a spark hazard. The electric heaters used to defrost the freezing coils are also a potential spark hazard (see Chapter 5, section 5.G.1). To ensure its effective functioning, a freezer should be defrosted manually when ice builds up.

Uncapped containers of chemicals should never be placed in a refrigerator. Caps should provide a vapor-tight seal to prevent a spill if the container is tipped over. Aluminum foil, corks, corks wrapped with aluminum foil, and glass stoppers usually do not meet these criteria, and, therefore, their use should be discouraged. The most satisfactory temporary seals are normally screw-caps lined with either a conical polyethylene insert or a Teflon insert. The best containers for samples that are to be stored for longer periods of time are sealed, nitrogen-filled glass ampoules. At a minimum, catch pans should be used for secondary containment.

Careful labeling of samples placed in refrigerators and freezers with both the contents and the owner's name is essential. Water-soluble ink should not be used, and labels should be waterproof or covered with transparent tape. Storing samples with due consideration of chemical compatibility is important in these often small, crowded spaces.

6.C.4 Stirring and Mixing Devices

The stirring and mixing devices commonly found in laboratories include stirring motors, magnetic stirrers, shakers, small pumps for fluids, and rotary evaporators for solvent removal. These devices are typically used in laboratory operations that are performed in a hood, and it is important that they be operated in a way that precludes the generation of electrical sparks. Furthermore, it is important that, in the event of an emergency, such devices can be turned on or off from a location outside the hood. Heating baths associated with these devices (e.g., baths for rotary evaporators) should also be spark-free and controllable from outside the hood. (See sections 6.C.1 and 6.C.5; also see Chapter 5, section 5.C.7.)

Only spark-free induction motors should be used in power stirring and mixing devices or any other rotating equipment used for laboratory operations. Although the motors in most of the currently marketed stirring and mixing devices meet this criterion, their on-off switches and rheostat-type speed controls can produce an electrical spark any time they are adjusted, because they have exposed contacts. Many of the magnetic stirrers and rotary evaporators currently on the market have this disadvantage. An effective solution is to remove any switches located on the device and insert a switch in the cord near the plug end; because the electrical receptacle for the plug should be outside the hood, this modification ensures that the switch will also be outside the hood. The speed of an induction motor operating under a load should not be controlled by a variable autotransformer.

Because stirring and mixing devices, especially stirring motors and magnetic stirrers, are often operated for fairly long periods without constant attention, the consequences of stirrer failure, electrical overload, or blockage of the motion of the stirring impeller should be considered. It is good practice to attach a stirring impeller to the shaft of the stirring motor by using lightweight rubber tubing. If the motion of the impeller becomes impeded, the rubber can twist away from the motor shaft. If this occurs, the motor will not stall. However, this practice does not always prevent binding the impeller. Hence, it is also desirable to fit unattended stirring motors with a suitable fuse or thermal-protection device. (Also see section 6.C.1.)

6.C.5 Heating Devices

Perhaps the most common types of electrical equipment found in a laboratory are the devices used to supply the heat needed to effect a reaction or a separation. These include ovens, hot plates, heating mantles and tapes, oil baths, salt baths, sand baths, air baths, hot-tube furnaces, hot-air guns, and microwave ovens. The use of steam-heated devices rather than electrically heated devices is generally preferred whenever temperatures of 100 °C or less are required. Because they do not present shock or spark risks, they can be left unattended with assurance that their temperature will never exceed 100 °C.

A number of general precautions need to be taken when working with heating devices in the laboratory. First, new or existing variable autotransformers should be wired (or rewired) as illustrated in Figure 6.3. The actual heating element in any laboratory heating device should be enclosed in a glass, ceramic, or insulated metal case in such a fashion as to prevent a laboratory worker or any metallic conductor from accidentally touching the wire carrying the electric current. This type of construction minimizes the risk of electric shock and of accidentally producing an electrical spark near a flammable liquid or vapor (see Chapter 5, section 5.G.1). It also diminishes the possibility that a flammable liquid or vapor will come into contact with any wire whose temperature may exceed its ignition temperature. If any heating device becomes so worn or damaged that its heating element is exposed, the device should be either discarded or repaired to correct the damage before it is used again. Because many household appliances (e.g., hot plates and space heaters) do not meet this criterion, they should not be used in a

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