TABLE 4.5 Additional Symbols Seen in the NFPA Diamond


Symbol

Meaning


ACID

Acid

ALK

Alkali

CORR

Corrosive

img

Radioactive


cool conditions (see Table 4.4). Some flammable liquids 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. High-performance liquid chromatography users generate acetonitrile–water mixtures that contain from 15–30% 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 is available for laboratory use only in metal containers. Carbon disulfide is almost as hazardous.

4.D.1.4 Causes of Ignition

4.D.1.4.1 Spontaneous Combustion

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.

4.D.1.4.2 Ignition Sources

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 Chapter 7, section 7.C). Whenever possible, open flames should be replaced by electrical heating. Because the vapors of most flammable liquids are heavier than air and capable of traveling considerable distances, 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 propagates 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.

4.D.1.4.3 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 4.6. When potassium ignites on addition to water, the metal is the reducing agent and water is the oxidant. If the hydrogen produced ignites, 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 hazardous combinations are treated further in section 4.D.2. (See Chapter 6, section 6.F, for a more detailed discussion on flammable substances.)

4.D.1.5 Special Hazards

Compressed or liquefied gases present fire hazards because the heat causes the pressure to increase and the container may rupture (Yaws and Braker, 2001). Leakage or escape of flammable gases produces an explosive atmosphere in the laboratory; acetylene, hydrogen, ammonia, hydrogen sulfide, propane, and carbon monoxide are especially hazardous.

Even if not under pressure, a liquefied gas is more concentrated than in the vapor phase and evaporates rapidly. Oxygen is an extreme hazard and liquefied air is almost as dangerous because nitrogen boils away first, leaving an increasing concentration of oxygen. Liquid nitrogen standing for a period of time may have condensed enough oxygen to require careful handling. When a liquefied gas is used in a closed system, pres-

TABLE 4.6 Examples of Oxidants


Gases: chlorine, fluorine, nitrous oxide, oxygen, ozone, steam

Liquids: bromide, hydrogen peroxide, nitric acid, perchloric acid, sulfuric acid

Solids: bromates, chlorates, chlorites, chromates, dichromates, hypochlorites, iodates, nitrates, nitrites, perchlorates, peroxides, permanganates, picrates




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