Faculty of Science

Department of Chemistry

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:

  • an oxidising atmosphere (usually air),
  • flammable gas or vapour at a concentration within the flammability limits of the substance and
  • a source of ignition.

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:-

Flammable Gases

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.

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Pyrophoric materials

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.

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Spontaneous Combustion

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.

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Flash Point

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.

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Ignition Temperature

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.

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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.

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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.

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