Many allotropic varieties of phosphorus have been described but it seems probable that the white and vio let forms are the only definite allotropes (see ALLOTROPY), the others being "solid solutions" (see CHEMISTRY, PHYSICAL) of these two. The ordinary yellowish phosphorus is really white when pure, and probably owes its slight colour to a trace of the violet form. The so-called red phosphorus (used for matches) is pro duced by heating white phosphorus to about 23o° C for F4 hours in an inert atmosphere, or to 300° C for a few minutes in closed vessels ; a trace of iodine expedites the change. Commercially, the process is carried out in iron pots with air-tight lids which, how ever, have a long narrow tube open to the air ; at first a little phos phorus burns, but this uses up all the oxygen, and the process thereafter continues in an inert atmosphere with no risk of explo sion. The product is ground under water, freed from unchanged white phosphorus by boiling with caustic soda, washed and dried. The same form is also produced by submitting ordinary phospho rus to the silent electric discharge, to sunlight or to ultra-violet light. Red and white phosphorus differ greatly : the red does not inflame until heated above 35o° C (hence its use in the match industry), is insoluble in all the solvents which dissolve the white, has a higher density (2.2), is stable to air and light, is much less chemically reactive (since its formation from white phosphorus is accompanied by liberation of much heat-3,700 cal. per 31 g.), and is non-poisonous.
Hittorf's "metallic" or violet phosphorus is formed by heating phosphorus with lead to redness in a sealed tube, and removing the lead by boiling with nitric acid or by electrolytic means. It is also obtained by heating white or red phosphorus for long periods at high temperatures, the proportion of violet phosphorus in creasing with time and temperature. This fact, together with the circumstance that the density of red phosphorus continually in creases with the duration and intensity of its heating, suggests that the red form is only an intermediate stage in the production of the violet. (See above.) This variety forms lustrous, heavy, nearly black, minute rhombohedra. A scarlet form was obtained by R. Schenk in 1905, and in 1921, P. W. Bridgman described a black graphitic variety which was obtained by heating ordinary phosphorus at 200° C under a pressure of 12,000 atmos., and
which was incombustible, very dense, and conducted electricity. Bridgman obtained also another modification of white phos phorus, with a transition point at — 76.9° C under atmospheric pressure. The relationships of these forms to white and violet phosphorus are indefinite. The various forms are probably poly merides, for the red phosphorus is at least as complex as
whereas the white is
Smits postulates two forms
and Ps as the basis of all varieties, and, as many of the conversions are very slow, the equilibria involved are probably complex.
(phosphoretted hydrogen),
a gas formed in the putrefaction of organic matter containing phosphorus, was first obtained (Gengembre, 1789) by the action of potash on phosphorus; this was spontaneously inflammable, whereas Davy obtained a hydride which was not spontaneously inflammable by heating phosphorous acid. In 1835 Le Verrier showed that the two gases were essentially identical, and that the inflammability was due to traces of another hydride,
When caustic alkalis are boiled with phosphorus, the impure gas is obtained, and it also results from the action of water on calcium phosphide (unless this is very pure), but the pure
may be obtained by heating phosphonium iodide with caustic potash. It is a colourless, very poisonous gas, with an offensive odour like that of rotting fish ; it can be liquefied at —86° C and solidified at —133°. It is only slightly soluble in water, burns with a luminous flame, fires spon taneously in air at about me', and combines violently with oxygen and the halogens. When passed over heated metals, it liberates hydrogen and forms the phosphide. It forms double compounds such as
and
gives phosphonium salts,
with the halogen acids, and decomposes to its elements when heated out of contact with air. The hydride
can be condensed as a liquid if the impure gas (above) is passed through U-tubes in a freezing mixture. It boils at 57*-58° C. and above this temperature it breaks down to P4H2, which also results when it is exposed to light or left in contact with hydrochloric acid. The hydride
is a solid obtained by the action of phosphorus trichloride on
or of water on phosphorus di-iodide.