1911 Encyclopædia Britannica/Bismuth

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BISMUTH, a metallic chemical element; symbol Bi, atomic weight 208.5 (O = 16). It was probably unknown to the Greeks and Romans, but during the middle ages it became quite familiar, notwithstanding its frequent confusion with other metals. In 1450 Basil Valentine referred to it by the name “wismut,” and characterized it as a metal; some years later Paracelsus termed it “wissmat,” and, in allusion to its brittle nature, affirmed it to be a “bastard” or “half-metal”; Georgius Agricola used the form “wissmuth,” latinized to “bisemutum,” and also the term “plumbum cineareum.” Its elementary nature was imperfectly understood; and the impure specimens obtained by the early chemists explain, in some measure, its confusion with tin, lead, antimony, zinc and other metals; in 1595 Andreas Libavius confused it with antimony, and in 1675 Nicolas Lemery with zinc. These obscurities began to be finally cleared up with the researches of Johann Heinrich Pott (1692–1777), a pupil of Stahl, published in his Exercitationes chemicae de Wismutho (1769), and of N. Geoffroy, son of Claude Joseph Geoffroy, whose contribution to our knowledge of this metal appeared in the Mémoires de l’académie française for 1753. Torbern Olof Bergman reinvestigated its properties and determined its reactions; his account, which was published in his Opuscula, contains the first fairly accurate description of the metal.

Ores and Minerals.—The principal source of bismuth is the native metal, which is occasionally met with as a mineral, usually in reticulated and arborescent shapes or as foliated and granular masses with a crystalline fracture. Although bismuth is readily obtained in fine crystals by artificial means, yet natural crystals are rare and usually indistinct; they belong to the rhombohedral system and a cube-like rhombohedron with interfacial angles of 92° 20′ is the predominating form. There is a perfect cleavage perpendicular to the trigonal axis of the crystals; the fact that only two (opposite) corners of the cube-like crystals can be truncated by cleavage at once distinguishes them from true cubes. When not tarnished, the mineral has a silver-white colour with a tinge of red, and the lustre is metallic. Hardness 2–21/2; specific gravity 9·70–9·83. The slight variations in specific gravity are due to the presence of small amounts of arsenic, sulphur or tellurium, or to enclosed impurities.

Bismuth occurs in metalliferous veins traversing gneiss or clay-slate, and is usually associated with ores of silver and cobalt. Well-known localities are Schneeberg in Saxony and Joachimsthal in Bohemia; at the former it has been found as arborescent groups penetrating brown jasper, which material has occasionally been cut and polished for small ornaments. The mineral has been found in some Cornish mines and is fairly abundant in Bolivia (near Sorata, and at Tasna in Potosi). It is the chief commercial source of bismuth.

The oxide, bismuth ochre, Bi2O3, and the sulphide, bismuth glance or bismuthite, are also of commercial importance. The former is found, generally mixed with iron, copper and arsenic oxides, in Bohemia, Siberia, Cornwall, France (Meymac) and other localities; it also occurs admixed with bismuth carbonate and hydrate. The hydrated carbonate, bismutite, is of less importance; it occurs in Cornwall, Bolivia, Arizona and elsewhere.

Of the rarer bismuth minerals we may notice the following:—the complex sulphides, copper bismuth glance or wittichenite, BiCu3S3, silver bismuth glance, bismuth cobalt pyrites, bismuth nickel pyrites or saynite, needle ore (patrinite or aikinite), BiCuPbS3, emplectite, CuBiS2, and kobellite, BiAsPb3S6; the sulphotelluride tetradymite; the selenide guanajuatite, Bi2Se3, the basic tellurate montanite, Bi2(OH)4TeOe; the silicates eulytite and agricolite, Bi4(SiO2)3; and the urnayl arsenate walpurgite, Bi(UO2),(OH)24(A3O4)4.

Metallurgy.—Bismuth is extracted from its ores by dry, wet, or electro-matallurgical methods, the choice depending upon the composition of the ore and economic conditions. The dry process is more frequently practised, for the easy reducibility of the oxide and sulphide, together with the low melting-point of the metal, renders it possible to effect a ready separation of the metal from the gangue and impurities. The extraction from ores in which the bismuth is present in the metallic condition may be accomplished by a simple liquation, or melting, in which the temperature is just sufficient to melt the bismuth, or by a complete fusion of the ore. The first process never extracts all the bisbuth, as much as one-third being retained in the matte or speiss; the second is more satisfactory, since the extraction is more complete, and also allows the addition of reducing agents to decompose any admixed bismuth oxide or sulphide. In the liquidation process the ore is heated in inclined cylindrical retorts, and the molten metal is tapped at the lower end; the residues being removed from the upper end. The fusion process is preferably carried out in crucible furnaces; shaft furnaces are unsatisfactory on account of the disintegrating action of the molten bismuth on the furnace linings.

Sulphuretted ores are smelted, either with or without a preliminary calcination, with metallic iron; calcined ores may be smelted with carbon (coal). The reactions are strictly analogous to those which occur in the smelting of galena (see Lead), the carbon reducing any oxide, either present originally in the ore or produced in the calcination and the iron combining with the sulphur of the bismuthite. A certain amount of bismuth sulphate is always formed during the calcination; this is subsequently reduced to the sulphide and ultimately to the metal in the fusion. Calcination in reverberatory furnaces and a subsequent smelting in the same type of furnace with the addition of about 3% of coal, lime, soda and fluorspar, has been adopted for treating the Bolivian ores, which generally contain the sulphides of bismuth, copper, iron, antimony, lead and a little silver. The lowest layer of the molten mass is principally metallic bismuth, the succeeding layers are a bismuth copper matte, which is subsequently worked up, and a slag. Ores containing the oxide and carbonate are treated either by smelting with carbon or by a wet process.

In the wet process the ores, in which the bismuth is present as oxide or carbonate, are dissolved out with hydrochloric acid, or, if the bismuth is to be extracted from a matte or alloy, the solvent employed is aqua regia or strong sulphuric acid. The solution of metallic chlorides or sulphates so obtained is precipitated by iron, the metallic bismuth filtered, washed with water, pressed in canvas bags, and finally fused in graphite crucibles, the surface being protected by a layer of charcoal. Another process consists in adding water to the solution and so precipitating the bismuth as oxychloride, which is then converted into the metal.

The crude metal obtained by the preceding processes is generally contaminated by arsenic, sulphur, iron, nickel, cobalt and antimony, and sometimes with silver or gold. A dry method of purification consists in a liquation on a hearth of peculiar construction, which occasions the separation of the unreduced bismuth sulphide and the bulk of the other impurities. A better process is to remelt the metal in crucibles with the addition of certain refining agents. The details of this process vary very considerably, being conditioned by the composition of the impure metal and the practice of particular works. The wet refining process is more tedious and expensive, and is only exceptionally employed, as in the case of preparing the pure metal or its salts for pharmaceutical or chemical purposes. The basic nitrate is the salt generally prepared, and, in general outline, the process consists in dissolving the metal in nitric acid, adding water to the solution, boiling the precipitated basic nitrate with an alkali to remove the arsenic and lead, dissolving the residue in nitric acid, and reprecipitating as basic nitrate with water. J. F. W. Hampe prepared chemically pure bismuth by fusing the metal with sodium carbonate and sulphur, dissolving the bismuth sulphide so formed in nitric acid, precipitating the bismuth as the basic nitrate, re-dissolving this salt in nitric acid, and then precipitating with ammonia. The bismuth hydroxide so obtained is finally reduced by hydrogen.

Properties.—Bismuth is a very brittle metal with a white crystalline fracture and a characteristic reddish-white colour. It crystallizes in rhombohedra belonging to the hexagonal system, having interfacial angles of 87° 40′. According to G. W. A. Kahlbaum, Roth and Siedler (Ziet. Anorg. Chem. 29, p. 294), its specific gravity is 9·78143; Roberts and Wrightson give the specific gravity of solid bismuth as 9·82, and of molten bismuth as 10·055. It therefore expands on solidification; and as it retains this property in a number of alloys, the metal receives extensive application in forming type-metals. Its melting-point is variously given as 268·3° (F. Rudberg and A. D. von Riemsdijk) and 270·5° (C. C. Person); commercial bismuth melts at 260° (Ledebur), and electrolytic bismuth at 264° (Classen). It vaporizes in a vacuum at 292°, and its boiling-point, under atmospheric pressure, is between 1090° and 1450° (T. Carnelley and W. C. Williams). Regnault determined its specific heat between 0° and 100° to be 0·0308; Kahlbaum, Roth and Siedler (loc. cit.) give the value 0·03055. Its thermal conductivity is the lowest of all metals, being 18 as compared with silver as 1000; its coefficient of expansion between 0° and 100° is 0·001341. Its electrical conductivity is approximately 1·2, silver at 0° being taken as 100; it is the most diamagnetic substance known, and its thermoelectric properties render it especially valuable for the construction of thermopiles.

The metal oxidizes very slowly in dry air at ordinary temperatures, but somewhat more rapidly in moist air or when heated. In the last case it becomes coated with a greyish-black layer of an oxide (dioxide (?)), at a red heat the layer consists of the trioxide (Bi2O3); and is yellow or green in the case of pure bismuth, and violet or blue if impure; at a bright red heat it burns with a bluish flame to the trioxide. Bismuth combines directly with the halogens, and the elements of the sulphur group. It readily dissolves in nitric acid, aqua regia and hot sulphuric acid, but tardily in hot hydrochloric acid. It is precipitated as the metal from solutions of its salts by the metals of the alkalis and alkaline earths, zinc, iron, copper, &c. In its chemical affinities it resembles arsenic and antimony; an important distinction is that it forms no hydrogen compound analogous to arsine and stibine.

Alloys.—Bismuth readily forms alloys with other metals. Treated with sodammonium it yields a bluish-black mass, BiNa3, which takes fire in the air and decomposes water. A brittle potassium alloy of silver-white colour and lamellar fracture is obtained by calcining 20 parts of bismuth with 16 of cream of tartar at a strong red heat. When present in other metals, even in very small quantity, bismuth renders them brittle and impairs their electrical conductivity. With mercury it forms amalgams. Bismuth is a component of many ternary alloys characterized by their low fusibility and expansion in solidification; many of them are used in the arts (see Fusible Metal).

Compounds.—Bismuth forms four oxides, of which the trioxide, Bi2O3, is the most important. This compound occurs in nature as bismuth ochre, and may be prepared artificially by oxidizing the metal at a red heat, or by heating the carbonate, nitrate or hydrate. Thus obtained it is a yellow powder, soluble in the mineral acids to form soluble salts, which are readily precipitated as basic salts when the solution is diluted. It melts to a reddish-brown liquid, which solidifies to a yellow crystalline mass on cooling. The Hydrate, Bi(OH)3, is obtained as a white powder by adding potash to a solution of a bismuth salt. Bismuth dioxide, BiO or Bi2O2, is said to be formed by the limited oxidation of the metal, and as a brown precipitate by adding mixed solutions of bismuth and stannous chlorides to a solution of caustic potash. Bismuth tetroxide, Bi2O4, sometimes termed bismuth bismuthate, is obtained by melting bismuth trioxide with potash, or by igniting bismuth trioxide with potash and potassium chlorate. It is also formed by oxidizing bismuth trioxide suspended in caustic potash with chlorine, the pentoxide being formed simultaneously; oxidation and potassium ferricyanide simply gives the tetroxide (Hauser and Vanino, Zeit. Anorg. Chem., 1904, 39, p. 381). The hydrate, Bi2O4·2H2O, is also known. Bismuth pentoxide, Bi2C5, is obtained by heating bismuthic acid, HBiO3, to 130°C.; this acid (in the form of its salts) being the product of the continued oxidation of an alkaline solution of bismuth trioxide.

Bismuth forms two chlorides: BiCl2 and BiCl3. The dichloride, BiCl2, is obtained as a brown crystalline powder by fusing the metal with the trichloride, or in a current of chlorine, or by heating the metal with calomel to 250°. Water decomposes it to metallic bismuth and the oxychloride, BiOCl. Bismuth trichloride, BiCl3, was obtained by Robert Boyle by heating the metal with corrosive sublimate. It is the final product of burning bismuth in an excess of chlorine. It is a white substance, melting at 225°–230° and boiling at 435°–441°. With excess of water, it gives a white precipitate of the oxychloride, BiOCl. Bismuth trichloride forms double compounds with hydrochloric acid, the chlorides of the alkaline metals, ammonia, nitric oxide and nitrosyl chloride. Bismuth trifluoride, BiF3, a white powder, bismuth tribromide, BiBr3, golden yellow crystals, bismuth iodide, BiI3, greyish-black crystals, are also known. These compounds closely resemble the trichloride in their methods of preparation and their properties, forming oxyhaloids with water, and double compounds with ammonia, &c.

Carbonates.—The basic carbonate, 2(BiO)2CO3·H2O, obtained as a white precipitate when an alkaline carbonate is added to a solution of bismuth nitrate, is employed in medicine. Another basic carbonate, 3(BiO)2CO3·2Bi(OH)3·3H2O, constitutes the mineral bismutite.

Nitrates.—The normal nitrate, Bi(NO3)3·5H2O, is obtained in large transparent asymmetric prisms by evaporating a solution of the metal in nitric acid. The action of water on this solution produces a crystalline precipitate of basic nitrate, probably Bi(OH)2NO3, though it varies with the amount of water employed. This precipitate constitutes the “magistery of bismuth” or “subnitrate of bismuth” of pharmacy, and under the name of pearl white, blanc d’Espagne or blanc de fard has long been used as a cosmetic.

Sulphides.—Bismuth combines directly with sulphur to form a, disulphide, Bi2S2, and a trisulphide, Bi2S3, the latter compound being formed when the sulphur is in excess. A hydrated disulphide, Bi2S2·2H2O, is obtained by passing sulphuretted hydrogen into a solution of bismuth nitrate and stannous chloride. Bismuth disulphide is a grey metallic substance, which is decomposed by hydrochloric acid with the separation of metallic bismuth and the formation of bismuth trichloride. Bismuth trisulphide, Bi2S3, constitutes the mineral bismuthite, and may be prepared by direct union of its constituents, or as a brown precipitate by passing sulphuretted hydrogen into a solution of a bismuth salt. It is easily soluble in nitric acid. When heated to 200° it assumes the crystalline form of bismuthite. Bismuth forms several oxysulphides: Bi4O3S constitutes the mineral karelinite found at the Zavodinski mine in the Altai; Bi6O3S4 and Bi2O3S have been prepared artificially. Bismuth also forms the sulphohaloids, BiSCl, BiSBr, BiSI, analogous to the oyxhaloids.

Bismuth sulphate, Bi2(SO4)3, is obtained as a white powder by dissolving the metal or sulphide in concentrated sulphuric acid. Water decomposes it, giving a basic salt, Bi2(SO4)(OH)4, which on heating gives (BiO)2SO4. Other basic salts are known.

Bismuth forms compounds similar to the trisulphide with the elements selenium and tellurium. The tritelluride constitutes the mineral tetradymite, Bi2Te3.

Analysis.—Traces of bismuth may be detected by treating the solution with excess of tartaric acid, potash and stannous chloride, a precipitate or dark coloration of bismuth oxide being formed even when only one part of bismuth is present in 20,000 of water. The blackish brown sulphide precipitated from bismuth salts by sulphuretted hydrogen is insoluble in ammonium sulphide, but is readily dissolved by nitric acid. The metal can be reduced by magnesium, zinc, cadmium, iron, tin, copper and substances like hypophosphorous acid from acid solutions or from alkaline ones by formaldehyde. In quantitative estimations it is generally weighed as oxide, after precipitation as sulphide or carbonate, or in the metallic form, reduced as above.

Pharmacology.—The salts of bismuth are feebly antiseptic. Taken internally the subnitrate, coming into contact with water, tends to decompose, gradually liberating nitric acid, one of the most powerful antiseptics. The physical properties of the powder also give it a mild astringent action. There are no remote actions.

Therapeutics.—The subnitrate of bismuth is invaluable in certain cases of dyspepsia, and still more notably so in diarrhoea. It owes its value to the decomposition described above, by means of which a powerful antiseptic action is safely and continuously exerted. There is hardly a safer drug. It may be given in drachm doses with impunity. It colours the faeces black owing to the formation of sulphide.