Arsenic
Title: Arsenic
CAS Registry Number: 7440-38-2
Additional Names: Grey arsenic; metallic arsenic; arsen (German)
Literature References: As; at. wt 74.92160; at. no. 33; valences 3, 5. Group VA (15) element, classified as a metalloid. Naturally occurring isotope (mass number): 75 (100%); known artificial, radioactive isotopes: 66-74; 76-87. Arsenic compds were described and used in antiquity, especially as poisons; their reduction to the element was known to medieval alchemists. Albertus Magnus credited with isolation of the element from the mineral orpiment in ~1250 A.D. First precise directions for the prepn of As found in Paracelsus' writings (ca. 1520). Arsenic probably occurs throughout the universe. Meteorites reported to contain from 0.0005 to 0.1% As. Occurrence in the earth's crust: 1.8 ppm. Found in nature to a small extent as the element; occurs mostly in minerals such as realgar (As4S4), orpiment (A2S3), arsenolite (As2O3). Commercial sources: as by-product in flue dusts from smelting copper, lead, cobalt and gold ores; by melting FeAs2 or FeAsS ores. Prepn of pure As by reduction with carbon (sugar charcoal) and sublimation in N2: Krepelka, Collect. Czech. Chem. Commun. 2, 255 (1930); E. H. Archibald, The Preparation of Pure Inorganic Substances (Wiley, New York, 1932) p 269. Other methods: Schenk in Handbook of Preparative Inorganic Chemistry vol. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) pp 591-593. Reviews: Gmelins, Arsenic (8th ed.) 17, 475 pp (1952); Smith, "Arsenic, Antimony and Bismuth" in Comprehensive Inorganic Chemistry vol. 2, J. C. Bailar, Jr. et al., Eds. (Pergamon Press, Oxford, 1973) pp 547-683; Chemistry of the Elements, N. N. Greenwood, A. Earnshaw, Eds. (Pergamon Press, New York, 1984) pp 637-697; S. C. Carapella, Jr. in Kirk-Othmer Encyclopedia of Chemical Technology vol. 3 (John Wiley & Sons, 4th ed., 1992) pp 624-633; G. O. Doak et al., ibid. pp 633-659. Review of carcinogenicity studies: IARC Monographs 23, 39-141 (1980); of toxicology and human exposure: Toxicological Profile for Arsenic (PB2000-108021, 2000) 468 pp. Book: "The Chemistry of Organic Arsenic, Antimony and Bismuth Compounds," S. Patai, Ed. (John Wiley & Sons, New York, 1994) 962 pp.
Properties: Allotropic forms: a-form, metallic, steel-gray, shiny, brittle, rhombohedral crystal structure; b-form, dark gray, amorphous sold, d 4.700, transforms to metallic form at 280°. Can be heated to burn in air with bluish flame, giving off an odor of garlic and dense white fumes of As2O3. Stable in dry air; loses its luster on exposure to humid air as surface oxidizes, forming a black modification + As2O3. Brinell hardness: 147; Mohs' scale: 3.5. d425 5.778. Sublimes760 615° without melting. mp 818° at 36 atm. Heat of vaporization 11.2 kcal/g-atom. Heat of sublimation 30.5 kcal/g-atom. Heat of fusion: 22.4 kcal/g-atom (Gmelins, loc. cit. pp 135-136). Also reported as: heat of fusion: 6.620 kcal/g-atom; heat of sublimation 7.63 kcal/g-atom: D. R. Stull, G. C. Sinke, "Thermodynamic Properties of the Elements" in Advances in Chemistry Series 18 (A.C.S., Washington, 1956) pp 11, 44. Latent heat of fusion: 27,740 J/mol K. Latent heat of sublimation: 31,974 J/mol K (Carapella). Specific heat (25°) 24.6 J/mol K. Dielectric constant = 10.23 at 20° and 60 cycles. Electrical and magnetic properties of crystalline As: Taylor et al., J. Phys. Chem. Solids 26, 69 (1965). Insol in water. Not attacked by cold H2SO4 or HCl; converted by HNO3 or hot H2SO4 into arsenous or arsenic acid. A yellow modification which has no metallic properties has been reported from sudden cooling of As-vapor. This yellow arsenic is converted back to the gray modification upon very short exposure to ultraviolet light.
Melting point: mp 818° at 36 atm
Density: d 4.700; d425 5.778
NOTE: In German and other languages Arsenik means arsenic trioxide.
CAUTION: Overexposure to arsenic and arsenic compounds has been associated with acute and chronic toxicity due to inhalation or ingestion. Organic forms are usually less harmful than inorganic forms. Direct contact can cause local irritation and dermatitis. Overexposure has been associated with an increased risk of skin, liver, bladder, kidney and lung cancer. See Toxicological Profile, loc. cit. Inorganic arsenic compounds are listed as known human carcinogens: Report on Carcinogens, Eleventh Edition (PB2005-104914, 2004) p III-18.
Use: In metallurgy for hardening copper, lead, nonferrous alloys; automotive body solder. In semiconductor materials. In the manufacture of low-melting glass. As wood preservative, herbicide, pesticide.

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