Physics & Chemistry

Arsenic

September 17, 2011, 4:56 am
caption Arsenic Ore. (Source:R.Weller/Cochise College)

Arsenic is an element (atomic number 33) classed as a semi-metal or metalloid. This means it has some properties of metals, and some properties of non-metals. Arsenic occurs in two distinct solid forms. One is a brittle, gray metal, while the other is a yellow, non-metallic form, rarely seen outside the laboratory. Arsenic and its compounds often have a garlic-like odor when crushed or when scratched with a hard object.

Elemental arsenic has very few uses. Nearly all the applications are as salts or oxides of arsenic. Arsenic compounds can be very toxic, and their uses are strictly controlled by health and environmental regulations. In the year 2010 a lifeform capable of metabolizing arsenic was found in Mono Lake, California. This discovery has implications for the possibility of finding extremophile organisms on other planets than Earth, which environments may have substantially different chemical composition from the Earth.

Name

Prior Element: Germanium

Next Element: Selenium

33

As

74.92
Physical Properties
Color gray
Phase at Room Temp. solid
Density (g/cm3) 5.778
Hardness (Mohs) 3.5
Melting Point (K) 886
Boiling Point (K) ---
Heat of Fusion (kJ/mol) 27.7
Heat of Vaporization (kJ/mol) ---
Heat of Atomization (kJ/mol) 302
Thermal Conductivity (J/m sec K) 50.2
Electrical Conductivity (1/mohm cm) 30.03
Source Arsenopyrite enargite
Atomic Properties
Electron Configuration [Ar]3d104s24p3
Number of Isotopes 1
Electron Affinity (kJ/mol) 78
First Ionization Energy (kJ/mol) 946.5
Second Ionization Energy (kJ/mol) 1797.8
Third Ionization Energy (kJ/mol) 2735.4
Electronegativity 2.18
Polarizability (Å3) 4.3
Atomic Weight 74.92
Atomic Volume (cm3/mol) 13
Ionic Radius2- (pm) ---
Ionic Radius1- (pm) ---
Atomic Radius (pm) 120
Ionic Radius1+ (pm) ---
Ionic Radius2+ (pm) ---
Ionic Radius3+ (pm) 72
Common Oxidation Numbers -3, +3, +5
Other Oxid. Numbers +2
Abundance
In Earth's Crust (mg/kg) 1.8
In Earth's Ocean (mg/L) 3.7-3
In Human Body (%) 0.00001%
Regulatory / Health
CAS Number 7740-38-2
OSHA Permissible Exposure Limit (PEL) TWA: 0.5 mg/m3
OSHA PEL Vacated 1989 No limits
NIOSH Recommended Exposure Limit (REL) No limits
Sources:

Mineral Information Institute
Jefferson Accelerator Laboratory
EnvironmentalChemistry.com
 

The name arsenic comes from the Greek word arsenikon, which means orpiment. Orpiment is a bright yellow mineral composed of arsenic sulfide (As2S3), and is the most highly-visible common arsenic mineral. Historians say that arsenic was discovered in 1250 C.E. by Albertus Magnus, a German monk who spent his life studying and classifying natural materials. It is believed that he heated soap and orpiment together and isolated elemental arsenic.

Sources

Arsenic metal very rarely occurs in its pure form in nature. The most common arsenic mineral is arsenopyrite, a compound of iron, arsenic, and sulfur. Several other, less-common minerals contain arsenic, including orpiment, realgar, and enargite, which are arsenic sulfides. Most arsenic is obtained not from an ore mineral of arsenic, but as a by-product in the treatment of gold, silver, copper, and other metal ores. In fact, environmental laws require that arsenic be removed from ores, so that it does not enter the environment in effluent gases, fluids, or solids.

Significant quantities of arsenic are associated with the copper-gold deposits in Chile, the Philippines, and many other countries. However, many countries produce by-product arsenic from smelting of metal ores. China is by far the largest producer, with Chile, Mexico, and Peru also important, and lesser production from about a dozen other countries with metal smelters.

The United States imports all the metallic arsenic and arsenic compounds that it consumes. Very little is recycled, except in waste from factories that make arsenic compounds.

Uses

Only about 5% of arsenic consumption is of the metallic element. Most of this is used to alloy (mix) arsenic with lead, copper, or other metals for specific uses. As a metalloid, arsenic is a semiconductor, like silicon. This means it conducts some electricity like a metal, but not all the electricity a true conductor like copper would conduct. Consequently, about 1/10 % of arsenic is consumed in the manufacture of gallium arsenide semiconductors for use in electronics. Some arsenic is also used in glass-making.

The majority of U.S. consumption is in the form of chromated copper arsenate (CCA), a chemical used as a wood preservative for telephone poles, fence posts, pilings, and foundation timbers. The CCA significantly reduces rot and eliminates wood destruction by termites, ants and other insects. However, the use of CCA is being phased out in the U.S., and a major decrease in the arsenic market is expected as a result.

Formerly the most important use of arsenic compounds, was as an insecticide sprayed in fields and orchards. This use has entirely disappeared in most countries, due to the poisonous nature of arsenic compounds. Arsenic contamination is a problem in some well-water and may be associated with mine drainage.

Arsenic is not recovered from any waste materials or in any recycling program. Some is recovered from runoff at wood treatment facilities.

Interestingly, a trace amount of arsenic is necessary for good health and growth of animals, including humans. 0.00001% is needed for growth and for a healthy nervous system.

Toxicity

Arsenic and many compounds containing arsenic are highly toxic to many animals, including humans.especially potent poisons. For example, this is important since there are numerous water supplies proximate to mines are contaminated by arsenic molecules. Arsenic disrupts Adenosine triphosphate (ATP) production through several mechanisms. Within the the citric acid cycle, arsenic inhibits lipoic acid which is a cofactor for pyruvate dehydrogenase;  by competing with phosphate, the arsenic uncouples oxidative phosphorylation, thereby inhibiting energy-linked reduction of Nicotinamide adenine dinucleotide, mitochondrial respiration, and ATP synthesis. Hydrogen peroxide production is also increased, which may form reactive oxygen species and oxidative stress. These metabolic interferences can lead to death from multi-system organ failure, likely from necrotic cell death. Post mortem necropsy reveals brick red coloured mucosa, due to severe hemorrhage. Although arsenic causes toxicity, it can also play a protective role in certain very small concentrations. (THIS ARTICLE DOES NOT ENCOURAGE USE OR EXPERIMENTATION WITH ARSENIC SUBSTANCES, DUE TO KNOWN HIGH  HEALTH RISKS)

Substitutes and Alternative Sources

A variety of alternative wood preservatives are available to replace CCA, as is plastic wood lumber.

Further Reading

  • Sabina C. Grund, Kunibert Hanusch, Hans Uwe Wolf. 2005.  Arsenic and Arsenic Compounds, Ullmann's Encyclopedia of Industrial Chemistry, Weinheim: Wiley-VCH,
  • Nicholas C.Norman. 1998. Chemistry of Arsenic, Antimony and Bismuth. Springer.  ISBN 9780751403893.
  • Egon Biberg, Nils Wiberg, and Arnold Frederick Holleman. 2001. Inorganic Chemistry. Academic Press. ISBN 9780123526519.
  • C.Michael Hogan. 2010. Heavy metal Encyclopedia of Earth. Topic ed. Emily Monosson. Ed.-in-Chief Cutler J.Cleveland. Washington DC.
  • Common Minerals and Their Uses, Mineral Information Institute.
  • More than 170 Mineral Photographs, Mineral Information Institute. 

 

Disclaimer: This article contains information that was originally published by the Mineral Information Institute. Topic editors and authors for the Encyclopedia of Earth have edited its content and added new information. The use of information from the Mineral Information Institute should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

 

Glossary

Citation

Institute, M. (2011). Arsenic. Retrieved from http://www.eoearth.org/view/article/150247

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