Red phosphorus is an allotrope of phosphorus. It is an amorphous polymeric red solid that is stable in air. It can be easily converted from

white phosphorus White phosphorus, yellow phosphorus, or simply tetraphosphorus (P4) is an allotrope of phosphorus. It is a translucent waxy solid that quickly yellows in light (due to its photochemical conversion into red phosphorus), and impure white phospho ...

under light or heating. It finds applications as

match A match is a tool for starting a fire. Typically, matches are made of small wooden sticks or stiff paper. One end is coated with a material that can be ignited by friction generated by striking the match against a suitable surface. Wooden matc ...

es and

fire retardant A fire retardant is a substance that is used to slow down or stop the spread of fire or reduce its intensity. This is commonly accomplished by chemical reactions that reduce the flammability of fuels or delay their combustion. Fire retardants ...

s. It was discovered in 1847 by Anton von Schrötter.

Structure

Red phosphorus is an amorphous form of phosphorus. Crystalline forms of red phosphorus include Hittorf's phosphorus and fibrous red phosphorus. The structure of red phosphorus contains the fragments illustrated below: Red phosphorus structure 1

Preparation

One method of preparing red phosphorus involves heating white phosphorus in an inert atmosphere like nitrogen or carbon dioxide, with

iodine Iodine is a chemical element; it has symbol I and atomic number 53. The heaviest of the stable halogens, it exists at standard conditions as a semi-lustrous, non-metallic solid that melts to form a deep violet liquid at , and boils to a vi ...

as catalyst. Another theoretically possible method of red phosphorus production is via light irradiation of white phosphorus. However, it has not been used industrially, likely due to the suspicious quality and unidentified structure the product.

Properties

Under standard conditions, red phosphorus is more stable than white phosphorus, but less stable than the thermodynamically stable black phosphorus. The standard enthalpy of formation of red phosphorus is −17.6 kJ/mol. Red phosphorus is kinetically most stable. Being

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

ic, red phosphorus is insoluble in

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s. It shows semiconductor properties. Due to such a kinetic stability, red phosphorus doesn't spontaneously ignite in air. It doesn't disproportionate in the presence of alkali, and is less reactive towards

halogen The halogens () are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would ...

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

, and

metals A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. These properties are all associated with having electrons available at the Fermi level, as against no ...

compared with

Applications

Red phosphorus can be used as a flame retardant in

resin A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...

s. Its mechanism of action involves the formation of

polyphosphoric acid In chemistry, a phosphoric acid, in the general sense, is a phosphorus acid, phosphorus oxoacid in which each phosphorus (P) atom is in the oxidation state +5, and is chemical bond, bonded to four oxygen (O) atoms, one of them through a double b ...

(the hydrogen atoms are from the resin) and char, which prevents flame propagation. However, for electronic/electrical systems, red phosphorus flame retardant has been effectively banned by major OEMs due to its tendency to induce premature failures. One persistent problem is that red phosphorus in epoxy molding compounds induces elevated leakage current in semiconductor devices. Another problem was acceleration of

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

reactions in PBT insulating material. Red phosphorus is used, along with abrasives, on the strike pads of modern safety matches. The match head, containing potassium chlorate, will ignite upon friction with the strike pad. However, the red color of the matchhead is due to addition of red dyes, and has nothing to do with red phosphorus content. Red phosphorus reacts with bromine and iodine to form phosphorus tribromide and phosphorus triiodide. Both are useful as halogenation agents, like replacing the hydroxyl group of alcohols. Phosphorus triiodide can also be used to produce hydroiodic acid after

. This reaction is notable in the illicit production of

methamphetamine Methamphetamine (contracted from ) is a potent central nervous system (CNS) stimulant that is mainly used as a recreational drug use, recreational or Performance-enhancing substance, performance-enhancing drug and less commonly as a secon ...

and Krokodil, where hydrogen iodide acts as a

reducing agent In chemistry, a reducing agent (also known as a reductant, reducer, or electron donor) is a chemical species that "donates" an electron to an (called the , , , or ). Examples of substances that are common reducing agents include hydrogen, carbon ...

. Red phosphorus is often used to prepare chemicals where the P-P bond is retained. Upon room temperature action with sodium chlorite, Na₂H₂P₂O₆ is formed.Phosphorus: Chemistry, Biochemistry and Technology, Sixth Edition, 2013, D.E.C. Corbridge, CRC Pres, Taylor Francis Group, Red phosphorus can be used as an elemental

photocatalyst In chemistry, photocatalysis is the acceleration of a photoreaction in the presence of a photocatalyst, the excited state of which "repeatedly interacts with the reaction partners forming reaction intermediates and regenerates itself after each ...

for hydrogen formation from the water. It has also been researched as a sodium ion battery anode.

Violet or Hittorf's phosphorus

Hittorf's phosphorus, or violet phosphorus, is one of the crystalline forms of red phosphorus. It adopts the following structure: Violetter Phosphor

Violet phosphorus can be prepared by sublimation of red phosphorus in a

vacuum A vacuum (: vacuums or vacua) is space devoid of matter. The word is derived from the Latin adjective (neuter ) meaning "vacant" or "void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressur ...

, in the presence of an iodine catalyst. It is chemically similar to red phosphorus. There are, however, subtle differences. Violet phosphorus ignites upon impact in air, while red phosphorus is impact stable. Violet phosphorus doesn't ignite in the presence of air upon room temperature contact with

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

, unlike red phosphorus. The reaction of red phosphorus and bromine alone does not generate a flame.

Fibrous red phosphorus

Fibrous red phosphorus is another crystalline form of red phosphorus. It is obtained along with

violet phosphorus Elemental phosphorus can exist in several allotropy, allotropes, the most common of which are white phosphorus, white and red phosphorus, red solids. Solid violet and black allotropes are also known. Gaseous phosphorus exists as diphosphorus and ...

when red phosphorus is sublimed in vacuum in the presence of

. It is structurally similar to violet phosphorus. However, in fibrous red phosphorus, phosphorus chains lie parallel instead of orthogonal, unlike violet phosphorus. Such a structure is depicted below: Red_phosphorus

Fibrous red phosphorus, similar to red phosphorus, displays activity as a photocatalyst.

References

{{reflist Allotropes Phosphorus