hazardous combinations are treated further in section 3.D.2.
Compressed or liquefied gases present hazards in the event of fire because the heat will cause the pressure to increase and the container may rupture (Braker and Mossman, 1980; Braker et al., 1988; Matheson Gas Products, 1983). 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.
Even if not under pressure, a substance in the form of a liquefied gas is more concentrated than in the vapor phase and may evaporate extremely rapidly. Oxygen is an extreme hazard. Liquefied air is almost as dangerous because nitrogen boils away first, leaving an increasing concentration of oxygen. Liquid nitrogen standing for some time may have condensed enough oxygen to require careful handling. When a liquefied gas is used in a closed system, pressure may build up. Hence adequate venting is required. If the liquid is flammable (e.g., hydrogen and methane), explosive concentrations may develop without warning unless an odorant has been added. Flammability, toxicity, and pressure buildup may become more serious on exposure of gases to heat.
Water reactive materials are those that react violently with water. Alkali metals (e.g., lithium, sodium, and potassium), many organometallic compounds, and some hydrides react with water to produce heat and flammable hydrogen gas, which can ignite or combine explosively with atmospheric oxygen. Some anhydrous metal halides (e.g., aluminum bromide), oxides (e.g., calcium oxide), and nonmetal oxides (e.g., sulfur trioxide) and halides (e.g., phosphorus pentachloride) react exothermically with water, and the reaction can be violent if there is insufficient coolant water to dissipate the heat produced.
For pyrophoric materials, oxidation of the compound by oxygen or moisture in air proceeds so rapidly that ignition occurs. Many finely divided metals are pyrophoric, and their degree of reactivity depends on particle size, as well as factors such as the presence of moisture and the thermodynamics of metal oxide or metal nitride formation. Many other reducing agents, such as metal hydrides, alloys of reactive metals, low-valent metal salts, and iron sulfides, are also pyrophoric.
Accidental contact of incompatible substances could result in a serious explosion or the formation of substances that are highly toxic or flammable or both. Many laboratory workers question the necessity of following storage compatibility guidelines. The reasons for such guidelines can be made obvious by reading descriptions of the condition of laboratories following California earthquakes in recent decades (see Pine, 1988, 1994). Those who do not live in seismically active zones should take these accounts to heart, as well. Other natural disasters and chemical explosions themselves can set off shock waves that empty chemical shelves and result in inadvertent mixing of chemicals.
Some compounds can pose either a reactive or a toxic hazard, depending on the conditions. Thus, hydrocyanic acid (HCN), when used as a pure liquid/ gas in industrial applications, is incompatible with bases because it is stabilized against (violent) polymerization by the addition of acid inhibitor. HCN can also be formed when cyanide salt is mixed with an acid. In this case, the toxicity of hydrogen cyanide gas, rather than the instability of the liquid, is the characteristic of concern.
Some general guidelines can be applied to lessen the risks involved with these substances. Concentrated oxidizing agents are incompatible with concentrated reducing agents. Indeed, either may pose a reactive hazard even with chemicals that are not strongly oxidizing or reducing. For example, sodium or potassium, strong reducing agents frequently used to dry organic solvents, are extremely reactive toward halocarbon solvents (which are not strong oxidizing agents). Strong oxidizing agents are frequently used to clean glassware. Clearly, it is prudent to use such potent reagents only on the last traces of contaminating material. Tables 3.9 and 3.10 are guides to avoiding accidents involving incompatible substances. Chemicals or classes of chemicals in one column can be hazardous when mixed with those opposite them in the adjacent column. The magnitude of the risk obviously depends on quantities. In ordinary laboratory use, chemical incompatibilities will not usually pose much, if any, risk if the quantity of the substance is small (a solution in an NMR tube or a microscale synthesis). However, storage of commercially obtained chemicals (e.g., in