White phosphorus is a highly reactive and toxic element that exists in three forms. It is used in the production of phosphoric acid and fertilizers, as well as in military applications such as incendiary weapons and smoke screens. Its use in areas with civilian populations is illegal, and attempts have been made to classify it as a chemical weapon.
The element phosphorus exists in three main forms, or allotropes – white, red and black – of which white phosphorus, made up of four phosphorus atoms arranged in a tetrahedral molecule, is the most reactive. It is a white, waxy solid that melts at 111.2°F (44°C) and boils at 536°F (280°C). In impure form, containing traces of red phosphorus, it often has a yellow color and is sometimes called yellow phosphorus. It is insoluble in water, but dissolves in some organic solvents. The white form of the element, unlike the others, is extremely flammable – it ignites in air at 86°C (30°F) – and highly toxic – the lethal dose to humans is estimated to be approximately 0.0018 ounces (0.05 grams ).
An interesting property of white phosphorus is its chemiluminescence in the presence of air: this is clearly visible as a green glow when viewed in the dark. The glow is thought to be related to partial oxidation of the steam. Despite having been known and studied since the element was first isolated in the 16th century, the exact chemical mechanism behind the glow remains, as of this writing, unclear. When exposed to light, the white form slowly converts to red phosphorus. In the laboratory it is normally stored underwater in tinted glass containers.
Phosphorus is produced industrially by reacting phosphate rocks with sand and coke at high temperatures. The sand reacts with the phosphate, forming calcium silicate and phosphorus pentoxide (P2O5), which is reduced by the coke to gaseous elemental phosphorus. This is then condensed into phosphorus in its white form.
Historically, white phosphorus was used in the manufacture of matches; however, safer alternatives are now being used, including some less toxic phosphorus compounds. Workers in match factories in the 19th century often suffered from a condition known as “fossany jaw,” a painful and debilitating jaw decay resulting from chronic exposure to phosphorus vapor. It was also previously used in some rat poisons.
Today its main uses are in the production of phosphoric acid, which is used in the food industry and in some cleaning products, and in the production of phosphate fertilizers. Much of it is converted into red phosphorus, which is non-toxic and less flammable. This form of phosphorus is used in some pyrotechnic mixtures for fireworks.
White phosphorus also has important military applications, such as in incendiary weapons, flares used to illuminate enemy positions, and smoke screens. Combustion in air produces phosphorus pentoxide (P2O5). This compound is extremely hygroscopic; that is, it rapidly absorbs moisture from the air, forming droplets of phosphoric acid. These droplets form a thick, white smoke that is very effective at hiding whatever is inside. For this reason, white phosphorus is used in smoke grenades to conceal troop movements.
The most controversial military use of white phosphorus has been in incendiary weapons. This substance burns fiercely, spewing globules of flaming, molten phosphorus that can ignite new fires and inflict severe burns on anyone in the vicinity. Because the ignition temperature is well below body temperature, burning material is difficult to extinguish: the flames can be smothered, but the phosphor will re-ignite as soon as it has access to oxygen. White phosphorus-based incendiary weapons were first used in World War I and have been employed in many subsequent conflicts. In military context, he is sometimes nicknamed “WP” or “Willie Pete.”
The use of WP in rockets and smokescreens and as an incendiary weapon against military targets is, as of 2011, permitted under international law; however, the use of incendiary weapons in areas with civilian populations is not. The United States, Israel, and Russia have been blamed for the illegal use of white phosphorus munitions in late 20th and early 21st century conflicts. Attempts, hitherto unsuccessful, have been made to classify WP as a chemical weapon, due to its toxicity and the irritating effects of the smoke produced by its combustion. These moves were countered on the grounds that the element’s toxicity is accidental and not the reason for its use.
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