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/>.