Pyrophoricity

Not to be confused with Porphyria or Porphyriticity.

"Pyrophorus" redirects here. For the bioluminescent beetle genus, see Pyrophorus (beetle).

A substance is pyrophoric (from Ancient Greek: πυροφόρος, pyrophoros, 'fire-bearing') if it ignites spontaneously in air at or below 54 °C (129 °F) (for gases) or within 5 minutes after coming into contact with air (for liquids and solids).^[1] Examples are organolithium compounds and triethylborane. 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 argon or (with a few exceptions) nitrogen. Class D fire extinguishers are designated for use in fires involving metals but not pyrophoric materials in general. A related concept is hypergolicity, in which two compounds spontaneously ignite when mixed.

Uses

The creation of sparks from metals is based on the pyrophoricity of small metal particles, and pyrophoric alloys are made for this purpose.^[2] Practical applications include 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 ferrous metals.

Handling

See also: Air-free technique

Small amounts of pyrophoric liquids are often supplied in a glass bottle with a polytetrafluoroethylene-lined septum. Larger amounts are supplied in metal tanks similar to gas cylinders, designed so a needle can fit through the valve opening. A syringe, carefully dried and flushed of air with an inert gas, is used to extract the liquid from its container.

When working with pyrophoric solids, researchers often employ a sealed glove box flushed with inert gas. Since these specialized glove boxes are expensive and require specialized and frequent maintenance, many pyrophoric solids are sold as solutions, or dispersions in mineral oil or lighter hydrocarbon solvents, so they can be handled in the atmosphere of the laboratory, while still maintaining an oxygen- and moisture-free environment. Mildly pyrophoric solids such as lithium aluminium 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 the material is returned to the container for storage.

Pyrophoric materials

Solids

• White phosphorus
• Alkali metals, especially potassium, rubidium, caesium, including the alloy NaK
• Finely divided metals (iron,^[3] aluminium,^[3] magnesium,^[3] calcium, zirconium,^{[citation needed]} uranium, titanium, tungsten, bismuth, hafnium, thorium, osmium, neodymium)
• Some metals and alloys in bulk form (cerium)
• Alkylated metal alkoxides or nonmetal halides (diethylethoxyaluminium, dichloro(methyl)silane)
• Potassium graphite (KC₈)
• Metal hydrides (sodium hydride, lithium aluminium hydride, uranium trihydride)
• Partially or fully alkylated derivatives of metal and nonmetal hydrides (diethylaluminium hydride, trimethylaluminium, triethylaluminium, butyllithium), with a few exceptions (i.e. dimethylmercury and tetraethyllead)
• Copper fuel cell catalysts (zinc oxide, aluminium oxide)^[4]
• Grignard reagents (compounds of the form RMgX)
• Used hydrogenation catalysts such as palladium on carbon or Raney nickel (especially hazardous because of the adsorbed hydrogen)
• Iron(II) sulfide: often encountered in oil and gas facilities, where corrosion products in steel plant equipment can ignite if exposed to air
• Lead and carbon powders produced from decomposition of lead citrate^[5][6]
• Uranium, as shown in the disintegration of depleted uranium penetrator rounds into burning dust upon impact with their targets; in finely divided form it is readily ignitable, and uranium scrap from machining operations is subject to spontaneous ignition^[7]
• Neptunium
• Several compounds of plutonium are pyrophoric, and they cause some of the most serious fires occurring in United States Department of Energy facilities^[8]
• Petroleum hydrocarbon (PHC) sludge

Liquids

• Diphosphane
• Metalorganics of main group metals (e.g. aluminium, gallium, indium, zinc, cadmium, etc.)
• Triethylborane
• tert-Butyllithium
• Diethylzinc
• Triethylaluminium

Gases

• Nonmetal hydrides (arsine, phosphine,^[i] diborane, germane, silane)
• Metal carbonyls (dicobalt octacarbonyl, nickel carbonyl)

Explanatory notes

1. Phosphine, PH₃ is only pyrophoric if impure, with P₂H₄ present.