Vapour density of phosphorus

Dumas determined the vapour densities of mercury, phosphorus, arsenic hydride, and stannic and titanium chlorides, the last two being redetermined in 1830, and the vapour density of phosphorus again (with those of iodine and sulphur), and again in 1832. The vapour density of phosphorus was 4-355 or 4 420 (air = i) and that of sulphur at 493° 6 495, at 506 " 6 512, at 524° 6 617 and 6 581. The vapour density of phosphorus, he found, corresponded with four times, that of sulphur with six times, and that of mercury with half, the chemical atomic weights. Since Dumas thought that the molecules of all elementary gases contain two atoms, he was unable to resolve these difficulties. 

The Densities of Phosphorus Vapour.—That phosphorus, in common with other non-metals, forms complex molecules in the gaseous state was established early in the nineteenth century by determinations of vapour density. At a temperature of 500° C. the relative density (air = l) is 4-35 8 at higher temperatures it falls, being 3-632 at 1484° C. and 3-226 at 1677° C.9 The molecule P4 requires a density of 4-294. The molecular complexity thus revealed is confirmed by the low value of the ratio of the specific heats, namely, 1-175 at 300° C.10... 

Mitscherlich measured vapour densities at higher temperatures, finding abnormally high values for sulphur, phosphorus, and arsenic, and the density of phosphorus pentachloride only half the normal value (see p. 219). He described the toxicological detection of white phosphorus by distillation in steam and the glow seen in the condenser. His researches on benzene and its derivatives (see p. 331), on etherification and contact action (see p. 262) and affinity (see p. 617) are described elsewhere. 

A modification of Dumas vapour density method was used by Mitscherlich, who measured higher temperatures (270°-700°) with an air thermometer, the cylindrical glass tube containing the substance being put inside an iron tube heated in a charcoal furnace. For 300" a metal bath was used. He determined the vapour densities of bromine, sulphur, phosphorus, arsenic, mercury, sulphur trioxide, phosphorus pentachloride, antimony pentachloride, calomel and other mercury salts, and arsenious oxide. Sodium and potassium vapours attacked glass. He used H2 = i as unit with H = i the number of atoms in an equal volume found were i for mercury, 2 for bromine, 6 for sulphur, 4 for phosphorus and arsenic. The densities of phosphorus pentachloride and of antimony pentachloride were half the normal values. Mitscherlich did not appreciate the consequences of Avogadro s hypothesis e.g. he says i vol. of... 

Johann Heinrich Biltz (Berlin, 26 May 1865-Breslau, 29 October or 2 November 1943), a pupil of Victor Meyer, professor in Breslau (1911), determined the vapour densities of stannous chloride, cuprous and silver chlorides, phosphorus, sulphur, selenium, tin, arsenic, antimony and bismuth, detecting the molecule Sg. His later work was largely on organic chemistry. His brother Eugen Wilhelm Biltz (Berlin, 8 March 1877-Heidelberg, 13 November 1943) was professor in Gottingen (1900), Clausthal (1908), and Hannover. He published an immense number of papers, on colloids, the conductivities of fused salts, the compounds of ammonia with salts, compounds of beryllium and other rarer metals, sulphides, phosphides and tellurides, etc., and the molecular volumes of solid compounds. ... 

Phosphorus(III) oxide dissolves in several organic solvents, for example benzene, carbon disulphide the molecular weight in these solvents corresponds to the formula P40(, as does the density of the vapour, and the structure is ... 

This oxide was originally given the formula P2O5 and called phosphorus pentoxide but the vapour density and structure indicate the formula P40,q. It is prepared by burning phosphorus in a plentiful supply of air or oxygen ... 

The vapour density2 is 16-1 (air = l) corresponding to the molecular formula Asl3 (15-8), but the vapour, which is yellow, generally contains the products of thermal decomposition (see below). The heat of formation, according to Berthelot,3 is (As, 8lgas) 28,800 calories and (As, 3lsona) 12,600 calories. From measurements of density and coefficient of expansion at low temperatures the molecular volume at 0° Abs. has been calculated 4 to be 98-2, a value which corresponds with that similarly derived for the molecular volume of phosphorus triiodide. 

The density of the vapour from red or yellow phosphorus is the same, and it corresponds with the mol. P4 hence, from the analogy between a vapour and a solute —1.10, 8—it might be inferred that the two varieties of phosphorus would become identical in a common solvent. If a soln. of yellow phosphorus in phosphorus tribromide—with a trace of iodine as catalytic agent—is kept between 170° and 190°, red phosphorus is gradually deposited. R. Schenck measured the cone, of the yellow phosphorus in soln. after the lapse of different intervals of time, and found the reaction to be bimolecular, but when allowance is made for the mechanical removal of the catalytic agent from the soln. by the precipitated red phosphorus, the reaction is unimolecular. 

According to H. Moissan, phosphorus trifluoride is a colourless gas which does not fume in air, and which can be condensed to a colourless, mobile liquid which does not attack glass. The vapour density is 3-022, and the value calculated for PF3 is 3-045. This formula is also in accord with H. Moissan s analysis. The mol. vol, of the gas at 0° and 760 mm. is 153-24 c.c. Phosphorus trifluoride does not change when heated to 500° for half an hour in an iron vessel, but in a glass vessel, or in... 

The density of the vapour is the same whether it is derived from white or red phosphorus, and at lower temperatures and not too low pressures corresponds to molecules P4. 

The composition has been established by the vapour density and by analysis (e.g. with silicon, see p. 87). The fluoride is a colourless gas which does not fume in the air, and is highly poisonous. It condenses to a colourless liquid at - 95° C. and freezes to a white solid at -160° C. The heat of formation is 106-2 to 109-7 Cals.8 per mol, therefore much greater than that of the trichloride, and the trifluoride also proves to be the more stable of the two compounds. It can be decomposed by electric sparks with deposition of phosphorus and formation of the pentafluoride, thus —... 

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