to flammable liquids or vapors. Mount variable auto-transformers on walls or vertical panels and outside laboratory chemical hoods; do not simply place them on laboratory benchtops.
Electrical input lines, including lines from variable transformers, to almost all laboratory heating devices have a potential of 110 V with respect to any electrical ground; always view these lines as potential shock and spark hazards. Connections from these lines to a heating device should be both mechanically and electrically secure and completely covered with insulating material. Do not use alligator clips to connect a line cord from a variable autotransformer to a heating device, especially to an oil bath or an air bath, because such connections pose a shock hazard. They also may slip off, creating an electrical spark and, perhaps, contacting other metal parts to create an additional hazard. Make all connections by using, preferably, a plug-and-receptacle combination, or wires with insulated terminals firmly secured to insulated binding posts.
Whenever an electrical heating device is used, either a temperature controller or a temperature-sensing device must be used that will turn off the electric power if the temperature of the heating device exceeds some preset limit. Similar control devices are available that will turn off the electric power if the flow of cooling water through a condenser is stopped owing to the loss of water pressure or loosening of the water supply hose to a condenser. Independent temperature sensors must be used for the temperature controller and shutoff devices. Fail-safe devices, which can be either purchased or fabricated, can prevent the more serious problems of fires or explosions that may arise if the temperature of a reaction increases significantly because of a change in line voltage, the accidental loss of reaction solvent, or loss of cooling. Use fail-safe devices for stills purifying reaction solvents, because such stills are often left unattended for significant periods of time. Temperature-sensing devices absolutely must be securely clamped or firmly fixed in place, maintaining contact with the object or medium being heated at all times. If the temperature sensor for the controller is not properly located or has fallen out of place, the controller will continue to supply power until the sensor reaches the temperature setting, creating an extremely hazardous situation. (See also Vignette 7.1.)
Hot plates, oil baths, and heating mantles that can melt and combust plastic materials (e.g., vials, containers, tubing) can cause laboratory fires, and the area around the equipment should be cleared of those hazards prior to use. Be aware that dry and concentrated residues can ignite when overheated in stills, ovens, dryers, and other heating devices.
(See section 7.C.1 for additional information.)
A researcher walking past a laboratory noticed a flame burning behind the closed sashes of the chemical fume hood. He determined that the oil in an oil bath was burning. There was no other equipment in the oil bath and no other chemicals were in the vicinity. The researcher turned off electrical service to the chemical fume hood using the red Crash button on the front and deemed it safe to attempt to extinguish the fire with a B/C extinguisher. When the sash was opened slightly to extinguish the fire, the flames flared through the opening and singed the researcher’s forehead and right forearm. The fire was extinguished immediately but continued to flare up because the oil was still above its autoignition temperature. A metal pan was placed over the oil bath to smother the fire.
An investigation determined that the thermocouple used by the oil bath temperature controller had fallen out of the oil bath. The controller, responding to the false temperature drop reading, continued to supply power to the bath, resulting in overheating and fire.
Electrically heated ovens are commonly used in the laboratory to remove water or other solvents from chemical samples and to dry laboratory glassware. Never use laboratory ovens to prepare food for human consumption.
Purchase or construct laboratory ovens with their heating elements and their temperature controls physically separated from their interior atmospheres. Small household ovens and similar heating devices usually do not meet these requirements and, consequently, should not be used in laboratories. With the exception of vacuum drying ovens, laboratory ovens rarely prevent the discharge of the substances volatilized in them into the laboratory atmosphere. The volatilized substances may also be present in sufficient concentration to form explosive mixtures with the air inside the oven (see Chapter 6, section 6.G). This hazard can be reduced by connecting the oven vent directly to an exhaust system. (See Vignette 7.2.)
Do not use ovens to dry any chemical sample that has even moderate volatility and might pose a hazard