Bismuth

BISMUTH, a metallic element ; it was probably unknown to the Greeks and Romans, but in the middle ages it became familiar, notwithstanding its frequent confusion with other metals. In 145o Basil Valentine referred to it by the name "wismut" ; 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 "bise mutum," 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 con fusion with tin, lead, antimony, zinc and other metals; in Andreas Libavius confused it with antimony, and in 1675 Nicolas Lemery with zinc. These obscurities began to be elucidated by the researches of Johann Heinrich Pott (1692-1777), a pupil of Stahl, published in his Exercitationes chemicae de Wismutho (1769), and of N. Geoffroy, whose contribution to our knowledge of this metal appeared in the Memoires de l'academie f rancaise for 1753. Torbern Olof Bergman reinvestigated its properties and determined its reactions; his account, published in his Opuscula, contains the first fairly accurate description of the metal. Its symbol is Bi, its atomic number 83, and its atomic weight 209.

Ores and Minerals.

The native metal is usually found either 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. Bismuth crystals, like those of arsenic and antimony, belong to the rhombohedral system, the three elements forming an isomorphous series. When not tar nished, the mineral has a silver-white colour with a tinge of red, and the lustre is metallic. Hardness 2-2 2 ; specific gravity 9.70 9.83. The slight variations in specific gravity are due to the pres ence of small amounts of arsenic, sulphur or tellurium, or to en closed 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. Considerable deposits of bismuth occur in South Australia, California, Ontario, Spain, southern China, Rhodesia and India. The mineral has been found in some Cornish mines and is fairly abundant in Bolivia ; the lat ter country is the chief commercial source of bismuth and the principal ore is a hydrated bismuth oxide.

The oxide, bismuth ochre, and the sulphide, bismuth glance or bismuthinite, 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.

Metallurgy.

The dry process of extraction is most frequently practised, for the easy reducibility of the oxide and sulphide, to gether with the low melting-point of the metal, renders it possible to effect a ready separation of the metal from the gangue and im purities. The extraction from ores in which the bismuth is present in the metallic condition may be accomplished by a simple liqua tion, 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 bismuth, 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 re ducing agents to decompose any admixed bismuth oxide or sul phide. In the liquation 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.

Sulphide 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 re ducing any oxide, either present originally in the ore or produced in the calcination, and the iron combining with the sulphur of the bismuthinite. 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, containing the sulphides of bismuth, copper, iron, antimony, lead and a little silver. The lowest layer of the molten mass is prin cipally 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 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 by heating with charcoal and sodium carbonate.

The crude metal obtained by the preceding processes is general ly contaminated by arsenic, sulphur, iron, nickel, cobalt and anti mony, and sometimes with silver or gold. A dry method of purification consists in a liquation on a hearth of peculiar con struction, which occasions the separation of the unreduced bismuth sulphide and the bulk of the other impurities. The wet refining process is tedious and expensive, and is only exceptionally, em ployed, 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 re precipitating as basic nitrate with water. Chemically pure bis muth is obtained 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, redissolving this salt in nitric acid, and then precipitating with ammonia. The bis muth hydroxide so obtained is finally reduced by hydrogen.

A considerable proportion of the bismuth manufactured in the U.S.A. is derived from anode slimes obtained in the Betts process for desilverizing lead. These slimes are fused with sodium hy droxide and carbonate to remove lead and arsenic and with sodium sulphide to extract copper. The residue, containing 94% bismuth with silver and gold, is cast into slabs and made the anode of a second electrolytic process with bismuth chloride electrolyte ; pure bismuth is deposited at the cathode, gold and silver being pre cipitated at the anode.

Properties.

Bismuth is a very brittle metal with a white crystalline fracture and a characteristic reddish-white colour. The specific gravity of solid bismuth is 9.82, and of molten bismuth is io.o55. The metal therefore expands on solidification; and as it retains this property in a number of alloys, the metal receives extensive application in forming type-metals. Bismuth melts at 271°C, is appreciably volatile at red heat and boils at about r,5oo°C. At raoo°C its vapour contains a mixture of monatomic and diatomic molecules. Its thermal conductivity is the lowest of all metals, being 18 as compared with silver as r,000; its co efficient of expansion between o° and roo° is o.001341. Its elec trical conductivity is approximately 1.2, silver at o° being taken as leo; it is the most diamagnetic substance known, and its ther moelectric properties render it especially valuable for the con struction of thermopiles.

The metal tarnishes very slowly in dry air at ordinary tempera tures, but somewhat more rapidly in moist air or when heated. At a bright red heat it burns with a bluish flame to the trioxide. Bismuth combines directly with the halogens, and with the ele ments 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, etc. In its chemical affinities it resembles arsenic and antimony; but its hydride (Baia?) is only obtained in minute traces by the action of nascent hydrogen on bismuth salts.

Alloys.

When present in other metals, even in very small quantity, bismuth renders them brittle and impairs their electrical conductivity. Bismuth is a component of many alloys charac terized by their low fusibility and expansion in solidification. Its eute,ctics with lead and with tin melt at 127°C and 133°C respec tively. Ternary and quaternary alloys have even lower melting points (see FUSIBLE METAL).

Compounds.—Bismuth trioxide, Bi203, occurs in nature as bis muth 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. The hydrate, Bi(OH)a, is obtained as a white powder by adding potash to a solution of a bismuth salt. When chlorine oxidizes bismuth tri oxide suspended in caustic potash, potassium bismuthate KBiOa is obtained. The alkali bismuthates are employed as oxidizing agents in analysis. The corresponding hydrate HBiOa has been isolated. Bismuth pentoxide, Bi3O5, is obtained by heating bis muthic acid, HBi0a, to r3o°C.

Bismuth trichloride, BiCla, obtained by Robert Boyle by heat ing the metal with corrosive sublimAte, is the final product of burning bismuth in an excess of chlorine. It is a white substance, melting at 225°-23o°C and boiling at 435°-441°. With excess of water, it gives a whice precipitate of the oxychloride, Bi0C1. Bis muth trichloride forms double compounds with hydrochloric acid, the chlorides of the alkaline metals, ammonia, nitric oxide and nitrosyl chloride. The other bismuth halides closely resemble the trichloride in their methods of preparation and their proper ties, forming oxyhalides, BiOX, with water, and double compounds with ammonia, etc.

Carbonates.—The basic carbonate, 2(Bi0K03.H20, 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 (Bi0)2CO3.2Bi(OH)3.31120, constitutes the min eral bismutite.

Nitrates.—The normal nitrate, Bi(NO3)3.5H20, 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 "sub nitrate 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, Bi,S, and a trisulphide, Bi,Ss, the latter compound being formed when the sulphur is in excess. Bismuth trisulphide, Bl„S3, constitutes the mineral bismuthinite, 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 2oo°C it as sumes the crystalline form of the mineral. Bismuth forms com pounds similar to the trisulphide with selenium and tellurium. The tritelluride constitutes the mineral tetradymite, Bi2Tea.

Bismuth sulphate, Bia(SO.,)a, is obtained as a white powder by dissolving the metal or sulphide in concentrated sulphuric acid. Water decomposes it, giving a basic salt, Bi,(SO4)(OH),, which on heating gives (Bi0)2SO4.

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. If bismuth is present heating on charcoal with potassium iodide and sulphur gives a brilliant scarlet incrustation. The blackish brown sulphide precipitated from bismuth salts by sulphuretted hydrogen is insoluble in ammonium sulphide, but is readily dis solved by nitric acid.

Pharmacology and Therapeutics.

In the older therapeutic applications of bismuth compounds the following salts have been employed, the benzoate, carbolate, carbonate, citrate (including the double ammonio citrate contained in liquor bismuthi et am monii citratis B.P.), oxide, salicylate, sulphocarbolate, subnitrate (basic nitrate), subgallate and tannate. These substances have an extended vogue in various disorders of the digestive tract on ac count of their ability to form a coating—probably bismuth oxy chloride—which protects the lining membranes from irritation. Such bismuth salts have also sedative, astringent and antiseptic properties; they are safe drugs owing to the absence of remote actions. The faeces are coloured black owing to the formation of bismuth sulphide.

Bismuth carbonate, oxychloride and subnitrates are employed for diagnostic purposes in X-ray examinations, when these com pounds serve to determine the outline of the oesophagus, stomach and other sinuses in different parts of the body.

A

compound known as "B.I.P.P.," extensively used as a dress ing for wounds during the World War, is composed of iodoform and the subnitrate made into a cream with liquid paraffin.

A more recent employment of bismuth compounds is in re placing mercurials in the treatment of syphilis. Neutral sodium bismuthyl tartrate and sodium potassium bismuthyl tartrate in solution, or more generally in suspension, have been injected intra muscularly. But as these applications are attended by local pain and by secondary effects on skin and mucous membranes, digestive tract and kidneys, a fine suspension of metallic bismuth has been recommended also for intramuscular injection. Considerable doses are given as the metal is slower than arsenic in the form of arsenobenzene in healing active signs of disease.

Bismuth hydroxide and salicylate have been similarly employed and the French Codex, Nouveau Supplement, P926, indicates quinine iodo-bismuthate with directions for preparing a suspen sion for injection.

See W. H. Martindale and W. W. Westcott, Extra Pharmacopoeia (1926); new ed., Vol. i., 1928. (G. T. M.)