desired temperature (100 °C or more) of the vessel being heated. These baths should be constructed so that the heating element is completely enclosed and the connection to the air bath from the variable autotransformer is both mechanically and electrically secure. These baths can be constructed from metal, ceramic, or, less desirably, glass vessels. If a glass vessel is used, it should be wrapped thoroughly with a heat-resistant tape so that if the vessel breaks accidentally, the glass will be contained and the bare heating element will not be exposed. Fluidized sand baths are usually preferred over air baths.

Tube furnaces are often used for high-temperature reactions under reduced pressure. The proper choice of glassware or metal tubes and joints is required, and the procedures should conform to safe practice with electrical equipment and evacuated apparatus.

(See also section 6.C.1 and Chapter 5, section 5.G.2.5.)

6.C.5.6 Heat Guns

Laboratory heat guns are constructed with a motor-driven fan that blows air over an electrically heated filament. They are frequently used to dry glassware or to heat the upper parts of a distillation apparatus during distillation of high-boiling materials. The heating element in a heat gun typically becomes red-hot during use and, necessarily, cannot be enclosed. Also, the on-off switches and fan motors are not usually spark-free. For these reasons, heat guns almost always pose a serious spark hazard (see Chapter 5, section 5.G.1). They should never be used near open containers of flammable liquids, in environments where appreciable concentrations of flammable vapors may be present, or in hoods used to remove flammable vapors. Household hair dryers may be substituted for laboratory heat guns only if they have three-conductor line cords or are double-insulated. Any hand-held heating device of this type that will be used in a laboratory should have ground-fault circuit interrupter (GFCI) protection to ensure against electric shock.

6.C.5.7 Microwave Ovens

To avoid exposure to microwaves, ovens should never be operated with doors open. Wires and other objects should not be placed between the sealing surface and the door on the oven's front face. The sealing surfaces must be kept absolutely clean. To avoid electrical hazards, the oven must be grounded. If use of an extension cord is necessary, only a three-wire cord with a rating equal to or greater than that for the oven should be used. To reduce the risk of fire in the oven, samples must not be overheated. The oven must be closely watched when combustible materials are in it. Metal containers or metal-containing objects (e.g., stir bars) should not be used in the microwave, because they can cause arcing.

Generally, sealed containers should not be heated in the oven, because of the danger of explosion. If sealed containers must be used, their materials must be selected carefully and the containers properly designed. Commercially available microwave acid digestion bombs, for example, incorporate a Teflon sample cup, a self-sealing Teflon O-ring, and a compressible pressure-relief valve. The manufacturer's loading limits must not be exceeded. For such applications, the microwave oven should be properly vented using an exhaust system. Placing a large item such as an oven inside a fume hood is not recommended.

Heating a container with a loosened cap or lid poses a significant risk. Microwave ovens can heat material (e.g., solidified agar) so quickly that, even though container lids may be loosened to accommodate expansion, the lid can seat upward against the threads and containers can explode. Screw-caps must be removed from containers being microwaved. If the sterility of the contents must be preserved, screw-caps may be replaced with cotton or foam plugs.

6.C.6 Ultrasonicators, Centrifuges, and Other Electrical Equipment

6.C.6.1 Ultrasonicators

The use of high-intensity ultrasound in the chemical laboratory has grown enormously during the past decade and has a diverse set of applications. Human exposure to ultrasound with frequencies of between 16 and 100 kilohertz (kHz) can be divided into three distinct categories: airborne conduction, direct contact through a liquid coupling medium, and direct contact with a vibrating solid.

Ultrasound through airborne conduction does not appear to pose a significant health hazard to humans. However, exposure to the associated high volumes of audible sound can produce a variety of effects, including fatigue, headaches, nausea, and tinnitus. When ultrasonic equipment is operated in the laboratory, the apparatus must be enclosed in a 2-cm-thick wooden box or in a box lined with acoustically absorbing foam or tiles to substantially reduce acoustic emissions (most of which are inaudible).

Direct contact of the body with liquids or solids subjected to high-intensity ultrasound of the sort used to promote chemical reactions should be avoided. (In contrast, ultrasound used for medical diagnostic imaging is relatively benign.) Under sonochemical conditions, cavitation is created in liquids, and it can induce high-energy chemistry in liquids and tissues. Cell death from

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