Phosphorus Violet

Tetraoxophosphoric acid, H3PO4 — This is prepared in the laboratory either by dissolving phosphorus(V) oxide in water (giving trioxophosphoric acid) and then heating to give the tetraoxo-acid or by heating violet phosphorus with 33% nitric acid, which oxidises it thus ... 

Any free bromine can be removed by passing the evolved gas through a U tube packed with glass beads covered with moist violet phosphorus (Figure 113). 

Hydrogen iodide is prepared in a similar way to hydrogen bromide, by the action of water on a mixture of iodine and violet phosphorus. TTie hydrogen iodide evolved may be collected by downward delivery or may be condensed (b.p. 238 K) it reacts with mercury and so cannot be collected over the latter. 

Similarly, phosphoric acid, H3PO4, is obtained from reaction of nitric acid with violet phosphorus. 

The acid also may be prepared by heating violet phosphorus with 33% nitric acid ... 

Four allotropes of phosphorus are known, the hexagonal /(-white, stable only below —77°C, the cubic a-white trap 44.1°C), the violet, and the black (which is thermodynamically the most stable). The a-white form is usually taken as the standard state. The violet is obtained by continued heating at 500°C of a solution of phosphorus in lead. When a-white phosphorus is heated to 250 C in the absence of air, a red variety (rap 590CC) is obtained which is believed to consist of a mixture of the a-white and violet allotropes, although the studies of the violet component in the mixture have shown that at least four polymorphic forms of red (violet) phosphorus exist. 

W. Hittorf prepared metallic or violet phosphorus by heating phosphorus in contact with lead for 10 hrs. at a temp, near 500°. The phosphorus dissolves in the lead at the high temp., and on cooling separates from the lead in the form of small, dark, reddish-violet, rhombohedral crystals. The crystals can be separated from the lead by treatment with dil. nitric acid, which dissolves only the lead. The crystals are further purified by boiling them with hydrochloric acid. A. Stock and F. Gomolka recommended the following procedure ... 

Bismuth may be used in place of lead, but it dissolves only one-fifth as much phosphorus, and the crystals obtained are less pure. The metals appear to be held in solid soln. Only very minute quantities of Hittorf s phosphorus are obtained by sublimation. According to L. Troost and P. Hautefeuille, the same variety is formed when red phosphorus is heated under press, to 580°. The work of A. Pedler, J. W. Retgers, and D. L. Chapman shows that this variety differs from ordinary red phosphorus only in the size and development of the crystals. Fine-grained red phosphorus is scarlet phosphorus, while coarse-grained red phosphorus is metallic or violet phosphorus. A number of other allotropes have been reported, but many of them are the result of a misinterpretation of facts, or of an incomplete knowledge of facts. 

A review of the alleged allotropes of phosphorus reduces their number to four, namely, the a- and/3-forms of yellow phosphorus, red or violet phosphorus, and black phosphorus. Most of the work of various investigators has been directed towards elucidating the nature of red phosphorus, and of the transformation of yellow to red phosphorus and conversely. Red phosphorus was formerly considered to be amorphous, and it was often called amorphous phosphorus. The term amorphous, however, here referred more to the general appearance of the powder rather than to its minute structure. J. W. Retgers 5 showed that the particles of ordinary red phosphorus are rhombohedral crystals, which are well developed in those of W. Hittorf s violet phosphorus. All four varieties are therefore crystalline. J. W. Terwen has reviewed this subject in a general way and M. Copisarow discussed the theory of allotropy,... 

H. W. B. Roozeboom suggested that the relations between white and red phosphorus may be like those of cyanogen, and that the vap. press, curve of liquid yellow phosphorus may terminate below the m.p. of the violet form he also said that it may be that liquid yellow phosphorus is super-cooled violet phosphorus. A. Smits and co-workers showed that liquid yellow phosphorus is to be taken as super-cooled liquid violet phosphorus. It is usual to consider under-cooled liquids as being still in the liquid state, and since yellow phosphorus crystallizes in the... 

P2vi2P and P4 2P2. R. Wegscheider and F. Kaufler argued that yellow and red phosphorus are not merely polymorphous, but that they are chemically different, because, if polymorphous, the liquid forms should be identical, and if violet phosphorus is the yellow phosphorus, or a sat. soln. of yellow phosphorus... 

A. Smits and S. 0. Bokhorst gave 589-5° at 43-1 atm. for the triple point of violet phosphorus. D. L. Chapman, and P. Jolibois estimated the triple point to be c. 600°. W. Marckwald and K. Helmholtz found the transition point of red to black phosphorus to be 575°. P. W. Bridgman found that the transition point of cubic or a-yellow phosphorus to hexagonal or /1-yellow phosphorus to be ... 

R. Lorenz and W. Herz studied some relations of the transition temp. A. Smits and S. C. Bokhorst gave 690-9° for the sublimation point of violet phosphorus. 

D. MacCrae and C. C. van Voorhis represented their measurements between 44-1° and 150° by log, p=7-9542—2757-5Z1-1 mm. L. Troost and P. Hautefeuille measured the vap. press, of liquid yellow phosphorus between 360° and 510°, and found that above 510°, the liquid was transformed so rapidly into red phosphorus that its vap. press, could not be measured. They also measured the vap. press, of red phosphorus between 360° and 577°. E. Riecke represented their values for solid red phosphorus by log10y=—45-01-f-16-28 log10 T—832jP-1 atm. and for liquid phosphorus, log10 p=—2-450+2-064 log10 T—1530 T 1 atm. P- Jolibois also measured the vap. press, of red and yellow phosphorus. A. Smits and S. C. Bokhorst gave for solid and liquid violet phosphorus from 308-5° to the critical temp., 695° ... 

They represented their results by logi0 p=19-2189 —3585-96T-1—3-59 logjq T mm. and on the assumption, rendered probable by the work of L. Troost and P. Hautefeuille, and A. Smits and co-workers, that under-cooled liquid violet phosphorus... 

It is assumed that violet phosphorus is the stable form but the metastable state of the element should have a higher vap. press, than the stable state, hence, it is further assumed that the mols. of the black and violet forms of phosphorus are not built up of the same kind of molecules. M. Centnerszwer found the partial press, of phosphorus vapour at 20° in an atm. of hydrogen is 0-0253 mm. oxygen, 0-0251 mm. carbon dioxide, 0-0312 mm. coal-gas, 0-0242 mm. air and iodobenzene, 0-0253 mm. W. Herz found that the constant c in the equation Bxj82 —TilT2-j-c(T2—Ti) varies from 0-001374 to 0-001990. Bx and 82 denote the b.p. of two liquids at any press., and Tx and Tz, the b.p. of the same liquids at another press. 

When white phosphorus is heated above 250°C in the absence of air and in the presence of a trace of iodine, which serves as a catalyst, the white allotrope is incompletely converted to violet phosphorus. Since this conversion is seldom complete, the resulting amorphous material consists of the white and violet allotropes and, because of its brownish-red appearance, is known as red phosphorus. 

Relatively little is known about the nature of violet phosphorus. It is best prepared by heating white phosphorus under high pressure or by crystallization from molten lead. Violet phosphorus has a metallic appearance, is nonflammable, and has a density of 2.69. 

Violet phosphorus (Hittorf s phosphorus) is a complex three-dimensional polymer in which each P atom has a pyramidal arrangement of three bonds linking it to neighboring P atoms to form a series of interconnected tubes, as shown in Fig. 15.3.3. These tubes lie parallel to each other, forming double layers, and in the crystal stmcture one layer has its tubes packed at right angles to those in adjacent layers. 

The latent heat of vaporisation of liquid yellow phosphorus at its boiling-point has been given as 4 Cals.2 or 3-89 Cals.3 This subject is further developed on p. 36, under Violet Phosphorus. ... 

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