Potassium selenate

Selenium trioxide, SeO, is white, crystalline, and hygroscopic. It can be prepared by the action of sulfur trioxide on potassium selenate or of phosphorous pentoxide on selenic acid. It forms selenic acid when dissolved in water. The pure trioxide is soluble in a number of organic solvents. A solution in Hquid sulfur dioxide is a selenonating agent. It is stable in very dry atmospheres at room temperature and on heating it decomposes first to selenium pentoxide [12293-89-9] and then to selenium dioxide. 

When fused with caustic soda or caustic potash, sodium selenate, or potassium selenate, Na2Se04, or K2Se04 is obtained. 

Perselenic Acid.—Electrolysis of a cold concentrated solution of potassium selenate containing some free selenie acid has been found to produce a similar effect to that observed with the corresponding sulphate.7 A potassium perselenate was formed at the anode, the chemical process probably being of the same nature as in the formation of perdisulphate, but on account of its instability the product was not obtainable in a higher degree of purity than approximately 75 per cent. 

Ag2Se04 (c). Metzner1 2 measured the heat of reaction of aqueous silver nitrate with aqueous potassium selenate to be 8.48. 

Reactions ofselenates To study these reactions use a 0- 1m solution of potassium selenate, K2Se04, or sodium selenate, Na2SeO4.10H2O. 

Potassium selenate (01m). Dissolve 22-1 g potassium selenate, K2Se04, in water and dilute to 1 litre. 

Potassium selenate, K2Se04.—Electrolytic oxidation of the solution of selenite formed by the interaction of selenious acid and potassium carbonate yields potassium selenate,7 rhombic crystals8 of density9 3-066 at 20° C. At the same temperature its solubility is 111 grams per 100 grams of water.10 Electrolytic oxidation of its solution acidified with selenic acid converts the selenate into potassium perselenate.11... 

The atomic positions of the paraelectric phase of crystalline NaNH4-Se04,2H20 have been determined by X-ray diffraction methods.300 The positions of the hydrogen atoms were unambiguously established by the n.m.r. spectrum of the partially deuteriated crystal. A low-temperature X-ray diffraction study301 of potassium selenate, K2Se04, has shown the existence of a superstructure in the low-temperature phases formed at 129.5 and 93 K. The superstructure is attributed to rather small atomic displacements parallel to the a-plane. 

Selenium trioxide cannot be prepared directly in a pure state by reaction between the elements. Dehydration of selenic acid by P4O10 under stringent conditions [5ITOU/DOS] and reaction of sulphur trioxide with potassium selenate [52LEH/KRU], followed by vacuum distillation, are two preferred methods of preparation. It is an extremely reactive solid at room temperature. 

Meyer and Aulich [28MEY/AUL] studied the solubility of potassium selenate in water as a function of temperature up to about 335 K and found the anhydrous salt, K2Se04(cr), to be the stable phase. A similar study by Friend [29FRI] between 273.2 and 370.0 K yielded a 1 to 2% higher solubility in the overlapping range. 

The standard Gibbs energy of formation of potassium selenate is calculated from the Gibbs energy of Reaction (V.196) and the selected values of AfG° for the ions to be ... 

SEL/ZUB] Selivanova, N. M., Zubova, G. A., Abramov, I. I., Kalinina, A. V., Sazykina, T. A., Physico-chemical properties of selenates. Properties of potassium selenate, Tr. Inst.-Mosk. Khim.-Tekhnol. Inst. im. D. I. Mendeleeva, 38, (1962), 21-25, in Russian. Cited on page 426. 

Table 3-2 and Figure 3-2 describe the health effects observed in humans and experimental animals associated with dose and duration of oral exposure to selenium and selenium compounds (i.e., elemental selenium dust, selenium dioxide dissolved in water [selenious acid], sodium selenate, sodium selenite, potassium selenate, and dietary selenium compounds, which include selenoamino acids). All doses for these compounds are expressed in terms of total selenium. Table 3-3 and Figure 3-3 describe health effects observed in laboratory animals following oral exposure to selenium sulfides (SeS2 and SeS) at varying doses and exposure durations. All doses for selenium sulfide compounds are expressed in terms of the compound, because selenium sulfide preparations often exist as a variable mixture of the mono-and disulfide forms, precluding accurate expression of the dose in terms of total selenium. 

The relative acute toxicities of sodium selenite, potassium selenite, sodium selenate, and potassium selenate in aqueous solution have been examined in mice (Pletnikova 1970). No significant differences among the toxicities of the potassium and sodium salts of selenium were apparent in this study. In another study, rats tolerated a dose of 1.05 mg selenium/kg/day administered in drinking water as potassium selenate for over 8 months with no deaths, but three of five females and one of three males died by the end of 1 year (Rosenfeld and Beath 1954). Decreased survival was reported in rats fed sodium selenate or selenite at 0.5 mg selenium/kg/day in a 2-year cancer study (Harr et al. 1967 Tinsley et al. 1967). No mortality was observed in hamsters fed 0.42 mg selenium/kg/day as sodium selenite in the diet for 82-142 weeks (Birt et al. 1986). 

Characteristic Sodium selenate Potassium selenate Sodium selenide Sodium selenite... 

Miscellaneous compounds Cesium trihydrogen selenite Lithium trihydrogen selenite Potassium selenate Methyl ammonium alum (MASD) Ammonium cadmium sulfate Ammonium bisulfate Ammonium sulfate Ammonium nitrate Colemanite Cadmium pyroniobite Gadolinium molybdate... 

This stated that compounds which crystallise in the same form (i.e. are isomorphous) are similar in atomic composition, and he quoted as examples the sodium phosphates and arsenates, in which an arsenic atom takes the place of a phosphorus atom. Mitscherlich used the fact that potassium selenate was isomorphous with potassium sulphate to deduce the formula of potassium selenate, and hence determine the atomic weight of selenium. Berzelius used the isomorphism of the sulphates and the chromates as evidence that the formula of anhydrous chromic acid was CrOj. He suggested that the lower oxide of chromium had a formula of Q2O3, and since this was isomorphous with the oxides of iron and aluminium, he was able to propose correct formulae for these compounds. 

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