Red Phosphorus

The production of this form can hardly escape observation, since it is present in the residue from the combustion of white phosphorus in the air.

When liquid phosphorus is heated in a closed vessel or in a non-oxidising atmosphere, such as one of carbon dioxide, the liquid gradually turns red and is then converted into a red solid. The change is rapid at 240° C. and very rapid just below the boiling-point, 280.5° C., of the white form. Red phosphorus was first prepared in this way by Schrotter in 1845, and its chief properties were then investigated.

The red form is chiefly amorphous, and of vitreous appearance and fracture. On prolonged heating it gradually turns violet and exhibits double refraction.

The transformation is greatly accelerated, and takes place at lower temperatures, in the presence of small quantities of iodine, and also of selenium. In the presence of AlCl₃ the transformation occurs in evacuated tubes below 100° C.

When white phosphorus is treated with liquid ammonia it is converted into red phosphorus with simultaneous formation of amide and nitride. White phosphorus, when dissolved in turpentine, phosphorus tribromide and certain other solvents, is converted into red phosphorus by heating for several hours at 290° C. Red phosphorus may be deposited when phosphorus vapour is suddenly cooled, and it was early shown that on account of its small vapour pressure this form might be condensed in the hotter part of a tube (at 300° C.) from vapour derived from white phosphorus in the cooler part of the tube. All varieties of phosphorus when strongly heated in sealed tubes may be converted into yellow liquids which deposit red crystals at about 550° C.

The red phosphorus prepared by moderate heating may be ground up with a solution of sodium hydroxide as in the technical preparation and may also be extracted with carbon disulphide until the extract is free from white phosphorus. The resulting preparation is considered to be the purest red phosphorus.

Density of Red Phosphorus

There is considerable variation in the values given by different investigators. A preparation which would answer most closely to the description of "pure" red phosphorus and prepared as described above has a density which varies only within the comparatively narrow limits of 2.18 and 2.23, and the value remains practically constant after long heating at 357° C. The density of ordinary red phosphorus may be taken then as 2.20±0.02. Cohen and Olie, to whom these results are due, regard red phosphorus as a solid solution of the white in the violet form. On long heating at 450° C. the density increased to a maximum, which was held to correspond with the maximum proportion of the violet form, at a value of about 2.30. Heating to higher temperatures, between 500° and 600° C., so that the phosphorus became liquid, in some cases increased the density to 2.34, but in others the density diminished, e.g. to 2.24. The increase in the density on heating above 360° C. was considered by Jolibois to be due to the production of the form which he called "pyromorphic" phosphorus, which was stable below 450° C., but he also considered that red phosphorus was a distinct allotropic form, with an interval of stability from 450° to 610° C.

Vapour Pressure of Red Phosphorus

The vapour pressures of red phosphorus are much lower than those of liquid phosphorus or of solid white phosphorus at all temperatures within the experimental range. The latter are therefore unstable forms, or are monotropic with respect to red phosphorus (and equally of course to the violet form).

Thus at 279° C. the vapour pressure of liquid phosphorus is 753 mm., while at 289° C. that of red phosphorus is 23 mm. The pressures of red phosphorus are not in equilibrium, but fall slowly as the solid changes into the more stable "pyromorphic" (violet) form.

The pressure of the latter, however, appears to increase more rapidly with rise of temperature than that of red phosphorus, as is seen from the following table, on account of which 400° C. was given as the transition temperature by Jolibois.

Red Phosphorus		''Pyromorphic'' Phosphorus.
t° C.	p (mm.).	t° C.	p (mm.).
289	23	345	20
398	755	422	710
400	798	430	1045

These measurements agree fairly well with those of Hittorf and of v. Schrotter.

Melting-point of Red Phosphorus

According to Chapman the melting-point of red phosphorus is 610° C.; according to Stock and Gomolka 620° to 625° C. It is not far removed from that of violet phosphorus. The variability in the melting-point, vapour pressure and density is explained if red phosphorus is a mixture of various forms and is not itself a modification of phosphorus. [A " modification " is a state of aggregation which can exist in inner equilibrium, and which therefore is able to behave in a unary manner.]

The Specific Heat of red phosphorus is less than that of white phosphorus, being 0.1698 to 0.1705 between 15° C. and 98° C.

The specific heats determined over other ranges of temperature are—

t° C.	0-51° C.	0-134° C.	0-199° C.
Specific heat	0.1829	0.2121	0.2162

Hardness of Red Phosphorus

This is greater than that of white phosphorus, being 3.5 on Moh's scale.