vide dilution is explosion proof and that sparking items are located outside the air stream.

  • Keep containers of flammable substances tightly closed at all times when not in use.

  • Use only refrigeration equipment certified for storage of flammable materials.

  • Use the smallest quantities of flammable substances compatible with the need, and, especially when the flammable liquid must be stored in glass, purchase the smallest useful size bottle.

5.F.2 Flammable Liquids

Flammable liquids burn only when their vapor is mixed with air in the appropriate concentration. Therefore, such liquids should always be handled so as to minimize the creation of flammable vapor concentrations. Dilution of flammable vapors by ventilation is an important means of avoiding flammable concentrations. Containers of liquids should be kept closed except during transfer of contents. Transfers should be carried out only in fume hoods or in other areas where ventilation is sufficient to avoid a buildup of flammable vapor concentrations. Spillage or breakage of vessels or containers of flammable liquids or sudden eruptions from nucleation of heated liquid can result in a sudden release of vapor, which will produce an unexpected quantity of flammable vapor.

Metal lines and vessels discharging flammable liquids should be grounded properly and also grounded to discharge static electricity. For instance, when transferring flammable liquids in metal equipment, avoid static-generated sparks by grounding and the use of ground straps. Development of static electricity is related closely to the level of humidity and may become a problem on very cold, dry winter days. When nonmetallic containers (especially plastic) are used, the contact should be made directly to the liquid with the grounding device rather than to the container. In the rare circumstance that static electricity cannot be avoided, all processes should be carried out as slowly as possible to give the accumulated charge time to disperse, or should be handled in an inert atmosphere.

Note that vapors of many flammable liquids are heavier than air and capable of traveling considerable distances along the floor. This possibility should be recognized, and special note should be taken of ignition sources at a lower level than that at which the substance is being used. Close attention should be given to nearby potential sources of ignition.

5.F.3 Flammable Gases

Leakage or escape of flammable gases can produce an explosive atmosphere in the laboratory. Acetylene, hydrogen, ammonia, hydrogen sulfide, propane, and carbon monoxide are especially hazardous. Acetylene, methane, and hydrogen have very wide flammability limits, which adds greatly to their potential fire and explosion hazard. Installation of flash arresters on hydrogen cylinders is recommended. Prior to introduction of a flammable gas into a reaction vessel, the equipment should be purged by evacuation or with an inert gas. The flush cycle should be repeated three times to reduce residual oxygen to about 1%.

(See section 5.H for specific precautions on the use of compressed gases.)

5.F.4 Catalyst Ignition of Flammable Materials

Palladium or platinum on carbon, platinum oxide, Raney nickel, and other hydrogenation catalysts should be filtered carefully from hydrogenation reaction mixtures. The recovered catalyst is usually saturated with hydrogen, is highly reactive, and, thus, inflames spontaneously on exposure to air. Especially for large-scale reactions, the filter cake should not be allowed to become dry. The funnel containing the still-moist catalyst filter cake should be put into a water bath immediately after completion of the filtration. Use of a purge gas (nitrogen or argon) is strongly recommended for hydrogenation procedures so that the catalyst can then be filtered and handled under an inert atmosphere.

5.G WORKING WITH HIGHLY REACTIVE OR EXPLOSIVE CHEMICALS

An explosion results when a material undergoes rapid reaction that results in a violent release of energy. Such reactions can occur spontaneously or be initiated and can produce pressures, gases, and fumes that are hazardous. Highly reactive and explosive materials used in the laboratory require appropriate procedures. In this section, techniques for identifying and handling potentially explosive materials are discussed.

5.G.1 Overview

Light, mechanical shock, heat, and certain catalysts can be initiators of explosive reactions. Hydrogen and chlorine react explosively in the presence of light. Examples of shock-sensitive materials include acetylides, azides, organic nitrates, nitro compounds, perchlorates, and many peroxides. Acids, bases, and other substances can catalyze the explosive polymerizations. The catalytic effect of metallic contamination can lead to explosive situations. Many metal ions can catalyze the violent decomposition of hydrogen peroxide.



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