Title: Selenium
CAS Registry Number: 7782-49-2
Literature References: Se; at. wt 78.96; at. no. 34; valences 2, 4, 6. Group VIA (16). Six stable isotopes: 74 (0.87%); 76 (9.02%); 77 (7.58%); 78 (23.52%); 80 (49.82%); 82 (9.19%); artificial, radioactive isotopes: 70-73; 75; 79; 81; 83-85; 87. Discovered in 1817 by Berzelius. Constitutes about 0.09 ppm of the earth's crust. Occurs in nature usually in the sulfide ores of the heavy metals; found in small quantities in pyrite; in the minerals clausthalite (PbSe), naumannite [(Ag,Pb)Se], tiemannite (HgSe); in selenosulfur. Essential trace element in human and animal diets; constituent of selenoproteins. Prepn: Waitkins et al., Ind. Eng. Chem. 34, 899 (1942). Purification: Nielsen, Heritage, J. Electrochem. Soc. 106, 39 (1959); Oberbacher, Schlier, US 2930678 (1960 to Norddeutsche Raffinerie). Symposium on organic selenium and tellurium compds: Y. Okamoto, W. H. H. Gunther, Eds., Ann. N.Y. Acad. Sci. 192, 1-225 (1972). Reviews of selenium and its compds: K. W. Bagnall, The Chemistry of Selenium, Tellurium and Polonium (Elsevier, New York, 1966) 200 pp; Bagnall in Comprehensive Inorganic Chemistry vol. 2, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 935-1008; Selenium, R. A. Zingaro, W. C. Cooper, Eds. (Van Nostrand, Reinhold, New York, 1974) 835 pp; E. M. Elkin in Kirk-Othmer Encyclopedia of Chemical Technology vol. 20 (Wiley-Interscience, New York, 3rd ed., 1982) pp 575-601. Reviews of nutritional properties and toxicology: E. J. Underwood, Trace Elements in Human and Animal Nutrition (Academic Press, New York, 1971) pp 323-368; Frost, Crit. Rev. Toxicol. 1, 467-514 (1972); Allaway, Cornell Vet. 63, 151-170 (1973); Frost, Lish, Annu. Rev. Pharmacol. 15, 259-284 (1975) of biochemistry: T. C. Stadtman, Annu. Rev. Biochem. 49, 93-110 (1980); idem, FASEB J. 1, 375-379 (1987). Review of role in human health: M. P. Rayman, Lancet 356, 233-241 (2000); of toxicology and human exposure: Toxicological Profile for Selenium (PB2004-100005, 2003) 457 pp.
Properties: Exists in several allotropic forms: amorphous, cryst or red, and gray or metallic. Liq is a brownish red, boils at 685° forming dark red vapors. Sol in dil aq caustic alkali solns; in aq potassium cyanide soln, in potassium sulfite soln. Burns in air with a bright blue flame forming the dioxide and emitting a characteristic odor resembling rotten horseradish. Combines directly with hydrogen, with the halogens (excluding iodine). Oxidized to selenious acid by nitric acid, to selenic acid by sulfuric acid. Reduces hot aqueous solutions of silver and gold salts with formation of silver selenide and metallic gold, respectively. Reacts with many metals.
Boiling point: boils at 685°
Derivative Type: Amorphous forms
Properties: Vitreous, black selenium; dark red-brown to bluish-black solid; formed when molten Se is cooled rapidly. d 4.28. Softens at 50-60° and becomes elastic at 70°. Red, amorphous form; formed by reduction of selenious acid in water; by condensation of Se vapor on a cold surface. d 4.26. Review of structural studies: Richter, Breitling, Z. Naturforsch. 26B, 1699-1708, 2074-2075 (1971). When freshly precipitated, reacts with water at 50° forming selenious acid and hydrogen. Sol in carbon disulfide, methylene iodide, benzene, or quinoline.
Density: d 4.28; d 4.26
Derivative Type: Crystalline or red
Properties: Two monoclinic forms; dark red, transparent crystals. a-Form prepd by slow evaporation of CS2 soln of Se; b-form by rapid evaporation of soln. mp below 200°. d (a-form) 4.46. Structure of a-form: Cherin, Unger, Acta Crystallogr. 28B, 513 (1972); see also Bagnall, loc. cit. Both forms are metastable and change into gray form on heating.
Melting point: mp below 200°
Density: d (a-form) 4.46
Derivative Type: Gray or metallic
Properties: Lustrous gray to black hexagonal crystals. The most stable form. d420 4.81. mp 217°. Mohs' hardness: 2.0. Latent heat of fusion 16.4 cal/g. Latent heat of vaporization 20.6 kcal/mol. Linear coefficient of thermal expansion per degree C = 37 ´ 10-6. Specific heat (28°): 0.084 cal/g/°C. Surface tension (217°): 92.5 dynes/cm. Thermal conductivity (25°): 0.0007-0.00183 cal/(cm)(°C)(sec). Insol in water, alcohol; very slightly sol in carbon disulfide (2 mg/100 ml at ord temp). Sol in ether. Conducts electricity and rectifies alternating current; the conductivity increases up to a thousand times on exposure to light.
Melting point: mp 217°
Density: d420 4.81
CAUTION: Potential symptoms of overexposure are irritation of eyes, skin, nose and throat; visual disturbance; headache; chills, fever; dyspnea, bronchitis; metallic taste, garlic breath, GI disturbance; dermatitis; skin and eye burns. See NIOSH Pocket Guide to Chemical Hazards (DHHS/NIOSH 97-140, 1997) p 276.
Use: As ingredient of toning baths in photography; as pigment in manuf ruby-, pink-, orange-, or red-colored glass; as metallic base in making electrodes for arc lights, electrical instruments and apparatus, as rectifier in radio and television sets; in selenium photocells, in semiconductor fusion mixtures, selenium cells, telephotographic apparatus; as vulcanizing agent in processing of rubber; as catalyst in determination of nitrogen by Kjeldahl method; for dehydrogenation of organic compds.
Therap-Cat-Vet: Nutritional factor (interrelationship with vitamin E), chiefly to prevent muscle degenerative diseases.

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