# 1911 Encyclopædia Britannica/Tellurium

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​TELLURIUM [Symbol Te, atomic weight 127.5 (O=16)], a chemical element, found to a certain extent in nature in the uncombined condition, but chiefly in combination with other metals in the form of tellurides, such, for example, as sylvanite, black tellurium, and tetradymite. Small quantities are occasionally met with in iron pyrites, and hence tellurium is found with selenium in the flue dust, or chamber deposits of sulphuric acid works. Tellurium was first recognized as a distinct element in 1798 by M. H. Klaproth. It may be obtained by heating tellurium bismuth with sodium carbonate, lixiviating the fused mass with water, filtering, and exposing the filtrate to air, when the tellurium is gradually precipitated as a grey powder (J. J. Berzelius). J. Farbaky (Zeit. angew. Chem., 1897, p. 11) extracts the element from black tellurium as follows:The ore is boiled with concentrated sulphuric acid, the solution diluted, hydrochloric acid added and the tellurium (together with selenium) precipitated by sulphur dioxide and the process repeated when a purer tellurium is obtained. B. Brauner (Monats., 1889, 10, p. 414) recommends the following method for the purification. The crude element is treated with aqua regia and then evaporated with an excess of hydrochloric acid, the solution diluted and the tellurium precipitated by a current of sulphur dioxide. The precipitated tellurium is then fused with potassium cyanide, the melt extracted with water and the element precipitated by drawing a current of air through the solution and finally distilled in a current of hydrogen.
Tellurium is a brittle silvery-white element of specific gravity 6.27. It melts at 452° C. and boils at 478° C. (F. Krafit, Ber., 1903, 36, p. 4344). When heated in a current of hydrogen it sublimes in the form of brilliant prismatic crystals. An amorphous form is obtained when tellurium is precipitated from its solutions by sulphur dioxide, this variety having a specific gravity 6.015. When heated in air, tellurium burns, forming the dioxide TeO$_2$. The element is insoluble in water, but dissolves in concentrated sulphuric acid forming a deep red solution.
Like sulphur and selenium, tellurium combines directly with hydrogen to form telluretted hydrogen, TeH$_2$, an extremely objectionable smelling and highly poisonous gas, which was first prepared by Sir H. Davy in 1810. It is best obtained by decomposing metallic tellurides with mineral acids. It is soluble in water, the solution gradually decomposing with deposition of tellurium; it also decomposes on exposure to light. It burns, and also, like sulphuretted hydrogen, precipitates many metals from solutions of their salts. It may be liquefied, the liquid boiling at 0° C., and on further cooling, it solidifies, the solid melting at −48° C. Many tellurides of metals have been examined by C. A. Tibbals (Jour. Amer. Chem. Soc., 1909, 31, p. 902) who obtained the sodium and potassium tellurides by the direct union of their component elements and others from these by precipitation. The tellurides of the alkali metals immediately decompose on exposure to air, with liberation of tellurium. Two chlorides are known, the dichloride, TeCl$_2$, and the tetrachloride, TeCl$_4$. They are both obtained by passing chlorine over tellurium, the product being separated by distillation (the tetrachloride is the less volatile). The dichloride is an amorphous, readily fusible, almost black solid. It is decomposed by water with formation of tellurium and tellurous acid: 2TeCl$_2$+3H$_2$O=Te+H$_2$TeO$_3$+4HCl. The tetrachloride is a white crystalline solid which is formed by the action of chlorine on the dichloride or by sulphur chloride on the element. It melts at 224° C, and is exceedingly hygroscopic. Water decomposes it with formation of tellurous acid and other products. It combines directly with sulphur trioxide to form a complex of composition TeCl$_4$⋅SO$_3$. The tetrabromide similarly gives TeOBr$_2$·5SO$_3$ (W. Prandtl, Zeit. anorg. Chem., 1909, 62, p. 237). Iodides are also known.
Two oxides of the element are definitely known, viz., the dioxide, TeO$_2$, and the trioxide, TeO$_3$, whilst a monoxide, TeO, has also been described. The dioxide is formed by burning tellurium in air or by warming it with nitric acid. It is a colourless crystalline solid which readily fuses to a yellow liquid. The trioxide is an orange coloured solid which is formed when telluric acid is strongly heated. Tellurous acid, H$_2$TeO$_3$, is obtained when the tetrachloride is decomposed by water, or on dissolving tellurium in nitric acid and pouring the solution into water. It is a colourless solid and behaves as a dibasic acid. The alkaline tellurites are soluble in water. It also gives rise to super-acid salts, such as KHTeO$_2$⋅H$_2$TeO$_3$; K$_2$TeO$_3$⋅3TeO$_2$. Telluric acid, H$_2$TeO$_4$, is obtained in the form of its salts when tellurium is fused with potassium carbonate and nitre, or by the oxidizing action of chlorine on a tellurite in alkaline solution. The free acid may be obtained by decomposing the barium salt with sulphuric acid and concentrating the solution, when a crystalline mass of composition H$_2$TeO$_4$⋅2H$_2$O separates. It is also formed when the dioxide is oxidized by hydrogen peroxide in caustic potash solution (A. Gutbier, Zeit. anorg. Chem., 1904, 40, p. 260), and perhaps best of all by oxidizing tellurium with a mixture of nitric and chromic acids. It crystallizes in prisms, which lose their water of crystallization at 160° C. The tellurates of the alkali metals are more or less soluble in water, those of the other metals being very sparingly or almost insoluble in water. Some tellurates exist in two forms, a colourless form soluble in water and acids, and a yellow form insoluble in water and acids. An oxychloride of tellurium has been described, but the investigations of V. Lenher (Jour. Amer. Chem. Soc., 1909, 31, p. 20) seem to negative its existence.
A considerable amount of work has been done on determinations of the atomic weight of tellurium, the earlier results giving the value 128. According to its position in the periodic classification of the elements one would expect its atomic weight to be less than that of iodine, instead of approximately equal, and on this account many efforts have been made to isolate another element from tellurium compounds, but none have as yet been successful. Recent investigations of the atomic weight are due to G. Gallo (Atti. R. Acad. Lincei, 1905 (iv.), 14, pp. 1, 23, 104), who, by a determination of the electrochemical equivalent of the element, arrived at the value 127.61; A. Gutbier (Ann., 1905, 342, p. 266) by reduction of the dioxide obtained 127.6; Marckwald, by determining the ratio of telluric acid to tellurium dioxide, obtained 126.85; H. B. Baker (Jour. Chem. Soc., 1907, 91, p. 1849), by determining the ratio of tellurium dioxide to oxygen and by analysis of tellurium tetra bromide, obtained 127.60, and V. Lenher (Jour. Amer. Chem. Soc., 1909, 31, p. 20), by heating the double salt, TeBr$_4$⋅2KBr, first in chlorine and finally in a current of hydrochloric acid to convert it into potassium chloride, obtained the value 127.55. P. E. Browning and W. R. Flint (Amer. J. Sci., 1909 (iv.), 28, p. 347) claim to have separated two substances (of atomic weights 126.49 and 128.85 respectively) from tellurium, by fractional precipitation of tellurium chloride with water, but in the opinion of H. B. Baker this would seem to point to the fact that the tellurium used was insufficiently purified, since his work showed that there was no difference between the first and last fractions (see Chem. Soc. Ann. Rep., 1909, 6, p. 39). Marckwald (Ber., 1903, 36, p. 2662) showed that the Joachimsthal pitchblende yields tellurium and a minute quantity of the strongly radioactive polonium which is precipitated by bismuth (see Radioactivity).