pyrophoric

Pyrophoric describes a substance that ignites spontaneously in air at or below room temperature. The term is most often applied to solids or liquids that react exothermically with oxygen rapidly enough to produce ignition without an external flame. Some materials only ignite when present as fine powders or slurries, or upon contact with a small amount of air, while others will ignite on contact with moisture or air at ambient conditions.

Common pyrophoric materials include alkali metals such as sodium, potassium, and lithium (often stored under mineral oil or inert gas to prevent ignition), finely divided magnesium and aluminum powders, and certain phosphorus allotropes such as white phosphorus. Highly reactive organometallic reagents are also pyrophoric, including triethylaluminum and diethylzinc, as well as organolithium reagents (R-Li, such as butyllithium) and Grignard reagents (RMgX).

Pyrophoric substances react vigorously with air to form oxides or other oxidized products, releasing heat that can ignite surrounding materials. Reactions can be spontaneous or require only a brief contact with air. The hazards include fires that can propagate rapidly, explosions in finely divided forms, and toxic fumes from oxidation products. They are moisture- and air-sensitive, and accidental exposure often leads to ignition without visible smoldering.

Handling typically requires an inert atmosphere and dry techniques, such as gloveboxes or Schlenk lines, and storage under inert gas or in oil. Use of appropriate personal protective equipment and trained procedures is essential. In the event of a fire involving pyrophorics, water should not be used; extinguishing media such as Class D dry powders or in some cases stored inert gas may be required. They are widely used in chemical synthesis and polymerization initiation, notably as initiators for certain polymerizations and as precursors to organometallic reagents.