Selenates and Tellurates

The ease with which the dipolarophile interacts with vinylacetylenes depends mainly on a spatial factor. The study of the reactions of alkylthiobuten-3-ynones-l and their selenic and telluric analogs with DPNT shows that, in this case, nitrilimine also acts as a nucleophilic agent with a nucleophilic center on the carbon atom of the 1,3-dipole and always adds to the terminal carbon of the enyne system to form l,3-diphenyl-5-/ -2-pyrazolenines. The oxidation of the latter with chloranil leads to alkynylpyrazoles (65ZOR51). 

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. 

Generally, oxo-selenates can be classified according to the oxidation state of the selenium atom as oxu-selenates(IV), and o.Yo-selenates(VI). The same applies for the respective tellurates. Besides this systematically correct naming, chemists usually use the terms selenites and tellurites instead of o. o-selenates(IV) and oxo-tellurates(IV), and selenates and tellurates instead of OAO-selenates(VI) and oxo-tellurates(VI). Therefore, both nomenclatures will be used in parallel throughout this chapter. Compared to the respective sulfur species, the oxo-selenate(IV) and oxo-tellurates(IV) ions are very stable so that numerous compounds with these anions have been prepared. 

Common routes for the synthesis of selenates and tellurates(IV and VI) are the reactions of metal oxides or carbonates with the repsective acids (see Section 4). The disadvantage of this procedure is, that one usually obtains hydrates or, at higher acid concentrations, acidic compounds. Because the oxides E03 and E02 (E=Se, Te) are solids under ambient conditions, solid-state reactions with the respective metal oxides are an alternative route to prepare the anhydrous compounds. 

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). 

Basic tellurium sulfate 2 Te02 S03 [12068-84-8]-, 2 Te02 Se03 selenate and tellurate, 2 Te02 Te03, are known. The sulfate is made by the slow evaporation of a Te02 solution in H2S04. It is stable up to 440—500°C and is hydrolyzed slowly by cold, and rapidly by hot water. 

Compare the physical and chemical properties of sulphurous, selenous, and tellurous acids (see Appendix 1). 

Selenous and tellurous acids can be obtained by the hydrolysis of the tetrahalides,... 

Selenate and tellurate also are potent inhibitors of methane formation, and we postulate that these compounds are reduced and methylated in a manner similar to that of arsenate. 

Selenic and Telluric Acids (for Sulfuric Acid see page 180)... 

The order of presentation of the uranium minerals will follow chemical groups. The U minerals are discussed first, followed by the niobates, tantalates and titanates. These two groups include the primary reduced minerals. The uranyl minerals are considered in the order hydrated oxides, silicates, phosphates and arsenates, vanadates, molybdates, sulphates, carbonates, and selenates and tellurates. Each section includes an evaluation of the known crystal chemistry and its effect on chemical variability and occurrence of mineral species. 

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