ARSENIC (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Arsenic compounds have been known since at least the days of Ancient Greece and Rome (thousands of years ago). They were used by physicians and poisoners. The compound most often used for both purposes was arsenic sulfide (As₂S₃).

Arsenic was first recognized as an element by alchemists. Alchemy was a kind of pre-science that existed from about 500 b.c. to about the end of the 16th century. People who studied alchemy—alchemists—wanted to find a way of changing lead, iron, and other metals into gold. They were also looking for a way to have eternal life. Alchemy contained too much magic and mysticism to be a real science, but alchemists developed a number of techniques and produced many new materials that were later found to be useful in modern chemistry.

SYMBOL
As

ATOMIC NUMBER
33

ATOMIC MASS
74.9216

FAMILY
Group 15 (VA)
Nitrogen

PRONUNCIATION
AR-se-nick

A small amount of arsenic is used in alloys. An alloy is made by melting and then mixing two or more metals. The mixture has properties different from those of individual metals. The most important use of arsenic in the United States is in wood preservatives.

Discovery and naming

Arsenic can be produced from its ores very easily, so many early craftspeople may have seen the element without realizing what it was. Since arsenic is somewhat similar to mercury, early scholars probably confused the two elements with each other.

Credit for the actual discovery of arsenic often goes to alchemist Albert the Great (Albertus Magnus, 1193-1280). He heated a common compound of arsenic, orpiment (As₂S₃), with soap. Nearly pure arsenic was formed in the process.

By the mid-seventeenth century, arsenic was well known as an element. Textbooks often listed methods by which the element could be made from its compounds.

Physical properties

Arsenic occurs in two allotropic forms. Allotropes are forms of an element with different physical and chemical properties. The more common form of arsenic is a shiny, gray, brittle, metallic-looking solid. The less common form is a yellow crystalline solid. It is produced when vapors of arsenic are cooled suddenly.

When heated, arsenic does not melt, as most solids do. Instead, it changes directly into a vapor (gas). This process is known as sublimation. However, under high pressure, arsenic can be forced to melt at about 814°C (1,500°F). Arsenic has a density of 5.72 grams per cubic centimeter.

Chemical properties

Arsenic is a metalloid. A metalloid is an element that has properties of both metals and non-metals. Metalloids occur in the periodic table on either side of the staircase line that starts between boron and aluminum.

When heated in air, arsenic combines with oxygen to form arsenic oxide (As₂O₃). A blue flame is produced, and arsenic oxide can be identified by its distinctive garlic-like odor.

Arsenic combines with oxygen more slowly at room temperatures. The thin coating of arsenic oxide that forms on the element prevents it from reacting further. Arsenic does not dissolve in water or most cold acids. It does react with some hot acids to form arsenous acid (H₃AsO₃) or arsenic acid (H₃AsO₄).

Occurrence in nature

Arsenic rarely occurs as a pure element. It is usually found as a compound. The most common ores of arsenic are arsenopyrite (FeAsS), orpiment (As₂S₃), and realgar (As₄S₄). These compounds are obtained as a by-product of the mining and purification of silver metal.

The abundance of arsenic in the Earth's crust is thought to be about 5 parts per million. That places it among the bottom third of the elements in abundance in the Earth's crust.

The world's largest producers of arsenic are China, Chile, Mexico, Belgium, Namibia, and the Philippines. The United States does not produce any arsenic.

Isotopes

One naturally occurring isotope of arsenic exists, arsenic-75. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

About 14 radioactive isotopes of arsenic are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

None of the isotopes of arsenic have any important commercial use.

Extraction

The process of recovering arsenic from its ores is a common one used with metals. The ore is first roasted (heated in air) to chemically convert arsenic sulfide to arsenic oxide. The arsenic oxide is then heated with charcoal (pure carbon ). The carbon reacts with the oxygen in arsenic oxide, leaving behind pure arsenic:

Uses

Arsenic is mostly used in compounds. A much smaller amount of the element itself is used in alloys. For example, certain parts of lead storage batteries used in cars and trucks contain alloys of lead and arsenic. Arsenic has also been used to make lead shot in the past. The amount of arsenic used in these applications is likely to continue to decrease. It is too easy for arsenic to get into the environment from such applications.

Minute amounts of arsenic are used in the electronics industry. It is added to germanium and silicon to make transistors. A compound of arsenic, gallium arsenide (GaAs), is also used to make light-emitting diodes (LEDs). LEDs produce the lighted numbers in hand-held calculators, clocks, watches, and a number of other electronic devices.

Compounds

Arsenic has a fascinating history as a healer and killer. Early physicians, such as Hippocrates (c. 460 b.c-370 b.c.) and Paracelsus (1493-1541), recommended arsenic for the treatment of some diseases. In more recent times, compounds of arsenic have been used to treat a variety of diseases, including syphilis and various tropical diseases.

Arsenic has a special place in the history of modern medicine. In 1910, German biologist Paul Ehrlich (1854-1915) invented the first drug that would cure syphilis, a sexually transmitted disease. This drug, called salvarsan, is a compound of arsenic. Its chemical name is arsphenamine.

Was a U.S. president poisoned?

On July 9, 1850, the twelfth president of the United States, Zachary Taylor (1784-1850), died in office. He had served as president for a little more than sixteen months. The cause of death was widely reported as gastroenteritis (an inflammation in the stomach and intestines). He had gotten sick after eating a mixture of cherries and buttermilk. But for years, historians wondered whether Taylor had been murdered—poisoned by arsenic!

On June 17, 1991, Taylor's remains were exhumed (removed from his grave) from a cemetery in Louisville, Kentucky. The late president's descendents agreed with historians that the possibility of poisoning existed. Samples of Taylor's hair and fingernails were taken to Oak Ridge National Laboratory, in Oak Ridge, Tennessee, for analysis.

Scientists used a process that measured the amount of arsenic in the tissue samples. Most human bodies do contain traces of arsenic. So the key issue was whether there would be more arsenic in the tissue samples than would be normal for someone who had been dead for 141 years. If there were, that would mean Taylor was probably poisoned; if not, death by natural causes was more likely.

The Kentucky medical examiner (a public official who studies corpses to find the cause of death) came to a conclusion. He said the amount of arsenic found in Taylor's samples was several hundred times less than what could be expected had the president been poisoned by arsenic. So while some still wonder whether Taylor was poisoned, arsenic was certainly not the chemical element used. And, more than likely, it was the cherries and buttermilk!

Compounds of arsenic have long been used for less happy purposes. Especially during the Middle Ages, they were a popular form of committing murder. At the time, it was difficult to detect the presence of arsenic in the body. A person murdered by receiving arsenic was often thought to have died of pneumonia.

The toxic properties of arsenic compounds made them useful as rat poison. However, they are seldom used for this purpose today. Safer compounds are used that do not present a threat to humans, pets, and the environment.

Today, the most important use of arsenic is in the preservation of wood. It is used in the form of a compound called chromated copper arsenate (CCA). CCA accounts for about 90 percent of all the arsenic used in the United States. It is added to wood used to build houses and other wooden structures. It prevents organisms from growing in the wood and causing it to rot. In 1996, about 19,200,000 metric tons of arsenic were used for this purpose. There is significant concern about the use of arsenic-treated wood in playground equipment and raised garden beds because of toxicity.

Health effects

Arsenic and its compounds are toxic to animals. In low doses, arsenic produces nausea, vomiting, and diarrhea. In larger doses, it causes abnormal heart beat, damage to blood vessels, and a feeling of "pins and needles" in hands and feet. Small corns or warts may begin to develop on the palms of the hands and the soles of the feet. Direct contact with the skin can cause redness and swelling.

The most important use of arsenic is in the preservation of wood.

Long term exposure to arsenic and its compounds can cause cancer. Inhalation can result in lung cancer. If swallowed, cancer is likely to develop in the bladder, kidneys, liver, and lungs. In large doses, arsenic and its compounds can cause death.

Arsenic

Arsenic (As) is a naturally occurring element that has been used in a variety of applications—in pesticides, as wood preservatives, and as a treatment for syphilis. Throughout history, arsenic was the most often used poison. Some historians believe that Nero used arsenic to poison Claudius in 54 c.e. Arsenic has also been the poison of choice in murder mysteries due to its easy availability in rat poison and insecticides.

In its elemental form, arsenic is a steel gray metal-like material. Combined with carbon and hydrogen, it forms organic arsenic compounds. When combined with oxygen, chloride, and sulfur, it forms inorganic arsenic compounds. Organic arsenic is generally less toxic than inorganic arsenic. Chromated copper arsenate (CCA), a pesticidal compound, has been widely used as a wood preservative. In February 2002 industry announced a voluntary decision to remove arsenic-treated lumber products (play structures, picnic tables, deck wood, etc.) from the market. By January 2004 the Environmental Protection Agency (EPA) will no longer allow CCA products for residential use. Although the EPA has not concluded that arsenic-treated wood poses an "unreasonable risk" to the public, arsenic is a known human carcinogen and any decrease in exposure from any source is desirable. Over 100,000 tons of arsenic are produced worldwide, most of which is a by-product of the smelting of metals such as copper and lead.

Arsenic occurs naturally in soils, rocks, water, and air. The burning of high-arsenic coal, the use of arsenical pesticides, and metals manufacturing has redistributed arsenic throughout the environment. Human exposure to arsenic occurs through ingestion of water and food contaminated with arsenic or the inhalation of contaminated air. The greatest human exposure to arsenic is through consumption of contaminated seafood. However, the arsenic in seafood is organic arsenic, which is low in toxicity. Ingestion of inorganic arsenic in drinking water represents the greatest health hazard.

Ingestion of large amounts of inorganic arsenic is extremely toxic and can be fatal. Although environmental levels of arsenic are much lower, exposure to arsenic in drinking water has been associated with increased risks of cancer of the bladder, kidney, skin, and lung. Noncarcinogenic effects include lesions of the skin; blackfoot disease, a peripheral vascular disorder; cardiovascular and neurological diseases; and possible adverse effects on the reproductive system. Recent research has shown arsenic to be an endocrine disruptor, blocking a steroid that regulates a number of biological processes.

Arsenic contamination of drinking water supplies is a worldwide problem. Areas where drinking water is of specific concern include Bangladesh, India, Hungary, Chile, China, Argentina, Taiwan, Ghana, Mexico, the Philippines, New Zealand, and the United States (primarily the western states). The World Health Organization (WHO) has established a guideline of 10 micrograms (μg) of arsenic per liter, or ten parts per billion (ppb), in drinking water. In February 2002 the EPA announced the new arsenic drinking water standard of ten ppb. By 2006 community drinking water systems across the United States must be in compliance. There are several methods to removing arsenic from drinking water, including:

• Coprecipitation, where iron binds with arsenic and the particles settle out or are removed.
• Adsorption, where arsenic adheres to aluminum or iron and can be removed.
• Membrane filtration, where arsenic is filtered out of the water.
• Ion exchange, where a chemical process exchanges chloride for arsenic.

The estimated cost for compliance of the new arsenic drinking water standard in the United States is approximately $177 million per year. The average cost per household is dependent upon the size of the community water system—the smaller the system, the higher the cost.

Arsenic has also been identified in hazardous waste sites within the United States. Scientists at the University of Florida's Institute of Food and Agriculture Sciences have identified the brake fern, Pteris vittata, which absorbs arsenic from the soil. The potential use of this fern to clean up arsenic from such sites is called phytoremediation, where plants and trees are used to extract contaminates from the soil. Many arsenic compounds dissolve in water, and thus, the cleanup of these waste sites would protect the underlying aquifers.

see also Bioremediation; Endocrine Disruption; Health, Human; Risk; Smelting; Water Treatment.

Bibliography

chappell, w.r.; abernathy, c.o.; and calderon, r.l., eds. (1999). arsenic exposure and health effects. new york: elsevier, 1999.

internet resources

agency for toxic substances and disease registry. "public health statement for arsenic." available from www.atsdr.cdc.gov.

world health organization. "arsenic in drinking water." fact sheet no. 210. available from www.who.org.

Betsy T. Kagey

ARSENIC

Arsenic has long been regarded as a dangerous poison and an environmental contaminant. But in the 1980s the focus on arsenic changed dramatically when approximately 3 million tube wells in Bangladesh and West Bengal, India, were found to be contaminated with that highly reactive chemical agent. By 2003 public health authorities estimated that as much as 40 million persons were being exposed to varying concentrations of the chemical in Bangladesh, plus another 3 million in West Bengal. The source of the arsenic came as a surprise to the toxic substance community in that the contamination was so widespread and came not from any industrial source but from rocks and sediment in the region's natural geological formations.

Arsenic is one of the most ubiquitous and paradoxical substances on Earth. In very small amounts, it is essential to life. In large amounts it is poisonous. While its inorganic forms are toxic, its organic forms are benign. Industrial arsenic is used for leather tanning, in pigments, glassmaking, fireworks, and medicinals, and as an additive that gives strength to metals. It is also a poison gas agent.

Arsenic's toxic effects vary according to exposure. Moderate levels (roughly 100 parts per billion and higher) can cause nausea and vomiting, decreased production of red and white blood cells, abnormal heart rhythm, and tingling in the hands and feet. Chronic exposure over time causes dark sores on hands, feet, and torso plus overall debilitation from damage to the cardiovascular, immune, neurological, and endocrine systems. Cancer can also occur after years of arsenic exposure at moderate to high levels.

After years of controversy over compliance costs, the U.S. Environmental Protection Agency in 2001 established a drinking water standard of 10 parts per billion, that was scheduled to go into effect in January 2006. The new rule supplanted the 50 ppb standard that had been in effect since 1975. The World Health Organization's has likewise adopted a 10ppb guideline. Arsenic readings in the Bangladesh/West Bengal groundwater frequently run from 200 ppb to 1,000 ppb. Deep wells, however, are not believed to be a problem.

Arsenic as both an industrial and natural pollutant is hardly a new phenomenon worldwide. High arsenic levels in air and water from mining and manufacturing operations from China to Peru have been well recognized though sporadically regulated for decades. Moreover, arsenic leached into waterways and aquifers from naturally occurring geological formations has been recorded in several regions. But because most of those areas are geographically remote, only the environmental toxicology community has taken much notice.

The ethics of arsenic control are vastly complex. The moment an environmental problem rises to crisis proportions in the industrial democracies of Europe and North America, the response is to assemble all possible mitigation techniques and human resources to attack the problem quickly. Nothing of the sort had happened in response to the disaster in South Asia, owing mainly to political graft, bureaucratic bloat, and the conflicting and poorly coordinated maze of national and international institutions whose involvement is required. The World Bank in 1998 issued a $32.4 million loan for the planning and execution of mitigation projects, but not until 2004 were the funds released for the project to begin.

As of 2005, the problem remained so widespread and Bangladesh was so lacking in resources that villagers themselves had to be taught to self-police and improve their water supplies by marking contaminated wells and using cheap and simple filtration techniques. For that to happen, inexpensive, mobile testing kits were needed and alternative sources of water had to be developed.

WIL LEPKOWSKI

SEE ALSO Heavy Metals.

BIBLIOGRAPHY

National Academy of Sciences. (2001). Arsenic in Drinking Water: 2001 Update. Washington, DC: National Academy of Sciences Press.

Patel, Priya Harish. (2001). "The Crisis of Capacity: A Case Study of Arsenic Contamination in Bangladesh and West Bengal." Cambridge, MA: Harvard University, Department of Environmental Science and Public Policy. Honors thesis.

INTERNET RESOURCES

Arsenic Crisis Information Center. Available at http://bicn.com/acic/.

Harvard University Environmental Science Group. Available at http://phys4.harvard.edu/~wilson/arsenic. Provides valuable information on the Bangladesh crisis.

Jadavpur University Department of Environmental Studies. Available at http://www.sos-arsenic.net.

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/>.