released, but rather with the remarkably high rate of a detonation reaction. A high-order explosion of even milligram quantities can drive small fragments of glass or other matter deep into the body. It is important to use minimum amounts of these hazardous materials with adequate shielding and personal protection. A compound is apt to be explosive if its heat of formation is more than about 100 calories per gram (cal/g) less than the sum of the heats of formation of its products. In making this calculation, a reasonable reaction should be used in order to yield the most exothermic products.

Scaling up reactions can introduce several hazards. The current use of microscale teaching methods in undergraduate laboratories unfortunately increases the likelihood that graduate students and others may be unprepared for a number of problems that can arise when a reaction is run on a larger scale. These include heat buildup and serious hazard of explosion from the use of incompatible materials. The rate of heat input and production must be weighed against that of heat removal. Bumping of the solution or a runaway reaction can result when heat builds up too rapidly. Exothermic reactions can "run away" if the heat evolved is not dissipated. When scaling up experiments, sufficient cooling and surface for heat exchange should be provided, and mixing and stirring rates should be considered. Detailed guidelines for circumstances that require a systematic hazard evaluation and thermal analysis are given in Chapter 5, section 5.G.

Another situation that can lead to problems is a reaction susceptible to an induction period; particular care must be given to the rate of reagent addition versus its rate of consumption. Finally, the hazards of exothermic reactions or unstable or reactive chemicals are exacerbated under extreme conditions, such as high temperature or high pressure used for hydrogenations, oxygenations, or work with supercritical fluids.

3.D.4 The Dirty Dozen

In laboratories carrying out moderate- to large-scale synthetic chemistry, it is generally recognized that certain substances tend to be responsible for more than their share of accidents (see also Chapter 5, section 5.G.6). In some laboratories these perennial "bad actors" are known as the "Dirty Dozen" (see Table 3.14). Although accident statistics for such laboratories show that most accidents lead to cut hands and back injuries (Kaufmann, 1990), enough workers have had incidents with these elements and compounds to make extreme caution advisable. Inappropriate mixing or handling of certain compounds can also produce hazardous toxic gases. Institutions might find it useful to prepare their own lists as part of their Chemical Hygiene Plans.

3.E PHYSICAL HAZARDS

3.E.1 Compressed Gases

Compressed gases can expose the worker to both mechanical and chemical hazards, depending on the gas. Hazards can result from the flammability, reactivity, or toxicity of the gas, from the possibility of asphyxiation, and from the gas compression itself, which could lead to a rupture of the tank or valve.

3.E.2 Nonflammable Cryogens

Nonflammable cryogens (chiefly liquid nitrogen) can cause tissue damage from extreme cold because of contact with either liquid or boil-off gases. In poorly ventilated areas, inhalation of gas due to boil-off or spills can result in asphyxiation. Another hazard is explosion from liquid oxygen condensation in vacuum traps or from ice plug formation or lack of functioning vent valves in storage Dewars. Because 1 volume of liquid nitrogen at atmospheric pressure vaporizes to 694 volumes of nitrogen gas at 20 °C, the warming of such a cryogenic liquid in a sealed container produces enormous pressure, which can rupture the vessel. (See Chapter 5, section 5.G, for detailed discussion.)

3.E.3 High-Pressure Reactions

Experiments carried out at pressures above one atmosphere can lead to explosion from equipment failure. Hydrogenation reactions are frequently carried out at elevated pressures. A potential hazard is the formation of explosive O2/H2 mixtures and the reactivity/pyrophoricity of the catalyst (see section 3.D). High pressures can also be associated with the growing use of supercritical fluids (see McHugh and Krukonis, 1994; Bright and McNally, 1992).

3.E.4 Vacuum Work

Precautions to be taken when working with vacuum lines and other glassware used at subambient pressure are mainly concerned with the substantial danger of injury in the event of glass breakage. The degree of hazard does not depend significantly on the magnitude of the vacuum because the external pressure leading to implosion is always one atmosphere. Thus, evacuated systems using aspirators merit as much respect as high-



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