The most common fire hazard in the laboratory is a flammable liquid or the vapour produced from such a liquid. For a fire to occur requires:
Under normal circumstances, oxygen or air will always be present and the best way to prevent a fire is to keep the vapour or gas away from sources of ignition. Some specific properties of flammable materials are:-
Leakage or escape of flammable gases can produce a serious explosive hazard in a laboratory. Acetylene, hydrogen, ammonia, hydrogen sulphide, propane and carbon monoxide are especially dangerous. Hydrogen leaking from a high-pressure cylinder can ignite spontaneously and the resulting flame can be almost invisible and so extremely dangerous.
Pyrophoric materials are those that ignite spontaneously in air below about 45°C. Consequently the main hazards arising from the use of such materials involve fire, either from direct contact with the pyrophoric material or as a result of secondary fires following ignition.
The most commonly used materials are alkyl lithiums, trialkylaluminium reagents and alkylboranes. t-BuLi is the most pyrophoric of the Li reagents but n-BuLi is also pyrophoric as a concentrated solution i.e.~ 10M.
These reagents are supplied in solution, in alkane, arene or ether solvents, the pyrophoric hazard increasing with concentration.
Some materials are prone to inflame spontaneously with no source of ignition. Normally this is the result of exothermic autoxidation within a large mass where heat cannot escape.
The flash point is the lowest temperature at which a liquid has a sufficient vapour pressure to form an ignitable mixture with air near the surface of the liquid. Many common organic liquids have flash points below room temperature e.g. acetone (-18° C) or diethyl ether (-45°C).
A Flammable Liquid is classified as one with a flash point of less than 55°C, a Highly Flammable Liquid (F) is one with a flash point of less than 21°C (a Highly Flammable solid is one which is spontaneously combustible in air at ambient temperature or one which readily ignites after brief contact with a flame or one which evolves highly flammable gases in contact with water or moist air) and an Extremely Flammable Liquid (F+) is one with a flash point less than 0°C and a boiling point of 35°C or less.
The ignition (sometimes called auto-ignition) temperature of a substance is the minimum temperature required to initiate or to cause self-sustained combustion independent of the heat source. A spark or flame is not necessary for ignition when a flammable vapour reaches its auto-ignition temperature. For diethyl ether this is 160° C and the material can be ignited by a hot plate.
Lower and Upper Explosive Limits
These limits define the range of concentrations in mixtures with air (or oxygen depending on definition) that will propagate a flame and cause an explosion. The lower values of these limits are normally well above levels legally allowed as ambient in laboratories and workplaces but can easily be exceeded following a spill.
Sources of Ignition
The most common sources of ignition in the laboratory are flames and heating elements but there are a number of less obvious electrical sources such as refrigerators, heat-guns, stirrers etc. It also must be remembered that vapours from a flammable liquid may be more dense than air and may spread over bench and floor surfaces to sources of ignition which are apparently remote.