Nomenclature
CAS number: 7782-49-2
Description and 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. Liquid 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.Derivative
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.Derivative
Crystalline or red.
Properties
Two monoclinic forms; dark red, transparent crystals.
α-Form prepd by slow evaporation of CS2 soln of Se; β-form
by rapid evaporation of soln. mp below 200°. d (α-form) 4.46. Structure of α-form: Cherin, Unger, Acta Crystallogr. 28B, 513 (1972); see also Bagnall, loc. cit. Both forms are metastable
and change into gray form on heating.Derivative
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.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.
Therapeutic Category (Veterinary)
Nutritional factor (interrelationship
with vitamin E), chiefly to prevent muscle degenerative diseases.