ARSENIC

Arsenic (As) is a silver-gray metal that gained much of its notoriety because of its historical use as a human poison (approximately 70 to 180 milligrams of arsenic is fatal to an adult). Arsenic is present in the earth's crust at an average concentration of 2 to 5 mg/kg, with low levels commonly found in the air, water, and soil. In the eighteenth and nineteenth centuries, arsenic was used as a preservative in animal hides, and as an ingredient in pigments, dyes, glass, pharmaceuticals, and pesticides.

In the first half of the twentieth century, arsenic was used in pharmaceuticals intended to treat syphilis (e.g., arsphenamine), skin diseases (e.g., Fowler's solution, a 1% potassium arsenate solution), and parasites (e.g., Pearson's Arsenical Solution). Arsenic is still used as an ingredient in pesticides, wood preservatives, copper and lead alloys, glass, semiconductor devices, and veterinary medicines.

Although arsenic is found in nature in its elemental form (arsenic metal), it occurs most commonly in inorganic or organic compounds. Common inorganic arsenic compounds are trivalent arsenic (e.g., arsenite, H₃AsO₃) and pentavalent arsenic (e.g., arsenate, H₂AsO₄, HAsO₄²). Common organic arsenic compounds are monomethyl arsonic acid (MMA), dimethyl arsinic acid (DMA, also known as cacodylic acid), and roxarsone.

Adverse health effects are dependent on the chemical form and physical state of the specific arsenic compound. In general, organic arsenic is less acutely toxic than inorganic arsenic. The health effects of arsenic are widely variable, and are primarily due to differences in the oxidation state of the two predominant forms: trivalent arsenite and pentavalent arsenate. Several organic arsenicals that accumulate in fish and shellfish are essentially nontoxic. Human exposure to arsenic compounds occurs primarily in occupational settings and by the ingestion of contaminated drinking water and seafood. Arsenic toxicity due to natural contamination of drinking water has been recently noted as a significant public health problem in Bangladesh. Predominant adverse health effects associated with acute arsenic exposure include fever, melanosis, hepatomegaly, cardiac arrhythmia, peripheral neuropathy, nephrotoxicity, diarrhea and vomiting, and, at sufficiently high doses (70 to 180 milligrams for an adult), death. Chronic exposure to arsenic may lead to neurotoxicity (evidenced by sensory changes, paresthesia, and muscle weakness), cancer (basal cell and squamous cell carcinoma of the skin, lung cancer, or bladder cancer), cardiovascular effects (including "blackfoot disease," so called because the soles of the feet and toes turn black with gangrene), skin disorders such as hyperpigmentation, and birth defects.

Arsine gas is a potent hemolytic agent. The International Agency for Cancer Research (IARC) and the U.S. Environmental Protection Agency (EPA) classify arsenic as a carcinogen based upon epidemiological evidence demonstrating a causal association between arsenic exposure and specific cancers, such as skin cancer and lung cancer. Arsenic can accumulate in hair and nails, and measurement of arsenic levels in these tissues may be a useful indicator of past exposures, while measurement of urine is considered a good indicator of current arsenic exposure. Arsenic is primarily excreted from the body in urine (30 to 85% of absorbed arsenic is excreted via urine). Scientists have puzzled for decades over arsenic's mechanism of carcinogenicity due to the discordance between the results of human and animal bioassays. Animals appear to be substantially less susceptible to arsenic-induced toxicity than humans. Investigations in animals have suggested that inorganic arsenic can be an essential trace element in some animals. In contrast, arsenic has not been determined to be an essential trace element in humans.

Margaret H. Whitaker

Bruce A. Fowler

(see also: Carcinogen; Heavy Metals )

Bibliography

International Agency for Research on Cancer (IARC) (1980). Some Metals and Metallic Compounds. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans. Vol. 23. Lyon, France: IARC.

National Research Council (1999). Arsenic in Drinking Water. Washington, DC: National Academy Press.

U.S. Environmental Protection Agency (2000). Arsenic Record. Integrated Risk Information Service (IRIS). Available at http://www.epa.gov/iris.

Arsenic

melting point: 817°C
boiling point: 613°C
density: 5.72 g/cm³
most common ions: As³⁺, AsO₂⁻

Arsenic is the twentieth most abundant element in Earth's crust, averaging a concentration of approximately 2 ppm. Arsenopyrite (FeAsS) is its most common mineral. Arsenic occurs widely in nature, and most abundantly in sulfide ores and the products of volcanic eruptions. Arsenic concentrations in rock and soil are highly variable; the highest concentrations are in hydrothermal sulfide mineralization areas.

Arsenic has two common oxidation states: +5, the predominant one, and the less thermodynamically stable +3. Arsenic has twenty-three isotopes ; of these, one (mass 75) is stable. The other isotopes have very short half-lives.

Trace amounts of arsenic occur in groundwater; it may cause human cancers at concentrations in drinking water of about 300 ppb. The U.S. Environmental Protection Agency (EPA) has proposed lowering the maximum allowable arsenic concentration in U.S. drinking water from 50 to 5 ppb. The latter lower limit is still controversial.

The properties of arsenic sulfides were known to physicians and "professional poisoners" in the fifth century b.c.e. Albertus Magnus (1193–1280) is credited with having isolated elemental arsenic by heating auripigment (As₂S₃) with soap.

Beneficial effects of arsenic compounds have been known for a very long time. Arsenic was important in the development of metallurgy at the beginning of the Bronze Age, and later as a pigment and as an incendiary warfare ingredient. Since ancient and classical times arsenic formulations have been prescribed to cure diseases.

Historically arsenic compounds were alchemical ingredients and the art of secret poisoning was a part of the social and political life of many societies. Arsenic toxicity resulted in the deaths of painters who mixed arsenic pigments.

Between 1850 and 1950 humans were habitually exposed to arsenic in medicine, food, air, and water. Consumer products of the period that contained arsenic included pigments, medicated soaps, embalming solutions, adhesive envelopes, glass, fly-powder, and rat poison.

Currently arsenic is a part of wood preservatives, some pesticides, non-ferrous alloys , and semiconductor manufacture. Arsenic may be released into the environment from metal smelting and coal burning.

see also Toxicity.

Jeffrey C. Reid

Bibliography

Agricola, Georgius (1556; modern edition 1950). De re metallica, tr. Herbert Clark Hoover and Lou Henry Hoover. New York: Dover.

Frankenberger, William T., Jr., ed. (2002). Environmental Chemistry of Arsenic. New York: Marcel Dekker.

Pinsker, Lisa M. (2001). "Arsenic." Geotimes 46(11):32–33.

Spencer, Jon F. (2000). "Arsenic in Ground Water." Arizona Geology 30(3):1–4.

Internet Resources

"TOXNET." National Institutes of Health. Available from <http://www.toxnet.nlm.nih.gov>.

"USGS—Minerals Information: Arsenic." U.S. Geological Survey. Available from <http://minerals.usgs.gov/minerals/pubs/commodity/arsenic/>.

arsenic , a semimetallic chemical element; symbol As; at. no. 33; at. wt. 74.92160; m.p. 817°C (at 28 atmospheres pressure); sublimation point 613°C; sp. gr. (stable form) 5.73; valence -3, 0, +3, or +5. Arsenic appears in several allotropic forms (see allotropy ); the stable form is a silver-gray, brittle crystalline solid that tarnishes rapidly in air, and at high temperatures burns to form a white cloud of arsenic trioxide. A yellow crystalline form and a black amorphous form are also known. Arsenic is a member of Group 5 of the periodic table . It combines readily with many elements: with hydrogen to form arsine, an extremely poisonous gas; with oxygen to form a pentoxide and the above-mentioned trioxide (As ₂ O ₃ or As ₄ O ₆ ), a deadly poison also called arsenic (III) oxide, arsenious oxide, white arsenic, or, simply, arsenic; with the halogens; and with sulfur.

The element is used with other metals to make hard, strong, corrosion-resistant alloys. Its compounds are used in pigments, animal poisons, insecticides (e.g., Paris green ), and poison gases (such as lewisite) for chemical warfare. They are also used in glassmaking, in calico and indigo printing, in tanning and taxidermy (as preservatives), and in pyrotechnics. Small quantities of arsenic added to lead in the manufacture of shot assure perfectly spherical pellets by delaying the solidification of the molten lead, and thereby allowing it to flow more readily; the arsenic also contributes hardness. A small amount of arsenic is added to germanium in the production of semiconductor devices such as transistors and integrated circuits.

A number of organic compounds of arsenic are used in medicine; the best known is Salvarsan, formerly used extensively in the treatment of syphilis and yaws. On the other hand, many arsenic compounds are strong poisons. Even in dilute concentrations that are not poisonous, as are found in some water supplies, arsenic may be carcinogenic. One delicate test for the presence of even minute quantities of arsenic in compounds is the Marsh test .

Arsenic occurs in many ores, including realgar, orpiment, and arsenopyrite , the chief commercial source. When it is prepared commercially from sulfide ores, e.g., arsenical pyrites, the ores are roasted (heated in the absence of air); the arsenic sublimes (passes directly from the solid to the gaseous state) and is condensed. In another method, white arsenic is reduced with carbon.

Although realgar, orpiment, and other arsenic minerals were known to the Greeks of Aristotle's time, the element itself was not. The "arsenic" so called by them and by the later alchemists was not true arsenic, but probably arsenic trioxide. The element was first described by Albertus Magnus in the 13th cent.

arsenic (symbol As) Semimetallic element of group V of the periodic table, probably obtained in 1250 by German chemist Albertus Magnus. Arsenic compounds are used as a poison, to harden lead, and to make semiconductors. Three allotropes are known: white arsenic, black arsenic, and a yellow nonmetallic form. Properties: at.no. 33; r.a.m. 74.9216; r.d. 5.7; m.p. 986°C (1806°F); sublimes 613°C (1135°F); most common isotope As⁷⁵ (100%).

arsenic (ar-sĕn-ik) n. a poisonous greyish metallic element producing the symptoms of nausea, vomiting, diarrhoea, cramps, convulsions, and coma when ingested in large doses. Arsenic was formerly used in medicine, the most important arsenical drugs being arsphenamine (Salvarsan) and neoarsphenamine, used in the treatment of syphilis and dangerous parasitic diseases. Symbol: As.

arsenic A toxic metal, which may have some metabolic functions, although deficiency is unknown and there are no estimates of possible requirements. Organic arsenic derivatives (arsenicals) have been used as pesticides. Arsenic can accumulate in crops treated with arsenical pesticides, and in fish and shellfish living in arsenic‐polluted water.

ar·se·nic • n. / ˈärs(ə)nik/ the chemical element of atomic number 33, a brittle steel-gray metalloid. (Symbol: As) • adj. (ar·sen·ic) / ärˈsenik/ of or relating to arsenic. ∎  Chem. of arsenic with a valence of five; of arsenic(V).

arsenic XIV. — (O)F. — L. arsenicum (arrhen-) — Gr. arsenikón (arrhen-), ult. — Iranian (cf. Pers. zirnīḳ, f. zar gold), but assoc. with arsenikós (arrhen-) male, f. ársēn, árrhēn male.
Also arsenic (cf. -IC) XIX, arsenical XVII, adjs.

As symbol for the element arsenic .

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