Red phosphorus: short process description
Red phosphorus is one of the allotropic forms of elemental phosphorus. It is largely amorphous and is considered a polymeric version of white phosphorus.
Commercial red phosphorus is normally produced by heating the white phosphorus at a temperature range of 250–350 °C for 40–50 hours in a closed furnace (exclusion of oxygen) and at ambient pressure.
After the polymerisation, the product is milled in presence of water then treated with an alkaline solution in order to remove traces of white phosphorus and finally filtered, washed and dried.
Although this special treatment is applied for removing white phosphorus in the production process, some mg/kg of white phosphorus still remain in the final product.
Specifications of commercial red phosphorus are presented in annex 1. The specifications state that the level of white phosphorus is < 200 mg/kg, but the present technology is able to reduce the white phosphorus content to < 100 mg/kg.
The term red phosphorus is used for describing a variety of different amorphous forms of the elemental phosphorus showing a range of colours from the orange to dark-violet. Such differences in colour can be explained by differences in:
• particle size of the powder,
• molecular weight,
• impurities normally present on the red phosphorus surface
Although the red phosphorus is largely amorphous, X-ray diffraction, optical microscopy and differential thermal analysis (DTA) have established the existence of several crystalline red varieties of pure elemental P in addition to the amorphous form. Normally the commercial red phosphorus is amorphous and the crystalline form is present only to a limited extent (< 10 % w) which is due to the ordered framework of different degrees of polymerisation.
Red phosphorus has been described as a complex three-dimensional polymer in which each P atom has a pyramidal arrangement of three bonds linking it to neighbouring P atoms.
It seems likely that all forms of red phosphorus are built from the pyramidal white phosphorus structure and that the polymer growth is terminated by the occluded impurities
such as halogen, oxygen or hydroxyl groups.
In conclusion, the amorphous red phosphorus probably consists of entirely random networks of P atoms terminated by oxygen or hydroxyl groups. This assumption is confirmed by NMR spectra of the solid red phosphorus material. Due to the fact that red phosphorus is a polymer, it is not a surprise that physico-chemical characteristics, the reactivity and the stability are far different from white phosphorus: the white phosphorus is crystalline, contains discrete P4 molecules, has a melting point of about 44°C, is very soluble in organic solvents like CS2 and benzene, is very unstable and spontaneously ignites in presence of air. Whereas white phosphorus is a very toxic product, the red phosphorus is not toxic (LD 50 oral rat is > 2000mg/kg) as expected on the basis of the polymeric structure.
Red phosphorus shows a totally different chemical behaviour than white phosphorus. Red phosphorus is a polymeric allotropic modification of phosphorus. White phosphorus consists of reactive P4 – tetrahedra (molecular weight: 124 g/mol) whereas red phosphorus has a polymeric structure of Pn . Consequently, the reactivity of red phosphorus is much lower than of white phosphorus. Yellow phosphorus has to be handled under water otherwise it will start to burn spontaneously. White phosphorus has a wax like appearance whereas red phosphorus is a red to violet coloured powder which can be handled in air. The vapour pressure of white phosphorus at 25 °C is 0.05 mbar whereas red phosphorus has no detectable vapour pressure at this temperature.
Due to these differences the reaction velocity of red phosphorus with water is much slower compared to white phosphorus, but the main reaction products are also phosphorus-containing acids. If the total amount of white phosphorus contained in commercial red phosphorus (upper limit 200 mg/kg) were to dissolve in water, the following
concentrations given in Table 2 would be achieved. One has to bear in mind that white phosphorus also reacts with water so that these calculated amounts of white phosphorus
can only be achieved theoretically. Laboratory experiments revealed that the yellow phosphorus contained in red phosphorus is not readily extractable with water.
If traces of white phosphorus are released from the red phosphorus, they probably quickly react to phosphorus containing acids – the same products that the red phosphorus itself liberates. The source of these phosphorus containing acids be it white or red phosphorus cannot be distinguished by chemical analysis, because the products themselves are identical and red phosphorus as a starting material is present in immense excess.
The concentration of obtained hydrolysis products from red phosphorus steadily increases with the amount of dispersed red phosphorus in water. However, the reaction of red phosphorus with water is extremely slow. The average amount of reaction products from 100 mg/L after 24 h calculated as phosphorus is about 0.7 %. This fraction rises very slowly up to a maximum of 3.7% of the nominal concentration of solid red phosphorus in water after 700 hours. In another experiment the soluble reaction compounds increased only up to 2.7% after 2 880 hours (4 months). These data show that red phosphorus does not dissolve as such in water which renders the concept of a maximum solubility unapplicable. Instead, a continuous but slow series of reaction occurs leading to phosphorus containing acids. The amount of products formed increases with the available amount of red phosphorus and time.
