to incorporate into many of the hot plates now in use. Warn laboratory personnel of the spark hazard associated with these hot plates. Set up any newly purchased hot plates to avoid electrical sparks. In addition to the spark hazard, old and corroded bimetallic thermostats in these devices can eventually fuse shut and deliver full continuous current to a hot plate. This risk can be avoided by wiring a fusible coupling into the line inside the hot plate. If the device does overheat, the coupling will melt and interrupt the current (see section 7.C.1).

On many brands of combined stirrer/hot plates, the controls for the stirrer and temperature control are not easily differentiated. Care must be taken to distinguish their functions. A fire or explosion may occur if the temperature rather than the stirrer speed is increased inadvertently.

7.C.5.3 Heating Mantles

Heating mantles are commonly used to heat round-bottom flasks, reaction kettles, and related reaction vessels. These mantles enclose a heating element in layers of fiberglass cloth. As long as the fiberglass coating is not worn or broken and no water or other chemicals are spilled into the mantle (see section 7.C.1), heating mantles pose minimal shock hazard. They are normally fitted with a male plug that fits into a female receptacle on an output line from a variable autotransformer. This plug combination provides a mechanically and electrically secure connection.

Always use heating mantles with a variable auto-transformer to control the input voltage. Never plug them directly into a 110-V line. Trained laboratory personnel should be careful not to exceed the input



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