Selenium, pure

The pure acid does not react in the cold with sulfur, selenium, tellurium, carbon, silver, copper, zinc, iron, chromium, or manganese, but slowly dissolves mercury and tin (20). At higher temperatures, lead, mercury, tin, and sulfur react rapidly, eg ... 

Pure selenium deficiency, without concurrent vitamin E deficiency, is not generally seen except in animals on experimental diets (113). In China, selenium deficiency in humans has been associated with Keshan disease, a cardiomyopathy seen in children and in women of child-bearing ages, and Kashin-Beck disease, an endemic osteoarthritis in adolescents (113). 

Selenium and precious metals can be removed selectively from the chlorination Hquor by reduction with sulfur dioxide. However, conditions of acidity, temperature, and a rate of reduction must be carefliUy controlled to avoid the formation of selenium monochloride, which reacts with elemental selenium already generated to form a tar-like substance. This tar gradually hardens to form an intractable mass which must be chipped from the reactor. Under proper conditions of precipitation, a selenium/precious metals product substantially free of other impurities can be obtained. Selenium can be recovered in a pure state by vacuum distillation, leaving behind a precious metals residue. 

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. 

Ghalcogenides. The reactions of pure zirconium turnings with threefold quantities of elemental sulfur, selenium, or tellurium give ZrS ... 

Betts Electrolytic Process. The Betts process starts with lead bullion, which may carry tin, silver, gold, bismuth, copper, antimony, arsenic, selenium, teUurium, and other impurities, but should contain at least 90% lead (6,7). If more than 0.01% tin is present, it is usually removed from the bullion first by means of a tin-drossing operation (see Tin AND TIN ALLOYS, detinning). The lead bullion is cast as plates or anodes, and numerous anodes are set in parallel in each electrolytic ceU. Between the anodes, thin sheets of pure lead are hung from conductor bars to form the cathodes. Several ceUs are connected in series. 

These are known as chemically pure (CP) cadmiums. With the development of other uses for cadmium and selenium, costs have risen substantially in recent years. Some cost reduction may be obtained by use of the cadmium Hthopones. These have the same relative shades but have been coprecipitated onto about 60% barium sulfate. The resulting extensions give better money value, if the higher pigment loading can be tolerated, with no loss in properties. 

Abundances of lUPAC (the International Union of Pure and Applied Chemistry). Their most recent recommendations are tabulated on the inside front fly sheet. From this it is clear that there is still a wide variation in the reliability of the data. The most accurately quoted value is that for fluorine which is known to better than I part in 38 million the least accurate is for boron (1 part in 1500, i.e. 7 parts in [O ). Apart from boron all values are reliable to better than 5 parts in [O and the majority arc reliable to better than I part in 10. For some elements (such as boron) the rather large uncertainty arises not because of experimental error, since the use of mass-spcctrometric measurements has yielded results of very high precision, but because the natural variation in the relative abundance of the 2 isotopes °B and "B results in a range of values of at least 0.003 about the quoted value of 10.811. By contrast, there is no known variation in isotopic abundances for elements such as selenium and osmium, but calibrated mass-spcctrometric data are not available, and the existence of 6 and 7 stable isotopes respectively for these elements makes high precision difficult to obtain they are thus prime candidates for improvement. 

The sulfiu can be piped long distances in liquid form or transported molten in ships, barges or rail cars. Alternatively it can be prilled or bandied as nuggets or chunks. Despite the vast bulk of liquid sulfur mined by the Frasch process it is obtained in very pure form. There is virtually no selenium, tellurium or arsenic impurity, and the product is usually 99.5-99.9% pure. ... 

The ethyl acetate solution is then washed with water, dried and evaporated. To remove any selenium still present, the residue is dissolved in 200 cc of methanol and mixed with 100 g of iron powder and 2 g of active carbon. The mixture is heated for 30 minutes with stirring under reflux, then filtered with suction, washed with methanol and the solution evaporated in vacuo. The residue is then chromatographed on 900 g of aluminum oxide. The residues of the evaporated benzene and ether fractions are treated with active carbon in methanol or acetone, evaporated again, and the residue recrystallized from a mixture of acetone and ether. There are obtained 17.5 g of pure 1-dehydro-17a-methyl-testosterone which melts at 163° to 164°C. 

Very little is known about chalcogenide halides of Group IVB elements. Although the existence of sulfide chlorides (45, 274, 329, 365) and of a selenide chloride (329) of titanium was claimed in early publications, their true composition, and even their existence, remains doubtful. They have usually been obtained by the reaction of titanium chlorides with sulfur and selenium, respectively, or with hydrogen sulfide. The synthesis of a pure compound, TiSClj, was published in 1959 (113). It is an intermediate of the reaction of TiCU with HjS. 

Commercial elemental sulfur is usually of bright-yellow color at 20 °C [36]. Pure orthorhombic a-Ss is, however, of greenish-yellow color at 20 °C but totally colorless at 77 K while commercial sulfur often remains pale-yellow at this temperature [59]. The reasons for this different behavior are twofold. Commercial samples are never pure Ss but besides traces of organic impurities they always contain Sy in concentrations of between 0.1 and 0.5% [59]. Sulfur found as a mineral in Nature sometimes also contains Sy but in addition traces of selenium are quite often present (up to 680 ppm Se, probably as SySe molecules) [60]. These minor components influence the color of the samples at ambient and low temperatures in the sense that a more orange-type of yellow ( egg-yellow ) is recognized. 

In addition to halogen bonded complexes or ionic salts, it is also possible for sulfur and selenium electron donors to form complexes in which the electron donor atom inserts into the X2 bond, giving a hypervalent donor atom with a T-shaped geometry. It has been recently reported [147] that for dibromine and selenium, this type of complex is favored over halogen bonded complexes. While no purely halogen bonded complex is reported for dibromine, there is one complex (IRABEI) in which one selenium atom of each of several selenanthrene molecules in the asymmetric unit does insert into a Br2 bond, but for one of the molecules, the other selenium atom forms a halogen bond with a Br2 molecule to form a simple adduct (A). 

Tertiary phosphine selenides have been prepared either by direct fusion of the tertiary phosphine with elemental selenium1 or by refluxing the phosphine with selenium in an inert solvent.2 In the case of a solid phosphine, such preparations often require numerous and time-consuming extractions to obtain the pure phosphine selenide. 

In contrast to sulfurous acid, H2S03, which is not known as pure substance, the respective selenium compound can be obtained in crystalline form. H2Se03 crystallizes with the non-centrosymmetric space group P2 2,2] and contains pyramidal H2Se03 molecules.54 Within the molecules the bond lengths Se-O and SeOH differ by about 10 pm and the molecules are connected to puckered layers by strong hydrogen bonds. 

Optically active selenoxides are known to be unstable toward racemization. An optically active selenoxide having a steroidal frame was obtained for the first time by Jones and co-workers in 1970.7 Enantiomeric selenoxides were prepared by Davis et al. in 1983,8 and an enantiomerically pure selenoxide was isolated for the first time by us in 1989.9 Many optically active selenoxides, which are kinetically stabilized by bulky substituents, were synthesized over the last two decades, and their stereochemistry and stability toward racemization were studied.3,5,10 Recently, some optically active selenoxides, which were thermodynamically stabilized by the intramolecular coordination of a Lewis base to the selenium atom, have been isolated. Optically active selenoxides 1 and 2 were obtained by optical resolution on chiral columns, and their stereochemistry and stability toward racemization under various conditions were clarified (Scheme 1).11,12... 

Hydrides of variable composition are not only formed with pure metals as solvents. A large number of the binary metal hydrides are non-stoichiometric compounds. Non-stoichiometric compounds are in general common for d,f and some p block metals in combination with soft anions such as sulfur, selenium and hydrogen, and also for somewhat harder anions like oxygen. Hard anions such as the halides, sulfates and nitrides form few non-stoichiometric compounds. Two factors are important the crystal structures must allow changes in composition, and the transition metal must have accessible oxidation states. These factors are partly related. FeO,... 

Many proteins, including many enzymes, contain hghtly bound metal ions. These may be inhmately involved in enzyme catalysis or may serve a purely structural role. The most common tightly bound metal ions found in metalloproteins include copper (Cu+ and Cu +), zinc (Zn +), iron (Fe + and Fe +), and manganese (Mn +). Other proteins may contain weakly bound metal ions that generally serve as modulators of enzyme activity. These include sodium (Na+), potassium (K+), calcium (Ca +), and magnesium (Mg +). There are also exotic cases for which enzymes may depend on nickel, selenium, molybdenum, or silicon for activity. These account for the very small requirements for these metals in the human diet. 

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