Tellurium

TELLURIUM, a chemical element and the third member of the sulphur and selenium family (symbol Te, atomic number 52, atomic weight 127.5). It is a complex element having three isotopes (q.v.) with atomic weights 126, 128 and 130. Tellurium is a brittle, silvery white, metallic substance of specific gravity 6.27; it melts at 452° C and boils at 1,390° C; its hardness is 2.3 (Mobs). It crystallises in hexagonal-rhombohedral prisms and is isomorphous with the stable grey modification of selenium (q.v.). Tellurium was first recognised as a distinct element in 1798 by M. H. Klaproth, although it had already attracted the notice of mineralogists and metallurgists who on account of its peculiar properties termed it "aurum paradoxum" or "metallum prob lematum." It is widely distributed although in small amounts.

Rarely found native as metallic tellurium, it more generally occurs in combination with metals in such minerals as sylvanite, petzite, tetradymite, and telluric ochre, The red tellurium sulphur of Japan contains 99.76% S, 0.17% Te, 0.06% Se and o.oi% As. Tellurium minerals are found in Germany, Colorado, California, Ontario, Mexico, South America and West Australia, often accompanying gold deposits. The chief sources of tellurium are the slimes from copper and lead refineries and the flue dusts from telluride gold ores. The slimes contain both tellurium and selenium, and simi lar processes are used in the extraction of both elements. The slimes from copper refining contain more selenium, those from lead yield more tellurium. The dusts or slimes are fused with sodium carbonate and nitrate, the melt being lixiviated with water. The alkaline liquors containing sodium tellurite and selenite are cautiously acidified with sulphuric acid when tellurium dioxide is precipitated. The dioxide may be reduced in the dry way with powdered charcoal or dissolved in hydrochloric acid and reduced with sulphur dioxide, when tellurium is precipitated. Further puri fication is effected by dissolving tellurium in dilute nitric acid (sp.gr. 1.255) and crystallising the basic nitrate, 2Te02,HNO3.

Applications.

Despite the labours of the chemical research school at Wisconsin University, no extensive use has been found for tellurium, although comparatively large supplies are available, it being estimated that in the U.S.A. alone 125,000 lb. of the metal could be produced without material alterations in existing plants. Most of the output is sold to makers of wireless equip ment, since metallic tellurium has some merit as a rectifier or crystal detector. A noteworthy application was the addition of diethyl telluride to motor spirit as an anti-knock material, but although effective this substance has been superseded by lead tetraethyl (see LEAD, COMPOUNDS OF ; ORGANO-METALLIC COM POUNDS : Fourth Series). Colloidal tellurium has been suggested as

an insecticide and fungicide in wood preservation. To a limited extent tellurium has been employed in colouring glass or porce lain, developing brown, blue or red shades. Acid solutions of the dioxide have been used as a dip for silver ornaments, giving a "platinum finish" to the metal. Soluble tellurium compounds are utilised in toning baths in photography. Tellurium and compounds have been tested therapeutically without marked success.

Physical Properties.

Tellurium is a poor conductor of heat and electricity and the latter property varies only slightly with change of illumination. Its electrical resistance of 200,000 mi crohms per cubic centimetre is the highest of any metal. Its yellow vapour has a density corresponding with the molecular formula Allotropy (q.v.) is far less definite than with se lenium and sulphur. There is an amorphous modification of tellurium, a crystalline variety, and colloidal preparations of tel lurium are made by reducing dilute solutions of telluric acid with hydrazine or sulphurous acid with or without protective colloids.

Compounds.

Hydrogen telluride or telluretted hydrogen, is a very unstable gas with a repulsive odour; its boiling point is o° C and it melts at —48° C. It is best prepared by de composing aluminium telluride, Al2Te3, with water or dilute hydrochloric acid. Tellurium furnishes two chlorides, both pre pared by direct combination of the metal with chlorine. The dichloride, is a brown amorphous mass which melts in definitely to a black liquid boiling at 327° C. The tetrachloride, obtained with excess of chlorine, is a snow-white, crystalline mass melting at about 22o° and boiling at 380° C. It is hydrolysed by water. Similar compounds are formed with bromine.

Tellurium dioxide is a white crystalline mass produced by ig niting telluric acid or by oxidising tellurium with nitric acid; it becomes yellow on heating, melts at dull red heat and distils in a vacuum at bright red heat. It is only slightly soluble in water but is amphoteric, dissolving in either aqueous acids or alkalis. Tel lurium trioxide, an orange-yellow, crystalline substance, is left on igniting telluric acid at a red heat. Telluric acid, a snow-white crystalline powder, is made by oxidising tellurium or its dioxide with either chromic acid or chlorates.