Many highly reactive chemicals can polymerize vigorously, decompose, condense, and/or become self-reactive. The improper handling of these materials may result in a runaway reaction that could become violent. Careful planning is essential to avoid serious accidents. When highly reactive materials are in use, emergency equipment should be at hand. The apparatus should be assembled in such a way that if the reaction begins to run away, immediate removal of any heat source, cooling of the reaction vessel, cessation of reagent addition, and closing of laboratory hood sashes are possible. Evacuation of personnel until the reaction is under control is advisable. A heavy, transparent plastic explosion shield should be in place to provide extra protection in addition to the hood window.

Highly reactive chemicals can lead to reactions with rates that increase rapidly as the temperature increases. If the heat evolved is not dissipated, the reaction rate can increase until an explosion results. Such an event must be prevented, particularly when scaling up experiments. Sufficient cooling and surface for heat exchange should be provided to allow control of the reaction. It is also important that the concentrations of the solutions used not be excessive, especially when a reaction is being attempted or scaled up for the first time. Use of too highly concentrated reagents has led to runaway conditions and to explosions. Particular care must also be given to the rate of reagent addition versus its rate of consumption, especially if the reaction is subject to an induction period.

Large-scale reactions with organometallic reagents and reactions that produce flammables as products and/or are carried out in flammable solvents require special attention. Active metals, such as sodium, magnesium, lithium, and potassium, are a serious fire and explosion risk because of their reactivity with water, alcohols, and other compounds containing acidic OH. These materials require special storage, handling, and disposal procedures. Where active metals are present, Class D fire extinguishers that use special extinguishing materials such as a plasticized graphite-based powder or a sodium chloride-based powder (Met-L-X®) are required.

Some chemicals decompose when heated. Slow decomposition may not be noticeable on a small scale, but on a large scale with inadequate heat transfer, or if the evolved heat and gases are confined, an explosive situation can develop. The heat-initiated decomposition of some substances, such as certain peroxides, is almost instantaneous. In particular, reactions that are subject to an induction period can be dangerous because there is no initial indication of a risk, but after the induction a violent process can result.

Oxidizing agents may react violently when they come in contact with reducing materials, trace metals, and sometimes ordinary combustibles. These compounds include the halogens, oxyhalogens, and peroxyhalogens, permanganates, nitrates, chromates, and persulfates, as well as peroxides (see also section 5.G.3). Inorganic peroxides are generally considered to be stable. However, they may generate organic peroxides and hydroperoxides in contact with organic compounds, react violently with water (alkali metal peroxides), or form superoxides and ozonides (alkali metal peroxides). Perchloric acid and nitric acid are powerful oxidizing agents with organic compounds and other reducing agents. Perchlorate salts can be explosive and should be treated as potentially hazardous compounds. "Dusts"—suspensions of oxidizable particles (e.g., magnesium powder, zinc dust, carbon powder, or flowers of sulfur) in the air—constitute a powerful explosive mixture.

Scale-up of reactions can create difficulties in dissipation of heat that are not evident on a smaller scale. Evaluation of observed or suspected exothermicity can be achieved by differential thermal analysis (DTA) to identify exothermicity in open reaction systems; differential scanning calorimetry (DSC), using a specially designed sealable metal crucible, to identify exothermicity in closed reaction systems; or syringe injection calorimetry (SIC) and reactive systems screening tool (RSST) calorimetry to determine heats of reaction on a microscale and small scale. (For an expanded discussion of identifying process hazards using thermal analytical techniques, see Tuma (1991).) When it becomes apparent that an exotherm exists at a low temperature and/or a large exotherm occurs that might present a hazard, large-scale calorimetry determination of exothermic onset temperatures and drop weight testing are advisable. In situations where formal operational hazard evaluation or reliable data from any other source suggest a hazard, review or modification of the scale-up conditions by an experienced group is recommended to avoid the possibility that an individual might overlook a hazard or the most appropriate procedural changes.

Any given sample of a highly reactive material may be dangerous. Furthermore, the risk is associated not with the total energy released, but rather with the remarkably high rate of a detonation reaction. A highorder explosion of even milligram quantities can drive small fragments of glass or other matter deep into the eye. It is important to use minimum amounts of hazardous materials with adequate shielding and personal protection.

Not all explosions result from chemical reactions. A dangerous, physically caused explosion can occur if a hot liquid is brought into sudden contact with a lower-boiling-point one. The instantaneous vaporization of the lower-boiling-point substance can be hazardous to

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