Arsenic

33 germanium ← arsenic → selenium P ↑ As ↓ Sb periodic table
General
Name, Symbol, Number	arsenic, As, 33
Chemical series	metalloids
Group, Period, Block	15, 4, p
Appearance	metallic gray
Atomic mass	74.92160(2) g/mol
Electron configuration	[Ar] 3d10 4s2 4p3
Electrons per shell	2, 8, 18, 5
Physical properties
Phase	solid
Density (near r.t.)	5.727 g/cm³
Liquid density at m.p.	5.22 g/cm³
Melting point	1090 K (817 °C, 1503 °F)
Boiling point	subl. 887 K (614 °C, 1137 °F)
Critical temperature	1673 K
Heat of fusion	(gray) 24.44 kJ/mol
Heat of vaporization	? 34.76 kJ/mol
Heat capacity	(25 °C) 24.64 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 553 596 646 706 781 874
Atomic properties
Crystal structure	rhombohedral
Oxidation states	±3, 5 (mildly acidic oxide)
Electronegativity	2.18 (Pauling scale)
Ionization energies (more)	1st: 947.0 kJ/mol
	2nd: 1798 kJ/mol
	3rd: 2735 kJ/mol
Atomic radius	115 pm
Atomic radius (calc.)	114 pm
Covalent radius	119 pm
Van der Waals radius	185 pm
Miscellaneous
Magnetic ordering	no data
Electrical resistivity	(20 °C) 333 nΩ·m
Thermal conductivity	(300 K) 50.2 W/(m·K)
Speed of sound (thin rod)	(r.t.) 8 m/s
Bulk modulus	22 GPa
Mohs hardness	3.5
Brinell hardness	1440 MPa
CAS registry number	7440-38-2
Notable isotopes
Main article: Isotopes of arsenic iso NA half-life DM DE (MeV) DP 73As syn 80.3 d ε - 73Ge γ 0.05D, 0.01D, e - 74As syn 17.78 d ε - 74Ge β+ 0.941 74Ge γ 0.595, 0.634 - β- 1.35, 0.717 74Se 75As 100% As is stable with 42 neutrons

Arsenic (IPA: (chemical symbol As, atomic number 33) is a notoriously poisonous metalloid. It comes in many allotropic forms, of which the yellow, black, and gray forms are regularly seen. Arsenic and its compounds are used as pesticides, herbicides, insecticides, and various alloys.

Some compounds—particularly Paris Green, calcium arsenate, and lead arsenate—have been used as agricultural insecticides and poisons. Other compounds, namely, orpiment and realgar, were formerly used as painting pigments, though they have somewhat fallen out of use due to their toxicity and reactivity.

Given its incredible potency as a poison and the ease with which it could be used surreptitiously, arsenic was used by murderers, including members of the ruling class, to gain wealth, position, and power. It was therefore called the "Poison of Kings and the King of Poisons."

Occurrence

Massive native arsenic.

The most common mineral of arsenic is arsenopyrite, also called mispickel (FeSAs). When this mineral is heated, the arsenic sublimes (goes directly to the vapor state), leaving ferrous sulfide. Other arsenic minerals include realgar, mimetite, cobaltite, and erythrite.

<>It is sometimes found native, but usually combined with silver, cobalt, nickel, iron, antimony, or sulfur.

<>The most important inorganic compounds of arsenic are white arsenic, orpiment, realgar, Paris Green, calcium arsenate, and lead hydrogen arsenate. In addition, arsenic occurs in various organic forms in the environment. Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolized to a less toxic form of arsenic through a process of methylation.

History

Arsenic has been known and used in Persia and elsewhere since ancient times. The word arsenic is borrowed from the Persian word زرنيخ Zarnikh, meaning "yellow orpiment." Zarnikh was borrowed by the Greek language as arsenikon. During the Bronze Age, arsenic was often included in the bronze (mostly as an impurity), which made the alloy harder.

Albertus Magnus is believed to have been the first to isolate the element in 1250. In 1649 Johann Schroeder published two ways of preparing it.

The alchemical symbol for arsenic is shown below.

As the symptoms of arsenic poisoning were somewhat ill-defined, and it could be used discreetly, arsenic was frequently used as an agent for murder. Its use for this purpose declined after the English chemist James Marsh perfected a sensitive chemical test (the "Marsh test") to detect its presence. A less sensitive but more general test is the Reinsch test.

Notable characteristics

In the periodic table, arsenic is located in group 15 (formerly group 5A), between phosphorus and antimony. It is thus a member of the nitrogen family of elements, sometimes called the pnictogens (or pnicogens). It lies in period 4, between germanium and selenium.

Elemental arsenic is found in two solid forms: yellow and gray/metallic, with specific gravities of 1.97 and 5.73, respectively. When heated rapidly, it oxidizes to arsenic trioxide; the fumes from this reaction have an odor resembling garlic. The element and some of its compounds can also sublimate upon heating, converting directly to a gaseous form.

As it is chemically very similar to phosphorus, arsenic will partly substitute for phosphorus in biochemical reactions, thereby poisoning the living organism.

Isotopes

Multiple isotopes of arsenic are known, but nearly all of them are radioactive, with extremely short half-lives. Only one isotope, ⁷⁵As, is stable.

Arsenic has been proposed as a "salting" material for nuclear weapons—that is, as an agent that could increase the radioactivity of the weapon's fallout for several hours. Such a weapon is not known to have ever been built, tested, or used.

Applications

• Lead hydrogen arsenate was used well into the twentieth century, as an insecticide on fruit trees. Regrettably, this application resulted in brain damage to those working the sprayers.

• Arsenic trioxide has been used as an herbicide, insecticide, and rodenticide.

• Scheele's Green (copper arsenite) is recorded as having been used in the nineteenth century as a coloring agent in sweets.

• Copper acetoarsenite was used as a green pigment known under many different names, including Paris Green and Emerald Green. It caused numerous arsenic poisonings.

• In the last half-century, monosodium methyl arsenate (MSMA), a less toxic organic form of arsenic, has replaced lead arsenate's role in agriculture.

• Wood treated with chromated copper arsenate ("CCA timber" or "Tanalith") was heavily used during the latter half of the twentieth century as a structural and outdoor building material, in areas where there was a risk of rot or insect infestation. Although this type of treatment has been banned in many countries (see Precautions below), CCA timber is still being used.

• During the eighteenth, nineteenth, and twentieth centuries, a number of arsenic compounds were used as medicines.
  • Arsphenamine (Salvarsan), developed by Paul Ehrlich, and neoarsphenamine (Neosalvarsan) were used for the treatment of syphilis and trypanosomiasis. These drugs have been superseded by modern antibiotics.
  • Arsenic trioxide has been used in various ways over the past 200 years, but most commonly in the treatment of cancer. In 2000, the U.S. Food and Drug Administration approved this compound for the treatment of patients with acute promyelocytic leukemia, in cases of resistance to the drug ATRA (all-trans retinoic acid). It was also used as Fowler's solution in treating psoriasis.

• Gallium arsenide is an important semiconductor material used in integrated circuits. Circuits made using the compound are much faster (and much more expensive) than those made using silicon. It can be used in laser diodes and LEDs to directly convert electricity into light.

• Also used in bronzing (imparting a bronze-like surface to materials) and fireworks.

Arsenic in drinking water

Arsenic contamination of groundwater has been reported to occur in various parts of the world. The most notable case, in the late twentieth century, was that of a massive epidemic of arsenic poisoning in Bangladesh and neighboring countries. This arsenic is of natural origin, being released from sediment into the groundwater due to anoxic (oxygen-deficient) conditions of the subsurface. The groundwater began to be used after Western non-governmental organizations began a massive, tube-well drinking-water program. The program was intended to prevent people from drinking bacterially contaminated surface waters, but it unfortunately failed to test for arsenic in the groundwater. Many other countries in South and Southeast Asia—such as Vietnam, Cambodia, and Tibet—are thought to have geological environments similarly conducive to the generation of high-arsenic groundwaters.

The northern United States, including parts of Michigan, Wisconsin, Minnesota and the Dakotas, are known to have significant concentrations of arsenic in groundwater.

Arsenic can be removed from drinking water through co-precipitation of iron minerals by oxidation and filtering. When this treatment fails to produce acceptable results, more of the arsenic can be removed by using specialized media to which arsenic will bind. Several such "adsorptive media systems" have been approved for point-of-service use, based on a study funded by the U.S. Environmental Protection Agency (EPA) and the National Science Foundation (NSF).

Precautions

Arsenic and many of its compounds are especially potent poisons. They inhibit the normal action of enzymes in the body and disrupt various metabolic pathways. These metabolic interferences lead to death from multi-system organ failure.

The World Health Organization recommends a limit of 0.01 milligrams per liter (mg/L) of arsenic in drinking water. Consumption of higher levels over long periods of time can lead to the condition known as arsenicosis—chronic arsenic poisoning. Its effects include changes in skin color, formation of hard patches on the skin, skin cancer, lung cancer, cancer of the kidney and bladder. It can also lead to gangrene.

The application of most concern to the general public is probably the use of CCA timber, mentioned above. After studies showed low-level leaching from in-situ timbers (such as children's playground equipment) into the surrounding soil, CCA timber was widely banned. Currently, the most serious risk is presented by the burning of such timber. Recent years have seen fatal animal poisonings and serious human poisonings resulting from the ingestion (directly or indirectly) of wood ash from CCA timber. (The lethal human dose is approximately 20 grams of ash). Scrap CCA construction timber continues to be widely burnt out of ignorance, in both commercial and domestic fires. Safe disposal of CCA timber remains patchy and seldom practiced. Some are concerned about the widespread landfill disposal of such timber.

The European Union has classified elemental arsenic and arsenic compounds as "toxic" and "dangerous for the environment." It lists arsenic trioxide, arsenic pentoxide, and arsenate salts as category 1 carcinogens.

Growing the Brake (fern) Pteris vittata will remove arsenic from the soil.

Compounds

• Arsenic acid (H₃AsO₄): This is the acid form of the arsenate ion, AsO₄^3-, a trivalent anion. The chemical properties of arsenate salts are similar to those of the phosphates. Arsenic acid is used in wood treatment, as an agent for finishing glass and metal, in the manufacture of dyestuffs and organic arsenic compounds, and in soil sterilants. It can moreover be used as an herbicide, insecticide, or rodenticide.

• Arsenous acid or arsenious acid (H₃AsO₃ or As(OH)₃): This weak acid occurs in aqueous solution. It can be prepared by the slow reaction of arsenic trioxide with water. Addition of base converts the acid to arsenite ions [AsO(OH)₂]^-, [AsO₂(OH)]^-2, and [AsO₃]^-3. Reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.

• Arsenic trioxide (As₂O₃): This is the main starting material for arsenic chemistry and the most important commercial compound of arsenic. It can be prepared by burning arsenic in air, or by the hydrolysis of arsenic trichloride, or by roasting arsenide minerals. It is an amphoteric oxide, but tends to exhibit mainly its acidic properties. It is a starting point for the manufacture of arsenic-based pesticides (sodium arsenite, sodium arsenate, and sodium cacodylate), herbicides, pharmaceuticals (Neosalvarsan), and veterinary products. It is also a starting material for the preparation of elemental arsenic, arsenic alloys, and arsenide semiconductors. It has been used as a wood preservative and as a decolorizing agent for glasses and enamels. It is also used to treat leukemia in patients who have not responded to other medications. Due to the toxic nature of arsenic, drugs containing this compound carry significant risks.

• Arsine (Arsenic Trihydride AsH₃): It is a flammable, pyrophoric (spontaneously igniting), and highly toxic gas that is soluble in water and various organic solvents. It is generally prepared by the reaction of As³⁺ sources with H⁻ equivalents. It can be decomposed to hydrogen gas and elemental arsenic, and it can be oxidized (with oxygen) to produce arsenic trioxide. Its reactions with fluorine and chlorine (or some of their compounds) are extremely dangerous, as they can result in explosions. AsH₃ is well known in forensic science because it is a chemical intermediate in the detection of arsenic poisoning. The old (but highly sensitive) Marsh test generates AsH₃ the presence of arsenic. Also, the The Gutzeit test for arsenic involves the reaction of AsH₃ with silver ions (Ag⁺). AsH₃ is used in the synthesis of semiconducting materials related to microelectronics and solid-state lasers. Several organic compounds based on arsine—such as lewisite (β-chlorovinyldichloroarsine), adamsite (diphenylaminearsine), Clark I (diphenylchlorarsine), and Clark II (diphenylcyanoarsine)—have been effectively developed for use in chemical warfare.

• Cadmium arsenide (Cd₃As₂)

• Gallium arsenide (GaAs): It is an important semiconductor and is used to make devices such as microwave frequency integrated circuits (Monolithic Microwave Integrated Circuits, or MMICs), infrared light-emitting diodes, laser diodes, and solar cells.

• Lead hydrogen arsenate (PbHAsO₄)

See also

• Chemical element
• Periodic table

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Arsenic

External links

• A Small Dose of Toxicology
• Case Studies in Environmental Medicine: Arsenic Toxicity
• National Pollutant Inventory - Arsenic
• WebElements.com – Arsenic
• Dartmouth Toxic Metals Research Program
• origen.net – CCA wood and arsenic: toxicological effects of arsenic
• Contaminant Focus: Arsenic by the U.S. Environmental Protection Agency.
• Environmental Health Criteria for Arsenic and Arsenic Compounds, 2001 by the World Health Organization.
• A summary of the above report by GreenFacts.

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