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Encyclopedia Britannica - Main :: TAV-THE |
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THALLIUM [symbol Ti, atomic weight 204.0 (0=16)1, a metallic chemical element. It was discovered in 1861 by Sir William Crookes, who, during a spectroscopic examination of the flue-dust produced in the roasting of seleniferous pyrites occurring at Tilkerode in the Harz, observed a green line foreign to all then known spectra. He concluded that the mineral
The chemical character of thallium presents striking peculiarities. Dumas once called it the " ornithorhynchus paradoxus of metals." As an elementary substance, it is very similar in its physical properties to lead; it resembles lead chemically inasmuch as it forms an almost insoluble chloride and an insoluble iodide. But the hydroxide of thallium, in most of its properties, comes very close to the alkali metals; it is strongly basic, forms an insoluble chloroplatinate, and an alum strikingly similar to the corresponding potassium compounds. Yet, unlike potassium or lead, it forms a feebly basic sesquioxide similar to manganic oxide
Traces of thallium exist in many kinds of pyrites, as used for vitriol-making. The only known mineral
For the extraction of the metal from chamber mud, the latter is boiled with water, which extracts the thallium as the sulphate. From the filtered solution the thallium is precipitated as the chloride by addition of hydrochloric acid, along, in general, with more or less of lead chloride. The mixed chlorides are boiled down to dryness with sulphuric acid to convert them into sulphates, which are then separated by boiling water, which dissolves only the thallium salt. From the filtered solution the thallium is re-covered, as such, by means of pure metallic zinc, or by electrolysis. The (approximately pure) metallic sponge obtained is washed, made compact by compression, fused in a porcelain
Metallic thallium is bluish white; it is extremely soft and almost devoid of tenacity and elasticity. Its specific gravity is 11.86. It fuses at 290 C.; at a white heat it boils and can be distilled in hydrogen gas. Its vapour density at 1728 corresponds to the molecule 1'12. Its salts colour the Bunsen flame a bright green. When heated in air it _is readily oxidized,with the formation of a reddish or violet vapour. When ex-posed to the air it becomes quickly covered with a film of oxide
chloric. Thallium forms two series of salts: thallous, in which the metal is monovalent; and thallic, in which it is trivalent. In the thallous series many analogies with lead compounds are observed; in the thallic some resemblance to aluminium andgold. Thallous hydroxide, TIOH, is most conveniently prepared by de-composing the solution of the sulphate with baryta water. It crystallizes from its solution in long yellow needles, T10H or T10H+H20, which dissolve readily in water, forming an intensely alkaline solution, which acts as a caustic, and like it greedily absorbs carbonic acid from the atmosphere. Unlike the alkalis, it readily loses its water at loo C. and even at the ordinary temperature, to form the oxide T120, which is black or black-violet.Thallic oxide, T10 or T1202, was obtained by 0. Rabe (Abst. J.C.S., 1907, ii. 769) by acting with hydrogen peroxide on an alkaline solution of thallous sulphate at low temperatures, an initial red precipitate rapidly changing into a bluish-black compound. It melts at 720 and decomposes rapidly above 800, giving oxygen and thallous oxide. Thallous chloride, TIC!, is readily obtained from the solution of any thallous salt, by the addition of hydrochloric acid, as a white precipitate similar in appearance to silver chloride, like which it turns violet in the light and fuses below redness into a (yellow) liquid which freezes into a horn-like flexible mass. It is also formed when the metal is burnt in chloride. The specific gravity of this " horn " thallium is 7.02. One part of the precipitated chloride dissolves at o C. in 500 parts of water, and in 70 parts at 100 C. It is less soluble in dilute hydrochloric acid. Carbonate of soda solution dissolves it pretty freely. Thallous iodide, Tll, is obtained as a yellow precipitate, which requires 16,000 parts of cold water for its solution, by the addition of potassium iodide to a solution of a thallous salt, or by the direct union of its components. The yellow crystals melt at 190, and when cooled down assume a red colour, which changes to the original
standing
ordinary carbonate. It is produced by the exposure of thallous hydrate to carbon dioxide, and therefore is obtained when the moist metal is exposed to the air. It forms resplendent monoclinic prisms, soluble in water. Thallous sulphate, TI2SO4, forms rhombic prisms. soluble in water, which melt at a red heat with decomposition, sulphur dioxide being evolved. It unites with sulphuric acid giving an acid salt, T1HSO4.3H2O, and with aluminium, chromium and iron sulphates to form an " alum." It also forms double salts of the type T12SO4(Mg,Fe,ZnSO4)6H20. Thallous sulphide, TI2S, is obtained as a black precipitate by passing sulphuretted hydrogen into a thallous solution. It is insoluble in water and in the alkalis, but readily dissolves in the mineral acids. On thallium sulphides see H. Mallon, Abst. J.C.S., 1907, ii. 770. Thallous nitrate, T1NO3, is obtained as white, rhombic prisms by crystallizing a solution of the metal, oxide, carbonate, &c., in nitric acid. Various thallous phosphates are known. The normal salt, T13PO4, is soluble in 200 parts of water, and may be obtained by precipitation. On thallous salts see W. Stortenbeker, Abst. J.C.S., 1907, H. 770. Thallic oxide, TI203, is obtained as a dark reddish powder, insoluble in water and alkalis, by plunging molten thallium into oxygen, or by electrolysing water, using a thallium anode. Titallic hydroxide, Tl(OH)3, is obtained as a brown precipitate by adding a hot solution of thallous chloride in sodium carbonate to a solution of sodium hypochlorite. On drying it has the composition TIO(OH). Hydrochloric acid gives thallous chloride and chlorine; sulphuric acid gives off oxygen; and on heating it first gives the trioxide and afterwards the monoxide. The hydroxide is obtained as brown hexagonal plates by fusing thallic oxide with potash to which a little water has been added. Thallic chloride. TIC13, isobtained by treating the monochloride with chlorine under water; evaporation in a vacuum gives colourless deliquescent crystals of TIC13H2O. By heating the metal or thallous chloride in chlorine, TICITlCI3 is obtained, which on further heating gives3TlC1TICI3. as a yellowish brown mass. The chloride when anhydrous is a crystalline mass which melts at 24. It forms several double salts, e.g. with hydrochloric acid and the alkaline chlorides, and also with nitrosyl chloride. The chlorine is not completely precipitated by silver nitrate in nitric acid solution, the ionization apparently not proceeding to all the chlorine atoms. Thallic iodide, TII3, is interesting on account of its isomorphism with rubidium and caesium tri-iodides, a resemblance which suggests the formula
Analysis.All thallium compounds volatile or liable to dissociation at the temperature of the flame of a Bunsen lamp impart to such flame an intense green colour. The spectrum contains a bright green of wave-length 5351. From solutions containing it as thallous salt the metal is easily precipitated as chloride, iodide, or chloroplatinate by the corresponding reagents. Sulphuretted hydrogen, in the presence of free mineral acid, gives no precipitate; sulphide of ammonium, from neutral solutions, precipitates TI2S as a dark brown or black precipitate, insoluble in excess of reagent. Thallic salts are easily reduced to thallous by means of solution of sulphurous acid, and thus rendered amenable to the above reactions. The chloroplatinate serves for the quantitative estimation. L. F. Hawley employs sodium thiostannate which precipitates thallium as T12SnSa, insoluble in water, and which may be dried on a Gooch filter at 105. It may be noted that all thallium compounds are poisonous.The atomic weight of thallium was determined very carefully by Crookes, who found T1=204.2 (0=16); this figure was con-firmed by Lepierre in 1893. End of Article: THALLIUM If you wish, you can link directly to this article.
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