Telluration

TEL, Et4pb. Lead tetraethyl, fellurates Formally salts containing oxy-anions of tellurium. Tellurales(VI) contain octahedral [TeO ] " species and derivatives of these. Tellurates(IV), tellurites, contain [TeOa] " species. 

Tellurium dioxide, Te02- Obtained by burning Te in air or heating tellurales(IV). Has rutile or brookile structures. Almost insoluble in water but gives tellurates(IV) with bases and, e g., TeCl4 with acids. 

Tellurous add, HzTeO, does not exist as TeOj is insoluble in water. Tellurates(/V), tellurites containing (TeO,) " species can be formed from TeO, and bases. 

Telluric acid, TefOH), is formed from Te in aqua regia and a chlorale(V). Forms tellurales, e.g. Na[TeO(OH)s], Na2[Te02(0H)4]. 

Selenium and tellurium react similarly, forming selenides and selenates(IV), and tellurides and tellurates(IV) respectively. Like the sulphide ion, S , the ions Se and Te form polyanions but to a much lesser extent. 

Tellurium trioxide, TeOa, is an orange yellow powder made by thermal decomposition of telluric(VI) acid Te(OH)g. It is a strong oxidising agent which will, like H2Se04, oxidise hydrogen chloride to chlorine. It dissolves in hot water to give telluric(VI) acid. This is a weak acid and quite different from sulphuric and selenic acids. Two series of salts are known. 

Both selenium hexafluoride and tellurium hexafluoride are more reactive than sulphur hexafluoride. Tellurium hexafluoride is slowly hydrolysed by water to telluric) VI) acid and on heating it decomposes to fluorine and the tetrafluoride. 

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

P. Grosse, Die Festktfrpereigenschaften von Tellur, Springer-Vedag, Berlin, Heidelberg, New York, 1969. 

The flow of circulating currents in the grounding conductors or ground of region two caused between two or more interconnected grounding stations, for a fault occurring in region one is termed the telluric effect. 

Telluric acid [7803-68-1] M 229.6, pKj 7.70, pKj 11.04 (Hnitric acid, then repeatedly from hot water (0.4mL/g). 

Fig. 15-3 Stray current interference of pipelines by telluric currents.

Other oxoacid salts of the alkali metals are discussed in later chapters, e.g. borates (p. 205), silicates (p. 347), phosphites and phosphates (p. 510), sulfites, hydrogensulfates, thiosulfates, etc. (p. 706) selenites, selenates, tellurites and tellurates (p. 781), hypohalites, halites, halates and perhalates (p. 853), etc. 

Selenous acid, 0=Se(0H)2, i.e. H2Se03, and tellurous acid, H2Te03, are white solids which can readily be dehydrated to the dioxide (e.g. in a stream of dry air). H2Se03 is best prepared by slow crystallization of an aqueous solution of Se02 or by oxidation of powdered Se with dilute nitric acid ... 

Alternatively, Te or Te02 can be oxidized by Cr03/HN03 or by 30% H2O2 under reflux. Acidification of a tellurate with an appropriate precipitating acid offers a further convenient route ... 

At least four series of periodates are known, interconnected in aqueous solutions by a complex series of equilibria involving deprotonation, dehydration and aggregation of the parent acid H5IO6 — cf. telluric acids (p. 782) and antimonic acids (p. 577) in the immediately preceding groups. Nomenclature is summarized in Table 17.24, though not all of the fully protonated acids have been isolated in the free state. The structural relationship between these acids, obtained mainly from X-ray studies on their salts, are shown in Fig. 17.24. H5IO6 itself (mp 128.5° decomp) consists of molecules of (HO)sIO linked into a three-dimensional array by O-H - O bonds (10 for each molecule, 260-278 pm). 

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