Pyrophoricity

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A pyrophoric substance is a substance that ignites spontaneously, that is, its autoignition temperature is below room temperature. Examples are iron sulfide and many reactive metals including uranium, when powdered or sliced thin. Pyrophoric materials are often water reactive as well and will ignite when they contact water or humid air. They can be handled safely in atmospheres of nitrogen or argon. Most pyrophoric fires should be extinguished with a class D fire extinguisher.

1 Uses
2 Safe handling of pyrophoric materials
3 List of pyrophoric materials
- 3.1 Pyrophoric gases
- 3.2 Pyrophoric liquids
4 External links

[edit] Uses

The creation of sparks from metals is based on the pyrophoricity of small metal particles. This can be useful, including: the sparking mechanisms in lighters and various toys, using ferrocerium; starting fires without matches, using a firesteel; the flintlock mechanism in firearms; and spark testing metals.

[edit] Safe handling of pyrophoric materials

[edit] Liquids

Small amounts of pyrophoric liquids are often supplied in a glass bottle with a PTFE lined septum, and larger amounts in metal tanks (similar to gas cylinders, but designed so a needle can fit through the valve opening.) For small amounts, a needle supplying a stream of inert gas is inserted in the septum. A syringe is flushed by inserting into the bottle above the level of liquid, and slowly withdrawing inert gas, no faster than the inert gas is being supplied. The needle is then removed, and the inert gas (and any outside air that was contained in the needle) is pushed out into the air. The needle is then re-inserted into through the septum, into the liquid, and a small amount of liquid is drawn into the syringe. The needle is then withdrawn to above the level of the liquid, and tilted so the pocket of inert gas rises to the needle end of the syringe. The pocket of inert gas is then pushed back into the bottle, so the syringe and needle contain only liquid. The needle is then re-inserted into the liquid, and a volume of liquid larger than desired is withdrawn. Then, the needle is inverted, and the excess liquid returned to the bottle, by pushing the barrel to the volume desired. The needle is then withdrawn to above the level of liquid, the syringe inverted so the needle end is facing upward, and some inert gas is withdrawn to provide a protective blanket over the reagent. The needle can then be completely removed from the reagent bottle, and should only be exposed to the air briefly before inserting the needle into a septum on a dried reaction flask and the contents injected, while holding the syringe vertically to first remove the inert gas, allowing accurate volume measurement. After injecting the reagent, inert gas is withdrawn from the reaction flask to blanket the small amount of pyrophoric material remaining inside. The syringe is then filled with a dry, inert solvent in which the reagent is soluble (usually hexane) and the contents quenched, at which point it is safe to clean the syringe with water. If using a non-disposable syringe, the syringe must then be rinsed successively with acetone and hexane, then placed in a desiccator or oven to remove traces of water.

[edit] Solids

Pyrophoric solids require the use of a sealed glove box flushed with inert gas. Glove boxes are expensive, and require maintenance. Thus, many pyrophoric solids are sold as solutions, or dispersions in mineral oil or lighter hydrocarbon solvents. Mildly pyrophoric solids (such as Lithium Aluminum Hydride and Sodium Hydride) can be handled in the air for brief periods of time, but the containers must be flushed with inert gas before storage.

[edit] Disposal of pyrophoric materials

Small amounts of pyrophoric materials and empty containers must be disposed of carefully, by quenching the residue. Less reactive substances can be disposed of by diluting heavily with an unreactive solvent like hexane, placing the container in a cooling bath, and adding water dropwise. More reactive substances can quenched by slowly adding the dilute solution to dry ice, then adding a mildly reactive substance which does not freeze in dry ice to the mixture (wet diethyl ether, acetone, isopropyl alcohol, and methanol are often used)

[edit] List of pyrophoric materials

Finely divided metals (magnesium, calcium, zirconium)
Alkali metals (sodium, potassium)
Metal hydrides or nonmetal hydrides (germane, diborane, sodium hydride, lithium aluminum hydride, uranium trihydride)
Grignard reagents (compounds of the form RMgX)
Partially or fully alkylated derivatives of metal and nonmetal hydrides (diethylaluminum hydride, trimethylaluminum, butyllithium, triethylboron)
Alkylated metal alkoxides or nonmetal halides (diethylethoxyaluminum, dichloro(methyl)silane)
Metal carbonyls (pentacarbonyliron, octacarbonyldicobalt, nickel carbonyl)
Used hydrogenation catalysts such as Raney nickel (especially hazardous because of the adsorbed hydrogen)
Phosphorus (white, or yellow)
Plutonium
Methanetellurol (CH₃TeH))