CHAPTER XXVI. THE NITROGEN FAMILY. PHOSPHORUS, ARSENIC, ANTIMONY, BISMUTH. A. Phosphorus. 343. Occurrence and Preparation of Phosphorus. Phosphorus is found in nature only in the combined form, chiefly in phosphates. The most abundant phosphate is calcium phosphate, Ca, (PO4)2. Calcium phosphate exists in the soil, and is taken up from it by plants. Animals consume phosphates in their food. Phosphorus is made from bone-ash, which is 60% to 70% Cas (PO4)2, or from natural phosphates. phosphate is heated with charcoal and sand in an electric furnace (Fig. 78). The The charge is fed into a hopper, C, with trap bottoms, E and F, to prevent the escape of phosphorus while the charge is being put in. The conveyor, G, forces the charge into the furnace. P D FIG. 78. HE F B The heat for the reaction is developed by the resistance which the current encounters between the electrodes, A and B. The slag is drawn off through D. Phosphorus vapor escapes through P, and is condensed under water. Crude phosphorus is purified by redistillation. It is then pressed, while liquid (under water), through a bone-ash filter. The phosphorus thus obtained is a white, transparent solid. About 3,000 tons are made annually. The partial equations for the reaction are, (3) (P2O5) +5 C →→→→→→ 2 P+5 CO. Hence the complete equation is, — Ca3(PO4)2+3 SiO2+5 C-3 CaSiO3+5 CO+2 P. 344. Properties. Phosphorus, like sulphur, exists in several allotropic forms. Ordinary or yellow phosphorus has a specific gravity of about 1.8, melts at about 45° C., and boils at 287° C. It is insoluble in water, but dissolves readily in carbon disulphide, CS2. It is very poisonous. Phosphorus derives its name, which means bearer of light' (cf. Latin, lucifer), from its property of phosphorescing, i. e., glowing in the dark. This phenomenon is caused by slow combustion on the surface of the phosphorus. Ordinary phosphorus ignites in air at 40° C., and burns with a hot flame to phosphorus trioxide and pentoxide. The spontaneous ignition of finely divided phosphorus has already been described (cf. § 31). Phosphorus unites readily with chlorine, bromine, and iodine even at the ordinary temperature. Two compounds of phosphorus and chlorine are possible, viz., the trichloride, PCl3, and MOLECULAR WEIGHT OF PHOSPHORUS. 321 the pentachloride, PCl5. Phosphorus trichloride is a liquid; phosphorus pentachloride, a crystalline solid. 2 P+3 Cl2 → 2 PCI3. 2 P+5 Cl2→→→→→ 2 PC15. 345. Red Phosphorus. A great difference exists between yellow phosphorus and the red modification. Red phosphorus is a reddish powder 2.2 times as heavy as water, infusible at red heat, unable to phosphoresce, insoluble in carbon disulphide, and not poisonous. It ignites at about 260° C. in air. Red phosphorus unites with the halogens only when heated. Red phosphorus is prepared by heating the ordinary form in closed iron tubes to 300° C. A small amount of the yellow phosphorus remains unchanged; this is removed by means of carbon disulphide, in which the red variety is insoluble. When a given amount of red phosphorus is burned, there is much less heat liberated than with an equal amount of the yellow form; the red has therefore much less energy than the yellow. This statement agrees with the known fact that when yellow phosphorus is changed into the red there is an evolution of heat. 346. Molecular Weight of Phosphorus. - The weight of a liter of phosphorus vapor is almost four times that of a liter of oxygen at the same temperature and pressure; consequently the molecular -weight of phosphorus, as a vapor, must be about 124, that is, about four times the atomic weight. The molecular formula of phosphorus vapor is thus written P4, just as that of oxygen is O2. 347. Matches. - Most of the phosphorus that is made is used to tip matches. The ordinary friction match, as made at present, consists of a splint of wood tipped, first, with sulphur, and then with a mixture containing some oxidizing agent, phosphorus, and an adhesive substance, like glue. The oxidizing agent may be potassium nitrate or chlorate, or the oxide of lead known as red lead, which has the formula Pb304. The chemical operations involved in lighting a match are essentially as follows: (1) The heat generated by rubbing the tip of the match against a rough surface causes the phosphorus to combine with the oxygen of the oxidizing agent in immediate contact with it. (2) The combustion of the phosphorus causes the sulphur to be raised to the kindling temperature of sulphur. (3) The burning of the sulphur raises the temperature of the wood to the kindling point; and the match burns. Safety matches have not the property of being easily ignited when rubbed; they require contact with a specially prepared surface. This surface is usually on the side of the match box, and consists of red phosphorus mixed with sand. The tip of the safety match generally contains antimony trisulphide (Sb2S3), an oxidizing agent, and glue. HYDROGEN PHOSPHIDE. 323 348. Hydrogen Phosphide (PH;). (PH3). Hydrogen phosphide, or phosphine, is a colorless gas which, as ordinarily made, is spontaneously combustible. The common method of preparing it is to heat a mixture of yellow phosphorus and a concentrated solution of sodium hydroxide. The equation is, 4 P+3 NaOH+3 H2O· H2 3 NaH2PO2+PH3. N FIG. 79. The apparatus (Fig. 79) consists of a generating flask containing the phosphorus and the sodium hydroxide solution. The stopper of the flask has two holes, one for a tube from a hydrogen generator and the other for a delivery tube ending under water. The air of the apparatus is first washed out by means of hydrogen (or illuminating gas); the gas then cut off, and the flask is heated. The escaping phosphine may be collected in a receiver, as shown in the figure, and this exposed to the air, or the bubbles of the gas may be allowed to escape through the water directly into the air. The material of the white smoke formed when phosphine burns is phosphorus pentoxide, water, and phosphoric acid. The equation for the combustion of phosphine is,— 2 PH3+4 02 → P2O5+3 H2O. |