White phosphorus incendiary

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This article is about the military applications of white phosphorus. For more general information, see Phosphorus.

White phosphorus is a common allotrope of the chemical element phosphorus which has found extensive military application as a smoke-screening agent and secondarily as an incendiary weapon. It is commonly referred to in military jargon as "WP" or "white phos". The Vietnam War era slang Willie Pete, Whiskey Pete or Wiley P is still occasionally heard.

1 Smoke-screening
2 Incendiary
- 2.1 First aid and Toxicity
3 Arms control status
4 Disadvantages
5 History
6 References

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Smoke-screening

Weight-for-weight, phosphorus is the most effective smoke-screening agent known, for two reasons:

It actually absorbs most of the screening mass from the surrounding atmosphere; and
The smoke particles are actually an aerosol, a mist of liquid droplets which are close to the ideal range of sizes for Mie scattering of visible light. This effect has been likened to three dimensional textured privacy glass—the smoke cloud does not obstruct an image, but thoroughly scrambles it. It also absorbs infrared radiation.

When phosphorus burns in air, it first forms phosphorus pentoxide:

P₄ + 5 O₂ → 2 P₂O₅

However phosphorus pentoxide is extremely hygroscopic (actually, deliquescent) and quickly absorbs even minute traces of moisture to form liquid droplets of phosphoric acid:

P₂O₅ + 3 H₂O → 2 H₃PO₄

Since an atom of phosphorus has an atomic mass of 31 but a molecule of phosphoric acid has a molecular mass of 98, the cloud is already 68% by mass derived from the atmosphere. (To put that another way, you have 3.2 kilograms of smoke for every kilogram of WP you started with.) But it can still absorb more; phosphoric acid itself is hygroscopic. Given time, the droplets will continue to absorb more water, growing larger and more dilute, until they reach equilibrium with the local water vapour pressure. In practice the droplets quickly reach a range of sizes very suitable for scattering visible light, and then start to dissipate due to wind or convection. The dilute phosphoric acid in the cloud may be mildly irritating to the eyes but with normal field concentrations and exposure it is not harmful; extended exposure can lead to damage of lungs and throat. The smoke may also contain traces of unburnt phosphorus. A respirator usually provides adequate protection.

USS Alabama hit by a white phosphorus bomb in bombing tests by General Billy Mitchell, September 1921.

Because of the great weight efficiency of WP smoke, it is particularly suited for applications where weight is highly restricted, such as hand grenades and mortar bombs. An additional advantage for hand smoke grenades—which are more likely to be used in an emergency—is that the WP smoke cloud forms very quickly, in fact in a fraction of a second. Because WP is also pyrophoric, most munitions of this type have a simple burster charge to split open the casing and spray fragments of WP through the air, where they ignite spontaneously and leave a trail of rapidly thickening smoke behind each particle. The appearance of this cloud forming is easily recognised; one sees a shower of burning particles spraying outward, followed closely by distinctive streamers of white smoke, which rapidly coalesce into a fluffy, very pure white cloud (unless illuminated by a coloured light source).

Various disadvantages of WP are discussed below, but one which is particular to smoke-screening is "pillaring". Because the WP smoke is formed from fairly hot combustion, the gasses in the cloud are hot, and tend to rise. Consequently the smoke screen tends to relatively quickly rise off the ground and form aerial "pillars" of smoke which are of little use for screening. Tactically this may be counteracted by using WP to get a screen quickly, but then following up with emission type screening agents for a more persistent screen. Some countries have begun using red phosphorus instead. Red phosphorus ("RP") burns cooler than WP and eliminates a few other disadvantages as well, but offers exactly the same weight efficiency. Other approaches include WP soaked felt pads (which also burn slower, and pose a reduced risk of incendiarism) and PWP, or plasticised white phosphorus.

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Incendiary

It is commonly believed that white phosphorus ignites spontaneously on contact with air at room temperature. This is not quite true; the autoignition temperature is actually about 30°C in humid air, and slightly higher in dry air. However at slightly lower temperatures WP will slowly surface oxidise, effectively smouldering, and will often warm up to the point where it will ignite. At any rate, the slightest degree of friction will easily ignite it, and it is practically guaranteed to be ignited by a burster charge, so for all intents and purposes it is pyrophoric.

Because of this, WP has long had a secondary role as an incendiary, either directly or more usually as a "first fire" material. Contrary to another popular myth, it does not burn particularly fiercely, especially in comparison to other incendiaries like thermite. As an incendiary, it is most effective against highly flammable targets like very dry vegetation or petrol, oils and lubricants. However a WP fire does have the special difficulty that if extinguished with water, even to the point of being quite cold, it may reignite later when it dries out and exposes the WP to the air again.

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First aid and Toxicity

Burns to persons struck by particles of burning WP are usually much less extensive than napalm or metal incendiary burns, but are complicated by the toxicity of phosphorus, the release of phosphoric acid into the wounds, and the possibility of small particles continuing to smoulder for some time if undetected.

The most immediate concern is to exclude air from wounds so as to extinguish any remaining burning particles—while the first aider takes care not to touch the WP particles. Usually, this is mostly practicably achieved by cutting off contaminated clothing and dumping it in a fire resistant contaminated waste bin, and applying soaked compresses to the wounds. Ideally the compresses should be soaked in a mild sodium bicarbonate solution to neutralise phosphoric acid. If the patient is to be transported, sufficient water must be provided to keep the bandages wet at all times.

As soon as practicable, remaining particles must be removed from the wounds. This is done by underwater debridement of burnt areas with tweezers or a blunt metal spatula. If available, irrigation with a fresh 1% solution of copper sulphate is advantageous as it reacts with any remaining phosphorus particles, coating them with a layer of copper phosphide. (This very dark material is easier to see, and also fluoresces under ultraviolet light, if available. Furthermore it provides a relatively inert coating.) Care must be taken to only irrigate briefly, however, and thoroughly rinse away the solution afterward, or there is a risk of copper poisoning through the wounds. If copper sulphate is not available, inspection of the wounds in a darkened room may reveal any missed pieces through phosphorescence.

Subsequently treat as for a burn, but seek expert medical advice to treat phosphorus poisoning. Avoid oily ointments until it is certain all phosphorus has been removed.

Afterward, ensure all particles of WP and contaminated clothing are decontaminated by incineration.

Chronic exposure to phosphorus over several months to years may lead to condition known as phossy-jaw.

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Arms control status

Use of white phosphorus is not specifically banned by any treaty, however the 1980 Convention on Conventional Weapons (Protocol III) prohibits the use of incendiary weapons against civilian populations or by air attack against military forces that are located within concentrations of civilians. [1] The United States is among the nations that have not signed this protocol.

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Disadvantages

See the pillaring above.

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History

WP is believed to have been used by Fenian arsonists in the 19th century in the form of a solution of WP in carbon disulfide. When the carbon disulfide evaporated, the WP would burst into flames, and probably also ignite the highly flammable carbon disulfide fumes. This mixture was known as "Fenian fire" and allegedly was also used by Wobbly extremists in the early 20th century.

Britain introduced her first factory built WP grenades in late 1916. However the phosphorus firm of Albright and Wilson suggested that the British government use a material similar to Fenian fire in several expedient incendiary weapons during the darkest hours of World War II. The only one actually fielded was the Grenade, No. 76 or Special Incendiary Phosphorus grenade, which consisted of a glass bottle filled with a mixture similar to Fenian fire, plus some latex (c.f. Molotov cocktail, Greek Fire). It came in two versions, one with a red cap intended to be thrown by hand, and a slightly stronger bottle with a green cap, intended to be launched from the Northover projector (a crude 2.5 inch blackpowder grenade launcher). Instructions on each crate of SIP grenades included the observations, inter alia:

Store bombs (preferably in cases) in cool places, under water if possible.

Stringent precautions must be taken to avoid cracking bombs during handling.

It was generally regarded as insanely dangerous to its own operators.

According to accounts of both U.S. soldiers and Iraqis, white phosphorous was used by the U.S. military in the November 2004 invasion of Falluja, injuring people (Iraq Analysis Group 2005).

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