# Red phosphorus

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 under light or heating. It finds applications as matches and fire retardants. 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:

## Preparation

One method of preparing red phosphorus involves heating white phosphorus in an inert atmosphere like nitrogen or carbon dioxide, with iodine 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 polymeric, red phosphorus is insoluble in solvents. 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 halogens, sulfur, and metals compared with white phosphorus.

## Applications

Red phosphorus can be used as a flame retardant in resins. Its mechanism of action involves the formation of polyphosphoric acid (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 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 hydrolysis. This reaction is notable in the illicit production of methamphetamine and Krokodil, where hydrogen iodide acts as a reducing agent.
Red phosphorus is often used to prepare chemicals where the P-P bond is retained. Upon room temperature action with sodium chlorite, Na$_2$H$_2$P$_2$O$_6$ is formed.
Red phosphorus can be used as an elemental photocatalyst 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:
Violet phosphorus can be prepared by sublimation of red phosphorus in a vacuum, 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, 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 when red phosphorus is sublimed in vacuum in the presence of iodine.
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:
Fibrous red phosphorus, similar to red phosphorus, displays activity as a photocatalyst.