Arsenic

2007 Schools Wikipedia Selection. Related subjects: Chemical elements

33 germanium ← arsenic → selenium P ↑ As ↓ Sb Periodic Table - Extended 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·cm−3
Liquid density at m.p.	5.22 g·cm−3
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−1
Heat of vaporization	? 34.76 kJ·mol−1
Heat capacity	(25 °C) 24.64 J·mol−1·K−1
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−1
	2nd: 1798 kJ·mol−1
	3rd: 2735 kJ·mol−1
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−1·K−1
Young's modulus	8 GPa
Bulk modulus	22 GPa
Mohs hardness	3.5
Brinell hardness	1440 MPa
CAS registry number	7440-38-2
Selected isotopes
Main article: Isotopes of arsenic iso NA half-life DM DE ( MeV) DP 73As syn 80.3 d ε - 73Ge γ 0.05 D, 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
References

Arsenic ( IPA: /ˈɑːsənɪk/, /ˈɑɹsənɪk/) is a chemical element in the periodic table that has the symbol As and atomic number 33. This is a notoriously poisonous metalloid that has many allotropic forms; yellow, black and gray are a few that are regularly seen. Arsenic and its compounds are used as pesticides, herbicides, insecticides and various alloys.

Notable characteristics

Arsenic is very similar chemically to its predecessor phosphorus, so much so that it will partly substitute for phosphorus in biochemical reactions and is thus poisonous. When heated rapidly it oxidizes to arsenic trioxide; the fumes from this reaction have an odour resembling garlic. Arsenic and some arsenic compounds can also sublimate upon heating, converting directly to a gaseous form. Elemental arsenic is found in two solid forms: yellow and gray/metallic, with specific gravities of 1.97 and 5.73, respectively.

Applications

Lead hydrogen arsenate has been used, well into the 20th century, as an insecticide on fruit trees (sometimes resulting in brain damage to those working the sprayers), and Scheele's Green has even been recorded in the 19th century as a coloring agent in sweets. In the last half century, monosodium methyl arsenate (MSMA), a less toxic organic form of arsenic, has replaced lead arsenate's role in agriculture.

The application of most concern to the general public is probably that of wood which has been treated with chromated copper arsenate ("CCA", or " Tanalith", and the vast majority of older " pressure treated" wood). CCA timber is still in widespread use in many countries, and was heavily used during the latter half of the 20th century as a structural, and outdoor building material, where there was a risk of rot, or insect infestation in untreated timber. Although widespread bans followed the publication of studies which showed low-level leaching from in-situ timbers (such as children's playground equipment) into surrounding soil, the most serious risk is presented by the burning of CCA 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 through ignorance, in both commercial, and domestic fires. Safe disposal of CCA timber remains patchy, and little practiced, there is concern in some quarters about the widespread landfill disposal of such timber.

During the 18th, 19th, and 20th centuries, a number of arsenic compounds have been used as medicines, including arsphenamine (by Paul Ehrlich) and arsenic trioxide (by Thomas Fowler). Arsphenamine as well as Neosalvarsan was indicated for syphilis and trypanosomiasis, but has been superseded by modern antibiotics. Arsenic trioxide has been used in a variety of ways over the past 200 years, but most commonly in the treatment of cancer. The Food and Drug Administration in 2000 approved this compound for the treatment of patients with acute promyelocytic leukemia that is resistant to ATRA. It was also used as Fowler's solution in psoriasis.

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

Other uses;

• Various agricultural insecticides and poisons.
• Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made using the compound are much faster (but also much more expensive) than those made in silicon. Unlike silicon it is direct bandgap, and so can be used in laser diodes and LEDs to directly convert electricity into light.
• Also used in bronzing and pyrotechny.

History

The word arsenic is borrowed from the Persian word زرنيخ Zarnikh meaning "yellow orpiment". Zarnikh was borrowed by Greek as arsenikon. Arsenic has been known and used in Persia and elsewhere since ancient times. As the symptoms of arsenic poisoning were somewhat ill-defined, it was frequently used for murder until the advent of the Marsh test, a sensitive chemical test for its presence. (Another less sensitive but more general test is the Reinsch test.) Due to its use by the ruling class to murder one another and its incredible potency and discreetness, arsenic has been called the Poison of Kings and the King of Poisons.

During the Bronze Age, arsenic was often included in the bronze (mostly as an impurity), which made the alloy harder.

Albertus Magnus(Albert the Great, 1193-1280) is believed to have been the first to isolate the element in 1250. In 1649 Johann Schroeder published two ways of preparing arsenic.

The alchemical symbol for arsenic is shown below.

In the Victorian era, arsenic was mixed with vinegar and chalk and eaten by women to improve the complexion of their faces, making their skin more fair to show they did not work in the fields. Arsenic was also rubbed into the faces and arms of women to improve their complexion.

Arsenic in drinking water

Arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh and neighbouring countries. It is estimated that approximately 57 million people are drinking groundwater with arsenic concentrations elevated above the World Health Organization's standard of 10 parts per billion. The arsenic in the groundwater is of natural origin, and is released from the sediment into the groundwater due to the anoxic conditions of the subsurface. This groundwater began to be used after western NGOs instigated a massive tube well drinking-water program in the late twentieth century. This program was designed to prevent drinking of bacterially-contaminated surface waters, but unfortunately failed to test for arsenic in the groundwater.(2) Many other countries in South East Asia, such as Vietnam, Cambodia, and Tibet, are thought to have geological environments similarly conducive to 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 ground water.

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, adsorptive arsenic removal media may be utilized. Several adsorptive media systems have been approved for point of service use in a study funded by the United States Environmental Protection Agency (U.S.EPA) and the National Science Foundation (NSF).

Magnetic separations of arsenic at very low magnetic field gradients have been demonstrated in point-of-use water purification with high–surface area and monodisperse magnetite (Fe3O4) nanocrystals. Using the high specific surface area of Fe3O4 nanocrystals the mass of waste associated with arsenic removal from water has been dramatically reduced.

Occurrence

Massive native arsenic

Arsenopyrite also called mispickel (FeSAs) is the most common mineral from which, on heating, the arsenic sublimes leaving ferrous sulfide. Other arsenic minerals include realgar, mimetite, cobaltite and erythrite.

The most important compounds of arsenic are white arsenic, orpiment, realgar, Paris Green, calcium arsenate, and lead hydrogen arsenate. Paris Green, calcium arsenate, and lead arsenate have been used as agricultural insecticides and poisons. Orpiment and realgar were formerly used as painting pigments, though they have somewhat fallen out of use due to their toxicity and reactivity. It is sometimes found native, but usually combined with silver, cobalt, nickel, iron, antimony, or sulfur.

In addition to the inorganic forms mentioned above, arsenic also occurs in various organic forms in the environment. Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised to a less toxic form of arsenic through a process of methylation.

Precautions

Arsenic and many of its compounds are especially potent poisons. Arsenic disrupts ATP production through several mechanisms including allosteric inhibition of the metabolic enzyme lipothiamide pyrophosphatase during glycolysis. At the level of the citric acid cycle, arsenic inhibits succinate dehydrogenase and by competing with phosphate it uncouples oxidative phosphorylation, thus inhibiting energy-linked reduction of NAD+, mitochondrial respiration, and ATP synthesis. Hydrogen peroxide production is also increased, which might form reactive oxygen species and oxidative stress. Arsenic kills by enzyme inhibtion because enzymes are the best documented targets of metals; in this case, it causes toxicity but can also play a protective role. These metabolic interferences lead to death from multi-system organ failure (see arsenic poisoning) probably from necrotic cell death, not apoptosis. A post mortem reveals brick red colored mucosa, due to severe haemorrhage.

Elemental arsenic and arsenic compounds are classified as " toxic" and "dangerous for the environment" in the European Union under directive 67/548/EEC.

The IARC recognizes arsenic and arsenic compounds as group 1 carcinogens, and the EU lists arsenic trioxide, arsenic pentoxide and arsenate salts as category 1 carcinogens.

Arsenic is known to cause arsenicosis due to its manifestation in drinking water, “the most common species being arsenate [HAsO₄^2- ; As(V)] and arsenite [H₃AsO₃ ; As(III)]”. The ability of arsenic to oxidized between As(III) and As(V) makes its availability in the environment possible. According to Croal, Gralnick, Malasarn, and Newman, “[the] understanding [of] what stimulates As(III) oxidation and/or limits As(V) reduction is relevant for bioremediation of contaminated sites (Croal). The study of chemolithoautotrophic As(III) oxidizers and the heterotrophic As(V) reducers can help the understanding of the oxidation and/or reduction of arsenic.

Compounds

• Arsenic acid (H₃AsO₄)
• Arsenous acid (H₃AsO₃)
• Arsenic trioxide (As₂O₃)
• Arsine (Arsenic Trihydride AsH₃)
• Cadmium arsenide (Cd₃As₂)
• Gallium arsenide (GaAs)
• Lead hydrogen arsenate (PbHAsO₄)

Isotopes

Arsenic has been proposed as a " salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of As-75, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope As-76 with a half-life of 1.0778 days and produce approximately 1.13 MeV of gamma radiation, significantly increasing the radioactivity of the weapon's fallout for several hours. Such a weapon is not known to have ever been built, tested, or used.