Difference between revisions of "Phosphorus"

Revision as of 17:33, 2 August 2015

White, red powder, red chunk and violet phosphorus. (left to right.)

Phosphorus is a chemical element with symbol P and atomic number 15. It is a solid nonmetal, consisting of two main allotrope forms, white and red. The white form is highly reactive in the atmosphere, the others much less.

Properties

Chemical

White phosphorus reacts with air and can spontaneously ignite, if heated slightly, forming phosphorus pentoxide. Red phosphorus is much more stable in air, but will react with halogens.

Phosphorus chloride, for example, is formed from the reaction of white phosphorus and chlorine gas

P₄ + 6 Cl₂ → 4 PCl₃

White phosphorus will react with nitric acid to produce phosphoric acid and nitrogen dioxide.

P + 5 HNO₃ → H₂O + H₃PO₄ + 5 NO₂

Physical

Phosphorus is solid at standard conditions and exhibits several allotrope forms:

• White phosphorus - also known as yellow phosphorus, WP or simply tetraphosphorus (P₄) is a white waxy solid, that becomes yellow when exposed to light. When exposed to oxygen, it glows green in the dark, is extremely flammable and pyrophoric upon contact with air, and is very toxic, having the potential to cause morbidity and death through high enough exposures. It it almost always stored underwater to prevent ignition. White phosphorus is highly soluble in carbon disulfide and disulfur dichloride, somewhat soluble in chloroform and benzene, and slightly soluble in diethyl ether. Evaporation of these solutions yields scarlet-colored phosphorus.
• Red phosphorus is an amorphous red solid at standard conditions, and is considerably more stable than the white form. It is formed by heating white phosphorus to 250 °C in inert conditions. It is insoluble in all solvents.
• Violet phosphorus - also known as monoclinic phosphorus, it is solid brown-violet. It is formed by heating red phosphorus in a sealed tube at 530 °C or white phosphorus at 500 °C in molten lead for 18 hours. Insoluble in all solvents, violet phosphorus has low reactivity and ignites in air at 300 °C.
• Black phosphorus - it is thermodynamically stable at standard conditions. The black form is obtained by heating white phosphorus under incredibly high pressures (12,000 atmospheres). It is similar to graphite in appearance and properties, as a flaky black solid that conducts electricity. It is the least reactive allotrope form.
• Diphosphorus - Unlike the other forms, it is gaseous. Diphosphorus can be obtained normally only under extreme conditions (from P₄ at 1100 K). Is is only stable between 1200 °C - 2000 °C.

Only the first two forms are of importance.

White phosphorus is soluble in many organic solvents, whereas the other varieties are insoluble in any solvent.

Availability

In the US, the sale of red and white phosphorus is restricted, as it can reduce elemental iodine to hydroiodic acid, which is a reagent effective for reducing ephedrine or pseudoephedrine to methamphetamine. It is classified as List I precursor chemicals under 21 CFR 1310.02 by the DEA.

Red phosphorus can be extracted from the striker strips found on match boxes, that consist of red phosphorus, ground glass and glue. The strips are soaked with acetone to remove the glue, then scraped from the sides. As the scraped paste also contains ground glass and other impurities, purification is required. Red phosphorus is not soluble in any solvents, so the purification is a bit more complicated. One way to remove the glass is to add a solution of sodium hydroxide to dissolve the glass, although this will take some time. The resulting RP is filtered, collected and dried. This method however requires a large number of striker strips (a typical matchbox has around 0.005g RP, thus it takes around 200 matchboxes to obtain 1 g of RP).^[1]It is much more efficient in the long run to invest in making your own phosphorus-producing apparatus to make white phosphorus. Buying huge amounts of matches is also likely to be noticed and attract attention, as it is something often done by drug manufacturers.

Legality

Phosphorus of any kind, especially red phosphorus, is highly restricted in the United States, where it is a DEA List I chemical. As such, phosphorus is illegal to distribute and those that possess any are highly suspect for illicit drug manufacture.

Preparation

Elemental phosphorus is difficult to prepare by the home chemist, because extracting it requires very high temperatures. A mixture of calcium phosphate, silicon dioxide and carbon reacts at 1500 °C to form calcium silicate, carbon monoxide and white phosphorus:

2 Ca₃(PO₄)₂ + 6 SiO₂ + 10 C → 6 CaSiO₃ + 10 CO + P₄

By replacing carbon with aluminium, the reaction can occur at lower temperatures:

3 Na₃PO₄ + 5 Al + 2 SiO₂ → 3/4 P₄ + 5 NaAlO₂ + 2 Na₂SiO₃

A less thermal method involves the reaction of calcium phosphate with sulfuric acid, which produces phosphoric acid. This is reduced with carbon and the phosphorus vapors are condensed underwater. ^[2]

Another method would require the thermal decomposition of copper phosphide:

4Cu₃P₂ → 4Cu₃P + P₄

The copper phosphide can be obtained by reducing copper phosphate with a metal, (aluminium e.g.).

The final product of all these methods produce the white form, so the condensation of the phosphorus must be done underwater.

Projects

• Fenian fire (WP solution self-ignition via evaporation of solvent)
• Metal phosphides
• Phosphorus pentachloride synthesis

Handling

Safety

The white phosphorus allotrope presents a significant hazard because it's pyrophoric and produces phosphorus pentoxide and phosphoric acid fumes as residue, that are potent irritants. Chronic white phosphorus poisoning leads to necrosis of the jaw called "phossy jaw". Ingestion of white phosphorus may cause a medical condition known as "Smoking Stool Syndrome".

Certain combinations of phosphorus with halides are extremely toxic. Organophosphate compounds are known nerve toxins as are its precursors.

Storage

White phosphorus should be stored in an inert atmosphere, underwater is best. The other type of phosphorus do not require special storage, and can be safely stored in open air.

Disposal

White phosphorus should be converted to phosphate ions, which are far less hazardous and can be safely disposed of.

References

1. ↑ http://williestop.tripod.com/generalchemdata/redphosphorus.html
2. ↑ Lange's Handbook of Chemistry

Relevant Sciencemadness threads

• Preparation of elemental phosphorus
• Question: Red phosphorus purification