Selenium Selenate

SYNS C.I. 77805 COLLOIDAL SELENIUM ELEMENTAL SELENIUM SELEN (POLISH) SELENIUM ALLOY SELENIUM BASE SELENIUM DUST SELENIUM ELExMEKTAL SELENIUM HOMOPOLYMER SELENIUM METAL POWDER, NON-PYROPHORIC (DOT) VANDEX... 

Trace amounts of Se are necessary for animal life, since one of the key enzymes of glutathione metabolism, namely glutathione peroxidase, contains 4 g-atoms of Se. The borderline between life and death, however, is fairly narrow. Thus, the minimum desiderable pasture content of Se for liverstock is 0.03 ppm, while continual ingestion of fodder contain 1-5 ppm will induce toxicity [112]. Se excretion occurs via urine (in the form of trimethyl selenium, selenates and Se-aminoacids), through the feces (in the form of Se-aminoacids) or sweat (as selenates and di- and trimethyl... 

Shennan DB. 1988. Selenium (selenate) transport by human placental brush border membrane vesicles. Br J Nutr 59(1) 13-19. 

Figure 4 Depth profiles for concentrations of total selenium and different chemical forms of selenium (selenate, selenite, and organic selenium compounds) in filtered seawater samples from the eastern tropical North Pacific Ocean (18° N, 108° W Oct.-Nov. 1981). Data are from Cutter GA and Bruland KW (1984) The marine biogeochemistry of selenium A reevaluation. Limnology and Oceanography 29 1179-1192.

Supplements. Selenium supplements are available in several forms. Sodium selenite and sodium selenate are inorganic forms of selenium. Selenate is almost completely absorbed, but a significant amount is excreted in the urine before it can be incorporated into proteins. Selenite is only about 50% absorbed, but is better retained than selenate, once absorbed. Selenomethionine, an organic form of selenium that occurs naturally in foods, is about 90% absorbed. Selenomethionine and selenium-enriched yeast, which mainly supplies selenomethionine, are also available as supplements. ... 

Discovery Berzelius found the element in 1817 in the lead chamber mud in a sulfuric acid plant. He named it selenium Selene, the moon) due to its similarity with tellurium (the earth)... 

Selenium. —Selenation and Oxidation. Three studies have appeared which are of general interest in the field. The first considers the fundamental reaction of PhSeCl addition to alkenes, where it is shown that the addition occurs initially in an anti-Markovnikoff fashion, isomerization occurring to the thermodynamically more stable isomer [RCH(Cl)CH2SePh] at higher temperatures.Oxidation and elimination of selenium provides a general alkene synthesis, and it has now been found that Bu OOH-alumina-THF is an efficient oxidation-elimination combination, and that addition of EtsN to a selenoxide prior to thermal elimination is advantageous. Thirdly, reductive removal of Se by Ph3SnH is preferable to the more conventional Raney nickel procedure. ... 

Selenates are poisonous, selenic acid, H2Se04. See selenium oxyacids. 

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. 

Phytoremediation is also being developed for dealing with soils contaminated with high levels of selenium in California again B.juncea seems to be particularly effective in accumulating the contaminant from soil, and all plants tested were more effective at removing selenate than selenite (92). This is an interesting contrast to bacterial systems, where selenite reduction is more commonly found than selenate reduction. 

The roasted pellets or extmdes are ground and leached in water. The hexavalent selenium dissolves as sodium selenate [13410-01 -0] Na2Se04. Sodium teUurate, being highly insoluble in the now very strongly alkaline solution, remains in the residue. The separation between selenium and tellurium is readily achieved, provided all tellurium is oxidized to the hexavalent state. 

Bina Selenides. Most biaary selenides are formed by beating selenium ia the presence of the element, reduction of selenites or selenates with carbon or hydrogen, and double decomposition of heavy-metal salts ia aqueous solution or suspension with a soluble selenide salt, eg, Na2Se or (NH 2S [66455-76-3]. Atmospheric oxygen oxidizes the selenides more rapidly than the corresponding sulfides and more slowly than the teUurides. Selenides of the alkah, alkaline-earth metals, and lanthanum elements are water soluble and readily hydrolyzed. Heavy-metal selenides are iasoluble ia water. Polyselenides form when selenium reacts with alkah metals dissolved ia hquid ammonia. Metal (M) hydrogen selenides of the M HSe type are known. Some heavy-metal selenides show important and useful electric, photoelectric, photo-optical, and semiconductor properties. Ferroselenium and nickel selenide are made by sintering a mixture of selenium and metal powder. 

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. 

Other inorganic selenium compounds include sodium selenocyanate [4768-87-0], NaSeCN, which is prepared by melting together selenium and sodium cyanide selenocyanogen [27151 -67-3] (SeCN)2 sodium selenosulfate [25468-09-1], Na2SeS02, which is prepared by dissolving selenium in aqueous sodium sulfite (acidification decomposes this compound) and selenate alums, eg, Al2(Se0 2 I SeO [13530-59-1]. 

Comprehensive accounts of the various gravimetric, polarographic, spectrophotometric, and neutron activation analytical methods have been pubHshed (1,2,5,17,19,65—67). Sampling and analysis of biological materials and organic compounds is treated in References 60 and 68. Many analytical methods depend on the conversion of selenium in the sample to selenous acid, H2Se02, and reduction to elemental selenium when a gravimetric deterrnination is desired. 

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 teUurous acids, the selenium may be quantitatively separated from the latter by performing the reduction in a solution which is 8 to 9.5 W with respect to hydrochloric acid. When selenic acid may also be present, the addition of hydroxylamine hydrochloride is recommended along with the sulfur dioxide. A simple method for the separation and deterrnination of selenium(IV) and molybdenum(VI) in mixtures, based on selective precipitation with potassium thiocarbonate, has been developed (69). 

Contact with elemental selenium does not injure the skin. Selenium dioxide, however, upon contact with water, sweat, or tears, forms selenous acid, a severe skin irritant. Selenium oxyhaHdes are extremely vesicant and cause bums when in contact with human skin (91,92). Hydrogen selenide affects the mucous membranes of the upper respiratory tract and the eyes (93). 

Silver Selenate. Silver selenate, Ag2Se04, is prepared from silver carbonate and sodium selenate (see Seleniumand selenium compounds). 

In mineral technology, sulfur dioxide and sulfites are used as flotation depressants for sulfide ores. In electrowinning of copper from leach solutions from ores containing iron, sulfur dioxide prereduces ferric to ferrous ions to improve current efficiency and copper cathode quaHty. Sulfur dioxide also initiates precipitation of metallic selenium from selenous acid, a by-product of copper metallurgy (326). 

X 16,000 = 3,608 pounds of selenium from load selenate (round to 3,600 pounds)... 

Selenium was isolated some 35 y after tellurium and, since the new element resembled tellurium, it was named from the Greek askrivr], selene, the moon. The discovery was made in 1817 by the Swedish chemist J. J. Berzelius (discoverer of Si, Ce and Th) and J. G. Gahn (discoverer of Mn) they observed a reddish-brown deposit during the burning of sulfur obtained from Fahlun copper pyrites, and showed it to be volatile and readily reducible to the new element. 

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