expense of an explosion-proof refrigerator, a modified sparkproof refrigerator is sometimes found in older laboratories and laboratories using very small amounts of flammable materials. However, a modified spark-proof refrigerator cannot meet the standards of an explosion-proof refrigerator. Where they exist, a plan to phase them out is recommended.

Sparkproof refrigerators must have had the following modifications:

   Interior light and switch mounted on the door frame, if present, have been removed.

   Contacts of the thermostat controlling the fan and temperature have been moved outside the refrigerated compartment.

Permanently attach a prominent sign warning against the storage of flammable substances to the door of an unmodified refrigerator. Frost-free refrigerators are not suitable for laboratory use, owing to the problems associated with attempts to modify them. Many of these refrigerators have a drain tube or hole that carries water (and any flammable material present) to an area adjacent to the compressor and thus present a spark hazard. The electric heaters used to defrost the freezing coils are also a potential spark hazard (see section 7.C.5). To ensure its effective functioning, defrost a freezer manually when ice builds up.

Never place uncapped containers of chemicals in a refrigerator. Caps 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 do not meet this criterion, and their use is discouraged. The most satisfactory temporary seals are normally screw caps lined with either a conical polyethylene 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, use catch pans for secondary containment.

Careful labeling of samples placed in refrigerators and freezers with both the contents and the owner’s name is essential. Do not use water-soluble ink; 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.

7.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 often used in laboratory chemical hoods, and they must be operated such that they do not provide an ignition source for flammable vapors. Consider the use of air-driven stirrers and other spark-free devices. Furthermore, it is important that, in the event of an emergency, such devices can be turned on or off from outside the laboratory chemical 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 7.C.1 and 7.C.5.)

Use only spark-free induction motors in power stirring and mixing devices or any other rotating equipment used for laboratory operations. In some cases these devices may be required by fire and electrical codes. 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 switch 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 chemical hood, this modification ensures that the switch will also be outside. Do not control the speed of an induction motor operating under a load 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, consider the consequences of stirrer failure, electrical overload, or blockage of the motion of the stirring impeller. In good practice a stirring impeller is attached to the shaft of the stirring motor with lightweight rubber tubing. If the motion of the impeller is impeded, the rubber can twist away from the motor shaft, and the motor will not stall. Because this practice does not always prevent binding of the impeller, it is also desirable to fit unattended stirring motors with a suitable fuse or thermal protection device. (Also see section 7.C.1.) Take care when attaching an impeller shaft to an overhead motor. If the attachment fails, the impeller shaft could fall through the bottom of a glass vessel below, risking flying glass and a spill.

7.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 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

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