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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
in Table 3.7 and in the LCSSs (see Appendix B). The data illustrate the range of flammability found for liquids commonly in use in laboratories. Dimethyl sulfoxide and glacial acetic acid (NFPA fire hazard ratings of 1 and 2, respectively) can be handled in the laboratory without great concern about their fire hazards. By contrast, both acetone (NFPA 3) and diethyl ether (NFPA 4) have flash points well below room temperature.
It should be noted, however, that tabulations of properties of flammable substances are based on standard test methods, which may have very different conditions from those encountered in practical laboratory use. Large safety factors should be applied. For example, the published flammability limits of vapors are for uniform mixtures with air. In a real situation, local concentrations that are much higher than the average may exist. Thus, it is good practice to set the maximum allowable concentration for safe working conditions at some fraction of the tabulated LEL; 20% is a commonly accepted value.
Among the most hazardous liquids are those that have flash points near or below 38 °C (100 °F) because these materials can be hazardous in the common laboratory environment. There is particular risk if their range of flammability is broad. It is important to note, as shown in Table 3.7, that some commonly used substances are potentially very hazardous, even under relatively cool conditions. Some flammable liquids will maintain their flammability even at concentrations of 10% by weight in water. Methanol and isopropyl alcohol have flash points below 38 °C (100 °F) at concentrations as low as 30% by weight in water. HPLC users generate acetonitrile/water mixtures that contain from 15 to 30% acetonitrile in water, a waste that is considered toxic and flammable and thus cannot be added to a sewer.
Because of its extreme flammability and tendency for peroxide formation, diethyl ether should be available for laboratory use only in metal containers. Carbon disulfide is almost as hazardous.
3.D.1.4 Causes of Ignition
Spontaneous ignition (autoignition) or combustion takes place when a substance reaches its ignition temperature without the application of external heat. The possibility of spontaneous combustion should always be considered, especially when storing or disposing of materials. Examples of materials susceptible to spontaneous combustion include oily rags, dust accumulations, organic materials mixed with strong oxidizing agents (e.g., nitric acid, chlorates, permanganates, peroxides, and persulfates), alkali metals (e.g., sodium and potassium), finely divided pyrophoric metals, and phosphorus.
Potential ignition sources in the laboratory include the obvious torch and Bunsen burner, as well as a number of less obvious, electrically powered, sources ranging from refrigerators, stirring motors, and heat guns to microwave ovens (see section 6.C). Whenever possible, open flames should be replaced by electrical heating.
The vapors of most flammable liquids are heavier than air and capable of traveling considerable distances. This possibility should be recognized, and special note should be taken of ignition sources situated at a lower level than that at which the substance is being used. Flammable vapors from massive sources such as spills have been known to descend into stairwells and elevator shafts and ignite on a lower story. If the path of vapor within the flammable range is continuous, as along a floor or benchtop, the flame will propagate itself from the point of ignition back to its source. Metal lines and vessels discharging flammable substances should be bonded and grounded properly to discharge static electricity. There are many sources of static electricity, particularly in cold, dry atmospheres, and caution should be exercised.
Oxidants Other Than Oxygen
The most familiar fire involves a combustible material burning in air. However, the oxidant driving a fire or explosion need not be oxygen itself, depending on the nature of the reducing agent. All oxidants have the ability to accept electrons, and fuels are reducing agents or electron donors (see Young, 1991).
Examples of nonoxygen oxidants are shown in Table 3.8. When potassium ignites on being added to water, the metal is the reducing agent and water is the oxidant. If the hydrogen produced is ignited, it becomes the fuel for a conventional fire, with oxygen as the oxidant. In ammonium nitrate explosions, the ammonium cation is oxidized by the nitrate anion. These