From Telluric Acid

Mixing equal volumes of aqueous 0.5 M telluric acid and 1.0 M 1,2-dihydroxybenzene and keeping the mixture at 0 for a week, produced 2,2 -spirobis[l,3,2-benzodioxatellurole] in 2% yield. The crystals were washed with water and dried at 20°. They melted with decomposition at 230° At a reaction temperature of 20° the yield is even lower and elemental tellurium is precipitated. 

The first step in this reaction must be a reduction of telluric acid to tellurous acid. The tellurous acid subsequently forms the ortho ester. The constitution of the product was ascertained by a single crystal X-ray analysis.  

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

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Marckwald, after a research with unsatisfactory material,4 prepared some pure tellurium dioxide from telluric acid which had been crystallised several hundred times.5 The dioxide was analysed volumetrically by oxidising to telluric acid by means of potassium permanganate, excess of the latter being determined by means of oxalic acid. The mean result obtained was 127-61. 

Abelian, J. 0. Preparation of Carrier-Free Iodine-131 from Telluric Acid by Irradiation with Neutrons. Anales Real Soc. Espan. FIs. Quim. (Madrid), Ser B. 535 (1964). (In Spanish). 19 15577... 

Hontessa, E. F. Carrier-Free I from Telluric Acid. Philipp. Nucl. 

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). 

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). 

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. 

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. 

Such effective masking may be traced to two factors (a) the inertness of the telluric acid complexes and (b) the near identity of the demands of oxidant and tellurate on the coordinating properties of the ligand. The decrease in reaction rate here is consistent with a very slow release of ligand from the tellurate complex as established by Edwards and his co-workers (1J). 

A number of substances, such as the most commonly used sulfur dioxide, can reduce selenous acid solution to an elemental selenium precipitate. This precipitation separates the selenium from most elements and serves as a basis for gravimetry. In a solution containing both selenous and tellurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 N with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxy] amine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and determination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

The use of sulphur dioxide as precipitant was first proposed by Berzelius, but accurate results by this method are only obtainable under special conditions. Complete precipitation does not take place from a strongly acid solution, and in the presence of other metals small amounts of these are liable to be carried down. In the presence of heavy metals such as copper, bismuth and antimony, the following procedure has been recommended 2 The tellurium is oxidised to telluric acid by the addition of ammonium perdisulphate in the presence of potassium hydroxide, excess of perdisulphate being subsequently removed by boiling. The heavy metals present are next removed by means of hydrogen sulphide. The tellurium may then readily be estimated by reduction with hydrogen chloride and precipitation with sulphurous acid. 

Electrolytic Methods.—According to Muller s experiments,3 tellurium is not deposited by electrolysis from solutions in which it is present in the sexavalent condition, so that telluric acid must first be reduced to tellurous acid. A weight of tellurous acid not exceeding 0-25 gram is dissolved in 175 c.c. of 2N H2S04 and electrolysed for two and a half hours between platinum electrodes. The grey deposit of tellurium is washed with water and alcohol and dried in a desiccator over sulphuric acid. In this method the average error is estimated to be 0T per cent. By this process tellurous acid may be estimated in the presence of telluric acid. After the tellurium from the tellurous acid has been removed, the solution is boiled with hydrochloric acid to reduce the telluric acid to tellurous acid and the estimation of the tellurium carried out as before.4... 

Volumetric Estimation.—Tellurium may be determined by oxidation from the tellurous to the telluric condition, using an excess of potassium dichromatc or permanganate and subsequently titrating the excess of oxidising agent with a standard solution of a suitable reducing agent.2 In order to obtain accurate results with the potassium dichromate titration, certain very definite steps in the procedure are essential, and it is necessary to control the course of the reaction, since hydrochloric and telluric acids interact with production of chlorine. 

Telluric acid may be determined iodometrically by reduction with either hydrogen bromide or hydrogen chloride.6 The substance is heated in a distillation llask with four times the theoretical quantity of potassium iodide in the presence of the acid, air being excluded from the apparatus by passing a stream of carbon dioxide. The liberated iodine is titrated in the receiver and in the residue.7... 

The tetra-iodide may also be formed from the dioxide and concentrated hydriodie acid. Telluric acid gives a similar result. The resulting halide may separate as such or in needles of an additive compound, HI.TeI4.8H20, which gives a residue of tetra-iodide when warmed to 50°— 60° C.5... 

Tellurous acid can be prepared from the residues from the electrolytic refining of copper by treating them with a solution of ammonia. On the addition of acetic acid to the resulting solution tellurous acid is obtained as a precipitate. When this precipitation is carried out in the cold the product obtained is readily soluble in alkali hydroxide, but if the precipitation takes place in a hot solution the product tends to be insoluble in the alkali hydroxides.2... 

A very pure product and almost theoretical yield may be obtained by the oxidation of tellurium tetrachloride with chloric acid. A slight excess of chloric acid is used to avoid the formation of any explosive oxides of chlorine. The addition of concentrated nitric acid causes precipitation of the telluric acid, which may be freed from chlorine and nitrogen oxides by drying in a vacuum. Prepared by this method the acid has the composition H2Te04.2H2O.8... 

That the foregoing classes of salts may be derived from different acids is quite feasible, for the second form of telluric acid described, allotelluric acid, obtained by heating the crystalline acid II6Te06 (p. 885), differs from the ortho-acid not only in its greater acidity but in its precipitation reactions.2... 

It is evident that telluric acid is notably different from what might be expected by analogy with sulphuric and selenic acids. One is hardly surprised, therefore, that isomorphism is exceedingly rare between the tellurates and the sulphates or selenates.3 The tellurates do not form alums, they do not as a rule form mixed crystals with the sulphates or selenates, the only fairly satisfactory case of mixed-crystal formation being with rubidium hydrogen sulphate and rubidium hydrogen tellurate. 

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