Telluric acid

It is perceptibly soluble in water to give a neutral solution. It is soluble both in sodium hydroxide solutions and in hydrochloric acid. It is converted into a basic tellurium sulfate by heating to the appearance of white fumes with sulfuric acid. 

Submitted by Frank C. Mathers, Charles M. Rice, Howard Broderick, AND Robert Forney  

Although similar to sulfur dioxide in many ways, tellu-rium(IV) oxide cannot be oxidized to the hexavalent state by nitric acid. While many of the stronger oxidizing agents will accomplish this oxidation, potassium permanganate in a nitric acid solution is preferred since the by-products of this reaction are suflSciently soluble in water and in nitric acid solution to make possible a satisfactory separation from the telluric acid which is only slightly soluble in water and very insoluble in concentrated nitric acid. 

Five grams of potassium permanganate is dissolved in 100 ml. of water and the resulting solution added slowly 

The clear solution is evaporated until its weight has been decreased to about 60 g. Ten milliliters of concentrated nitric acid is added and the solution allowed to stand for 24 hours. The mother liquor is decanted from the crystals, evaporated to a weight of 25 g., and again treated with 10 ml. of concentrated nitric acid. A second crop of crystals is obtained after an additional 24 hours. 

Compounds containing hexavalent tellurium in which the tellurium atom is bonded via one or more heteroatoms to at least one carbon atom can be considered to be condensation products formed from orthotelluric acid and compounds with at least one active hydrogen atom in the molecule. 

Orthotelluric acid is a crystalline, well-characterized compound. All the hexavalent tellurium compounds discussed in this chapter are derivatives of orthotelluric acid. Telluric acid (H2TCO4), corresponding to sulfuric acid, is not known. Organic derivatives of such a telluric acid similar, for instance, to dimethyl sulfate, are also unknown. 

Irgolic Organo-tellurium Compounds without a C,Te-Bond 

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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]. 

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. 

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

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 ... 

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). 

Tellur-sfiure, /. telluric acid, -schwefelkohlen-stoff, m. carbon sulfide telluride, CSTe. -sil-ber, n. silver telluride. -silberblei, n. lead silver telluride, Min.) sylvanite. -silber-blende,/. sylvanite stutzite. 

Selenic acid, H2Se04, is a strong acid (/fai 2) and the solubility of its salts parallels that of the corresponding sulfates. It is formed by the oxidation of selenous acid or elemental selenium with strong oxidizing agents in the presence of water. Telluric acid, H2Te04, or tellurates are obtained by oxidation of tellurides,... 

Te (tellurium, brown black, amorphous), Te02 (tellurous anhydride, white), Te02 hydr. (tellurous acid, H2Te03 or Te02-H20, white, amorphous), TeOs hydr. (orthotelluric acid, H6Te06 or Te03 3H20, colorless, cubic or monoclinic). 

H2Te (hydrogen telluride, colorless), HTe (hydrogen telluride ion, colorless), Te (telluride ion, colorless), Te (ditelluride ion, red), Te" + (tellurous ion), HTeOj (telluryl ion), HTeO (acid tellurite ion, colorless), TeO " (tellurite ion, colorless), H2Te04 (telluric acid, colorless), HTeOJ (acid tellurate ion, colorless), TeO (tellurate ion, colorless). 

Lingane and Niedrach have claimed that the h-VI states of tellurium (or selenium) are not reduced at the dropping electrode under any of the conditions of then-investigation however, Norton et al. [42] showed that under a variety of conditions, samples of telluric acid prepared by several different procedures do exhibit well-defined (though irreversible) waves, suitable for the analytical determination of the element. The reduction of Te(H-VI) at the dropping electrode was found coulometri-cally to proceed to the -II state (whereas selenate, Se(-i-VI), was not reduced at the dropping electrode in any of the media reported). 

The electrodeposition of tellurium and silver has been investigated in dilute aqueous solutions of tellurous acid and Ag " ions (concentrations in the order of 10 to 10 " M) in 0.1 M HCIO4 [164], In particular, cyclic voltammetry experiments were conducted with rotating glassy carbon disk electrodes in baths with various concentration ratios of Ag(I) and Te(IV) precursors, and their outcome was discussed in terms of the voltammetric features. For a Ag(I)/Te(IV) ratio close to 0.8, formation of quasi pure silver telluride, Ag2Te, was reported. The authors, based on their measurements and on account of thermodynamic predictions, assumed that silver is deposited first on the electrode (Ag" + e Ag), and then Te(IV) is reduced on the previous silver deposit with formation of Ag2Te according to the reaction... 

Oxyacids containing selenium and tellurium are known, but they and their salts are generally less important than the sulfur compounds. When selenium and tellurium halides hydrolyze, the solutions contain selenous and tellurous acids. 

Although selenic and telluric acids contain the central atom in the +6 oxidation state, they are very different. The properties of selenic acid, H2Se04, are very similar to those of H2S04, and many of their salts are similar. The oxyacid that contains Te in the +6 state is HsTeOs which can also be written as Te(OH)s. This acid can be prepared from Te or Te02 by suitable oxidation reactions, and it can be also obtained as a solid hydrate. As expected from the formula, telluric acid is a weak acid, although some salts can be obtained in which one or more protons are replaced. 

The pure form of tellurium burns with a blue flame and forms tellurium dioxide (TeO ). It is brittle and is a poor conductor of electricity. It reacts with the halogens of group 17, but not with many metals. When it reacts with gold, it forms gold telluride. Tellurium is insoluble in water but readily reacts with nitric acid to produce tellurous acid. If inhaled, it produces a garlic-like odor on one s breath. 

Telluric acid can be prepared by reducing barium tellurate with sulfuric acid ... 

Also, telluric acid can be prepared by oxidation of tellurium or tellurium dioxide with a strong oxidizing agent such as hydrogen peroxide, sodium peroxide, chromic acid, or potassium permanganate in nitric acid. Molecular equations for overall reactions are shown helow ... 

At cold temperatures at about 1°C, telluric acid crystallizes as tetrahy-drate. 

Tellurium burns in air with a greenish-blue flame. The combustion product is dioxide, Te02, the most stable oxide of the metal. Tellurium also forms other oxides the monoxide, TeO, the trioxide, TeOs, and the pentoxide, Te205. Monoxide has not yet been obtained in solid form. Like sulfur and selenium, tellurium forms oxyacids. Such oxyacids include orthotelluric acid, HeTeOe and tellurous acid, H2Te03, in which the metal is in +6 and +4 valence states respectively. 

An acidic solution of tellurium (IV) or tellurium (VI) is treated with sulfur dioxide and hydrazine hydrochloride. Tellurium precipitated from solution can be estimated by gravimetry. Selenium interferes with this test. A volumetric test involves converting tellurium to tellurous acid and oxidizing the acid with excess ceric sulfate in hot sulfuric acid in the presence of Cr3+ ion as catalyst. The excess ceric sulfate is measured by titration with a standard solution of ferrous ammonium sulfate. 

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