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Nitrogen, N . . 14.04

Antimony, Sb . . 120
Phosphorus, P. . 31.0

Bismuth, Bi . . 208.5 Arsenic, As . . 75.0 In this family of elements we have a gradual transition from the non-metals to the metals. Nitrogen and phosphorus may be considered as typical non-metallic elements, both as regards their physical and chemical properties. The third member, arsenic, begins to exhibit metalline properties ; its specific gravity is more than three times as high as that of phosphorus, and it possesses considerable metallic lustre; arsenic is called a metalloi i on this account. Antimony is still more metallic in its character, possessing most of the physical attributes of a true metal, while in bismuth all non-metallic properties cease altogether to exist.

All these elements form more than one compound with oxygen, of which the following may be compared

N,03 ; (P.,O3)2 ; (As,O3)2 ; Sb.,03 ; B1,Oz.
N.O,; P.04; -- Sb.,0,; Bi.O.

N2O5; P,03; As,Oz; Sb,05; Bi,Oz. The oxides (which in the case of nitrogen and phosphorus are strongly acidic in their nature, combining with water to form acids) gradually become less and less acidic and more basic as the series is traversed.

Thus, nitrogen pentoxide, N.,0, unites violently with water to form nitric acid, which with bases yields nitrates. Antimony pentoxide is insoluble in water, and no antimonic acid has been isolated, although its salts, the antimonates, are known. The oxides of antimony, on the other hand, begin to exhibit basic properties and unite with acids, forming salts in which the antimony functions as the base.

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In the case of the last element the acidic nature of the oxides is entirely lost ; no bismuth compounds being known corresponding to antimonates or arsenates, while these oxides unite with acids in the capacity of bases, giving rise to bismuth salts.

Four of the elements of this group unite with hydrogen, forming similarly constituted compounds, NH3, P113, AsH3, SbHz.

The stability of these compounds gradually decreases as we pass from nitrogen to antimony. Antimony hydride has never been obtained free from other gases, while no similar bismuth compound is known. Ammonia is alkaline and strongly basic, and unites readily with acids to form ammonium salts. Phosphorus hydride has no alkaline character, and is only feebly basic. It combines, however, with the halogen acids to form phosphonium chloride, bromide, and iodide, PH,CI, PH,Br, PH 1, analogous to ammonium chloride, bromide, and iodide. The hydrides of arsenic and antimony exhibit no basic character. All the elements of this group unite with chlorine, giving rise to the compounds

NC13, PC13, AsCl3, SbCl3, BiCla, which also exhibit a gradation in their properties; thus, nitrogen trichloride is an extremely unstable liquid, exploding with extraordinary violence upon very slight causes, while the analogous bismuth compound is a perfectly stable solid.

The boiling-points of these compounds show a gradual increase with the increasing atomic weight of the element ; thus, nitrogen chloride boils at 71°, phosphorus trichloride at 78°, arsenic trichloride at 130.2", and antimony trichloride at 200°.

The elements arsenic, antimony, and bismuth are isomorphous, and their corresponding compounds are also isomorphous.

The first member of this family, namely, nitrogen, has been already treated in Part II. as one of the four typical elements studied in that section of the book. It occupies a position in relation to the other members of the family very similar to that of oxygen towards sulphur, selenium, and telluriurn.

Symbol, P. Atomic weight=31.0. Vapour density=62.0.

Molecular weight - 124.0.
History.-- Phosphorus was first discovered by the alchemist
Brand of Hamburg (1669), who obtained it by distilling a mixture

of sand with urine which had been evaporated to a thick syrup. The process, however, was kept secret. Robert Boyle (1680) discovered the process of obtaining this element, but the method was not published till after his death. Until the year 1771, when Scheele published a method by which phosphorus could be obtained from bone ash, this element was looked upon as a rare chemical curiosity. The name phosphorus was not first coined for this element: it had been in previous use to denote various substances known at that time, which had the property of glowing in the dark. To distinguish the element it was called Brand's phosphorus, or English phosphorus,

Occurrence.-Phosphorus has never been found in nature in the free state. In combination with oxygen and metals, as phosphates, it is very widely distributed, especially as calcium phosphate. The following are some of the commonest natural phosphates

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Calcium phosphate is present in all fertile soils, being derived from the disintegration of rocks : the presence of phosphates in soil has been shown to be essential to the growth of plants. From the vegetable it passes into the animal kingdom, where it is chiefly present in the urine, brain, and bones. Bones contain about 60 per cent. of calcium phosphate, to which they entirely owe their rigidity.

Mode of Formation.- Manufacture. The chief source of phosphorus is bone ash, a material obtained by burning bones, and which consists of nearly pure calcium phosphate, Caz(PO4). Other varieties of calcium phosphate, such as sombrerite and apatite, are also employed, as well as phosphates of other metals, such as the Redonda phosphates, which consist of phosphates of iron and alumina. The bone ash, in fine powder, is first decomposed by means of sulphuric acid, specific gravity 1.5 to 1.6. This operation is performed in large circular wooden vessels, resembling a brewer's “mash tun,” provided with an agitator, and into which high pressure steam can be driven. Finely-ground bone ash and sulphuric acid, in charges of a few cwts. at a time, are alternately stirred into the decomposer, until from four to five tons of

phosphate have been introduced, with sufficient acid to convert the whole of the lime into calcium sulphate, according to the equation

Caz(PO4)2+3H,50,=3CaSO4+2H3PO4. The contents of the decomposer are next run out into filtering tanks, and the phosphoric acid is then concentrated to a syrup, in large lead-lined pans through which steam-coils pass, the liquor being constantly agitated by a mechanical stirrer.

The concentrated liquor is next mixed, either with sawdust, or with coarsely-ground charcoal, or coke, and the mixture completely dried by being heated in a cast-iron pot, or in a muffle, to a

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dull red heat. During this process the tribasic phosphoric acid (or orthophosphoric acid), H3PO4, is converted by loss of water into metaphosphoric acid, HPO,–

H2PO4=H,O+HPOZ. The charred mixture is then distilled in bottle-shaped retorts of Stourbridge clay, about 3 feet long, and having an internal diameter of 8 inches. A number of these retorts, usually twenty-four, are arranged in two tiers, in a galley furnace, as seen in section in Fig. 118. The empty retorts are first gradually raised to a bright red heat, and a charge of the mixture is then quickly introduced. Bent

pieces of 2-inch walleable iron pipe are then luted into the mouths of the retorts connecting them with the pipes, D D'. These pipes dip into troughs of water, E E', which run along the entire length of the furnace, and in which the phosphorus condenses. The temperature of the furnace is then raised to a white heat, when decomposition of the metaphosphoric acid commences, and phosphorus begins to distil over. The process is continued for about sixteen hours. The change that goes on is mainly represented by the following equation-

4HPO3 +12C =12C0+2H2+4P. The crude product, which is usually dark red or black in appear

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Fig. 119.

ance, is first melted under hot water and thoroughly stirred, in order to allow the greater part of the rougher suspended matters to rise to the surface. The mass is then allowed to resolidify. The exact processes by which phosphorus is further purified on a manufacturing scale are guarded as trade secrets ; one method that has been in use consists in treating the phosphorus while melted under water with a mixture of potassium dichromate and sulphuric acid, whereby some of the impurities are oxidised and others are caused

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