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Prudent Practices in the Laboratory: Handling and Disposal of Chemicals
known. Solvent labels, LCSSs, or other sources of information can be consulted to learn the flash point, vapor pressure, and explosive limit in air of each chemical handled. While all flammable substances should be handled prudently, the extreme flammability of some materials requires additional precautions.
To ensure that laboratory workers respond appropriately, they should be briefed on the necessary steps to take in case of a fire. The laboratory should be set up in such a way that the locations of fire alarms, pull stations, fire extinguishers, safety showers, and other emergency equipment are marked and all laboratory personnel alerted to them (see section 5.C.11 below). Exit routes in case of fire should be reviewed. Fire extinguishers in the immediate vicinity of an experiment should be appropriate to the particular fire hazards. Proper extinguishers must be used because fires can be exacerbated by use of an inappropriate extinguisher. Telephone numbers to call in case of an accident should be readily available.
Scale-up of reactions from those producing a few milligrams or grams to those producing more than 100 g of a product may represent several orders of magnitude of added risk. The attitudes, procedures, and controls applicable to large-scale laboratory reactions are fundamentally the same as those for smaller scale procedures. However, differences in heat transfer, stirring effects, times for dissolution, and effects of concentration and the fact that substantial amounts of materials are being used introduce the need for special vigilance for scaled-up work. Careful planning and consultation with experienced workers to prepare for any eventuality are essential for large-scale laboratory work.
Although it is not always possible to predict whether a scaled-up reaction has increased risk, hazards should be evaluated if the following conditions exist:
The starting material and/or intermediates contain functional groups that have a history of being explosive-e.g., N-N, N-O, N-halogen, O-O, and O-halogen bonds-or that could explode to give a large increase in pressure.
A reactant or product is unstable near the reaction or work-up temperature. A preliminary test consists of heating a small sample in a melting point tube.
A reaction is delayed; that is, an induction period is required.
Gaseous by-products are formed.
A reaction is exothermic. What can be done to provide cooling if the reaction begins to run away?
A reaction requires a long reflux period. What will happen if solvent is lost owing to poor condenser cooling?
A reaction requires temperatures below 0 °C. What will happen if the reaction warms to room temperature?
In addition, thermal phenomena that produce significant effects on a larger scale may not have been detected in smaller-scale reactions and therefore could be less obvious than toxic and/or environmental hazards. Thermal analytical techniques should be used to determine whether any process modifications are necessary.
5.C.10Responsibility for Unattended Experiments and Working Alone
Generally, it is prudent to avoid working alone at the bench in a laboratory building. Individuals working in separate laboratories outside of working hours should make arrangements to check on each other periodically, or ask security guards to check on them. Experiments known to be hazardous should not be undertaken by a worker who is alone in a laboratory. Under unusually hazardous conditions, special rules may be necessary.
Laboratory operations involving hazardous substances are sometimes carried out continuously or overnight with no one present. It is the responsibility of the worker to design these experiments so as to prevent the release of hazardous substances in the event of interruptions in utility services such as electricity, cooling water, and inert gas. Laboratory lights should be left on, and signs should be posted identifying the nature of the experiment and the hazardous substances in use. If appropriate, arrangements should be made for other workers to periodically inspect the operation. Information should be posted indicating how to contact the responsible individual in the event of an emergency.