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arsenic

 
(är'sə-nĭk) pronunciation
n.
  1. (Symbol As) A highly poisonous metallic element having three allotropic forms, yellow, black, and gray, of which the brittle, crystalline gray is the most common. Arsenic and its compounds are used in insecticides, weed killers, solid-state doping agents, and various alloys. Atomic number 33; atomic weight 74.922; valence 3, 5. Gray arsenic melts at 817°C (at 28 atm pressure), sublimes at 613°C, and has a specific gravity of 5.73.
  2. Arsenic trioxide.
adj. ar·sen·ic (är-sĕn'ĭk)
Of or containing arsenic, especially with valence 5.

[Middle English arsenik, from Old French, from Latin arsenicum, from Greek arsenikon, yellow orpiment, alteration of Syriac zarnīkā, from Middle Persian *zarnīk, from Old Iranian *zarna-, golden.]


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Arsenic (gray) with realgar (red) and orpiment (yellow)
(click to enlarge)
Arsenic (gray) with realgar (red) and orpiment (yellow) (credit: Courtesy of the Joseph and Helen Guetterman collection; photograph, John H. Gerard)
Nonmetallic to semimetallic chemical element, chemical symbol As, atomic number 33. It exists uncombined in two stable (and several unstable) allotropes, one gray and one yellow, but is more often found in nature as the sulfide or oxide. The elemental form is used to form alloys of metals (especially lead), and certain semiconductors are made from crystals of gallium arsenide (GaAs). Arsenious oxide (arsenic trioxide or white arsenic, As2O3) is used in pesticides, as a pigment, and as a preservative of hides and wood; this is the poisonous "arsenic" (see arsenic poisoning) in detective stories. Arsenic pentoxide (As2O5) is also used in insecticides, herbicides, metal adhesives, and pigments.

For more information on arsenic, visit Britannica.com.

A chemical element, symbol As, atomic number 33. Arsenic is found widely distributed in nature (approximately 5 × 10−4 of the Earth's crust). It is one of the 22 known elements composed of only one stable nuclide, 3375As; the atomic weight is 74.92158. There are 17 other radioactive arsenic nuclides known.

There are three polymorphic modifications of arsenic. The yellow cubic α-form is made by condensing the vapor at very low temperatures. The black β-polymorph is isostructural with black phosphorus. Both these modifications revert to the stable γ-form, gray or metallic, rhombohedral arsenic, on heating or exposure to light. The metallic form is a moderately good thermal and electric conductor and is brittle, easily fractured, and of low ductility.

Arsenic is found native as the mineral scherbenkobalt, but generally occurs among surface rocks combined with sulfur or metals such as Mn, Fe, Co, Ni, Ag, or Sn. The principal arsenic mineral is FeAsS (arsenopyrite, mispickel); other metal arsenide ores are FeAs2 (löllingite), NiAs (nicolite), CoAsS (cobalt glance), NiAsS (gersdorffite), and CoAs2 (smaltite). Naturally occurring arsenates and thioarsenates are common, and most sulfide ores contain arsenic. As4S4 (realgar) and As4S6 (orpiment) are the most important sulfur-containing minerals. The oxide, arsenolite, As4O6 is found as the product of the weathering of other arsenical minerals, and is also recovered from flue dusts collected during the extraction of Ni, Cu, and Sn from their ores; it also results when the arsenides of Fe, Co, or Ni are roasted in air or oxygen. The element may be obtained by roasting FeAsS or FeAs2 in the absence of air or by reduction of As4O6 carbon, when As4 may be sublimed away.

Elemental arsenic has few uses. It is one of the few minerals available in 99.9999+% purity, which is largely used in the laser material GaAs and as a doping agent in the manufacture of various solid-state devices. Arsenic oxide is used in glass manufacture. The arsenic sulfides are used as pigments and in pyrotechnics. Dihydrogen arsenate is used in medicine, as are several other arsenic compounds. Most of the medicinal uses of arsenic compounds depend on their toxic nature. See also Antimony; Periodic table; Phosphorus.


A toxic metal, which may have some metabolic functions, although deficiency is unknown and there are no estimates of possible requirements. Organic arsenic derivatives (arsenicals) have been used as pesticides. Arsenic can accumulate in crops treated with arsenical pesticides, and in fish and shellfish living in arsenic-polluted water.

Highly toxic in large amounts, some people believe that very small amounts of this metallic greyish element may be essential for health, but no deficiency signs have been described. Total daily intake averages about 90 micrograms, but its role in the body is unknown. It occurs especially in foods such as fish and shellfish. Inorganic arsenic is more toxic than organic forms.


from Syriac
This word originated in Turkey

As an element of the English language, arsenic made a modest debut in about the year 1386. It was a minor ingredient of a lesser-known story by a major author, Geoffrey Chaucer. In the Canon's Yeoman's tale, one of the last of the Canterbury Tales, the apprentice of a fugitive alchemist describes his master's laboratory. After inventorying the "vessels made of earth and glass" and other equipment used by the alchemist, the Canon's Yeoman lists his chemical supplies, including "waters rubifying, bull's gall, arsenic, sal ammoniac, and brimstone."

The arsenic mentioned by the Canon's Yeoman was probably what chemists nowadays call arsenic trisulphide, a bright yellow substance used as a pigment and for tanning. There was also the highly poisonous white arsenic or arsenic trioxide, mentioned in English as early as 1605. This is the arsenic used by murderers like the Brooklyn ladies in the Broadway play and 1944 Cary Grant movie Arsenic and Old Lace. White arsenic is slightly sweet, so the ladies masked its flavor in blackberry wine. Arsenic is not a wise choice if you wish to conceal poisoning, because it leaves traces that remain in the body of the victim for years. One mark of slow arsenic poisoning is "Mees' lines," white transverse lines on the fingernails.

In small amounts, though, white arsenic is said to be good for you; it stimulates the production of red blood cells. Once taken as a nutritional supplement in Alpine countries, it supposedly gave people ruddy complexions and increased their ability to work.

There is also just plain arsenic, which smells like garlic when it evaporates. As is the symbol for this semi-metallic element arsenic, No. 33 in the periodic table.

Linguistically, arsenic has a compound history too. English got the name from French; French got it from Latin; Latin got it from Greek. Greek seems to have taken it from Syriac zarnika, and though this word evidently goes back to Middle Persian and Old Iranian, Syriac is the oldest attested form.

Syriac is a language that became extinct a thousand years ago, although it is still used as a literary language by followers of the Syrian Orthodox and Syrian Catholic churches. It was spoken in a corner of present-day Turkey as well as Iraq and Syria. The alchemical name for mercury, azoth (1477), also traces itself back to Syriac. That language may also be the source, via Hebrew and German, for schwa (1895), the name given by linguists to an unstressed vowel, symbolized by an upside-down e.



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.

(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.

— MARGARET H. WHITAKER; BRUCE A. FOWLER




[Ma]

A steel-grey semi-metallic element (As) that is highly toxic but which occurs naturally in many copper deposits and which was later sought after and added to bronze by early European metalworkers in order to improve the casting properties of their metal.

arsenic (är'sənĭk), a semimetallic chemical element; symbol As; at. no. 33; at. wt. 74.92160; m.p. 817°C (at 28 atmospheres pressure); sublimation point 613°C; sp. gr. (stable form) 5.73; valence −3, 0, +3, or +5. Arsenic appears in several allotropic forms (see allotropy); the stable form is a silver-gray, brittle crystalline solid that tarnishes rapidly in air, and at high temperatures burns to form a white cloud of arsenic trioxide. A yellow crystalline form and a black amorphous form are also known. Arsenic is a member of Group 5 of the periodic table. It combines readily with many elements: with hydrogen to form arsine, an extremely poisonous gas; with oxygen to form a pentoxide and the above-mentioned trioxide (As2O3 or As4O6), a deadly poison also called arsenic (III) oxide, arsenious oxide, white arsenic, or, simply, arsenic; with the halogens; and with sulfur.

The element is used with other metals to make hard, strong, corrosion-resistant alloys. Its compounds are used in pigments, animal poisons, insecticides (e.g., Paris green), and poison gases (such as lewisite) for chemical warfare. They are also used in glassmaking, in calico and indigo printing, in tanning and taxidermy (as preservatives), and in pyrotechnics. Small quantities of arsenic added to lead in the manufacture of shot assure perfectly spherical pellets by delaying the solidification of the molten lead, and thereby allowing it to flow more readily; the arsenic also contributes hardness. A small amount of arsenic is added to germanium in the production of semiconductor devices such as transistors and integrated circuits.

A number of organic compounds of arsenic are used in medicine; the best known is Salvarsan, formerly used extensively in the treatment of syphilis and yaws. On the other hand, many arsenic compounds are strong poisons. Even in dilute concentrations that are not poisonous, as are found in some water supplies, arsenic may be carcinogenic. One delicate test for the presence of even minute quantities of arsenic in compounds is the Marsh test.

Arsenic occurs in many ores, including realgar, orpiment, and arsenopyrite, the chief commercial source. When it is prepared commercially from sulfide ores, e.g., arsenical pyrites, the ores are roasted (heated in the absence of air); the arsenic sublimes (passes directly from the solid to the gaseous state) and is condensed. In another method, white arsenic is reduced with carbon.

Although realgar, orpiment, and other arsenic minerals were known to the Greeks of Aristotle's time, the element itself was not. The "arsenic" so called by them and by the later alchemists was not true arsenic, but probably arsenic trioxide. The element was first described by Albertus Magnus in the 13th cent.


Known carcinogen. Highest concentrations are in seafood, rice and rice cereal, mushrooms, and poultry. Root crops, such as carrots, onions, and potatoes, are the most vulnerable.


As
Hexagonal -- scalenohedral

Environment

In ore veins in crystalline rocks.

Crystal description

Crystals, which are almost unknown, are rhombohedrons resembling cubes. The commonest appearance is in rounded mammillary or botryoidal crusts or granular masses.

Physical properties

White. Luster metallic; hardness 3Ɖ; specific gravity 5.7; fracture uneven; cleavage basal (rarely seen, since crystals are rare). Brittle.

Composition

Arsenic, usually relatively pure, sometimes with a little antimony.

Tests

It is tin-white in color, brittle, and volatilizes completely under the blowpipe, giving off a garlic odor and not melting.

Distinguishing characteristics

Can be confused with antimony (which melts) and with the antimony-arsenic compound allemontite (which gives off white arsenic fumes and forms a metallic globule that takes fire and burns). The color and total volatilization distinguish it from most other similar substances.

Occurrence

In metal ore veins, but not common. Most collection specimens are ancient botryoidal crusts from Saxony. In France it has been found at Sainte-Marie-aux-Mines, Alsace. It has been found in masses in Arizona. Small balls of crystals are found in a decomposed rhyolite in the Akadani Mine, Fukui Prefecture, s. Honshu, Japan. Allemontite (AsSb) occurs in similar veins and brittle, white, metallic grains have been found in pegmatites.



Devil's Dictionary:

arsenic

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A cynical view of the world by Ambrose Bierce


n.

A kind of cosmetic greatly affected by the ladies, whom it greatly affects in turn.

    "Eat arsenic?  Yes, all you get,"
        Consenting, he did speak up;
    "'Tis better you should eat it, pet,
        Than put it in my teacup."
                                                             Joel Huck



symbol: As; a semi-metal element of group 15 of the IUPAC periodic table; atomic number 33; relative atomic mass 79.41. It exists in three forms: yellow, grey, and black. The yellow form, comparable to white phosphorus, is less stable than the other forms and is converted into the grey (metallic) form by heat. The black form is more stable than the yellow form but is metastable with respect to the grey form. Arsenic is required in trace amounts by some organisms, but generally it acts as an antimetabolite.

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A chemical element, atomic number 33, atomic weight 74.92, symbol As. Arsenic compounds have been widely used in veterinary medicine, but they have been replaced for the most part by antibiotics, which are less toxic and equally effective. Still used in homeopathy. Some of the arsenicals are used for infectious diseases, especially those caused by protozoa, and some skin disorders and blood dyscrasias also are still treated with arsenic compounds. Since arsenic is highly toxic it must be administered with caution. The antidote for arsenic poisoning is dimercaprol (BAL). See also arsenical.

  • a. bushSenna floribunda, S. occidentalis.
  • copper–chrome–a. wood preservative — see wood preservative.
  • a. deficiency — evidence on the response to arsenic supplementation of the diet suggests that it may exert a beneficial effect on patients by controlling deleterious intestinal organisms.
  • inorganic a. poisoning — can occur after ingestion or cutaneous absorption. Acute poisoning is manifested by abdominal pain, diarrhea and dehydration. Chronic poisoning shows a syndrome of emaciation, chronic diarrhea, poor haircoat and greatly reduced productivity.
  • organic a. poisoning — arsanilate poisoning in pigs is characterized by blindness and incoordination and a high recovery rate; poisoning by 4-hydroxyphenyl arsenic acid also in pigs causes a syndrome of tremor and incoordination but only if the affected animals are exercising at the time.
  • a. poisoning — see inorganic arsenic poisoning, organic arsenic poisoning (above).
  • a. trioxide — AsO3, pollutant on pasture from roasting of arsenical and some iron ores.
Random House Word Menu:

categories related to 'arsenic'

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Random House Word Menu by Stephen Glazier
For a list of words related to arsenic, see:

germaniumarsenicselenium
P

As

Sb
Appearance
metallic grey
General properties
Name, symbol, number arsenic, As, 33
Pronunciation /ˈɑrsənɪk/ ar-sə-nik,
also /ɑrˈsɛnɪk/ ar-sen-ik when attributive
Element category metalloid
Group, period, block 154, p
Standard atomic weight 74.92160(2)
Electron configuration [Ar] 4s2 3d10 4p3
Electrons per shell 2, 8, 18, 5 (Image)
Physical properties
Phase solid
Density (near r.t.) 5.727 g·cm−3
Liquid density at m.p. 5.22 g·cm−3
Sublimation point 887 K, 615 °C, 1137 °F
Triple point 1090 K (817°C), 3628 [2] kPa
Critical point 1673 K, ? MPa
Heat of fusion (grey) 24.44 kJ·mol−1
Heat of vaporization  ? 34.76 kJ·mol−1
Molar heat capacity 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
Oxidation states 5, 3, 2, 1,[3] -3
(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 119 pm
Covalent radius 119±4 pm
Van der Waals radius 185 pm
Miscellanea
Crystal structure trigonal[1]
Magnetic ordering diamagnetic[4]
Electrical resistivity (20 °C) 333 nΩ·m
Thermal conductivity 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
Most stable 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% 75As is stable with 42 neutrons
· r

Arsenic (play /ˈɑrsɨnɪk/ ar-sə-nik is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.[5] Arsenic is a metalloid. It can exist in various allotropes, although only the grey form has important use in industry.

The main use of metallic arsenic is for strengthening alloys of copper and especially lead (for example, in car batteries). Arsenic is a common n-type dopant in semiconductor electronic devices, and the optoelectronic compound gallium arsenide is the most common semiconductor in use after doped silicon. Arsenic and its compounds, especially the trioxide, are used in the production of pesticides (treated wood products), herbicides, and insecticides. These applications are declining, however.[6]

Arsenic is notoriously poisonous to multicellular life, although a few species of bacteria are able to use arsenic compounds as respiratory metabolites. Arsenic contamination of groundwater is a problem that affects millions of people across the world.

Contents

Characteristics

Physical characteristics

Crystal structure common to Sb, AsSb and grey As

The three most common allotropes are metallic grey, yellow and black arsenic, with grey being the most common.[7] Grey arsenic (α-As, space group R3m No. 166) adopts a double-layered structure consisting of many interlocked ruffled six-membered rings. Because of weak bonding between the layers, grey arsenic is brittle and has a relatively low Mohs hardness of 3.5. Nearest and next-nearest neighbors form a distorted octahedral complex, with the three atoms in the same double-layer being slightly closer than the three atoms in the next.[8] This relatively close packing leads to a high density of 5.73 g/cm3.[9] Grey arsenic is a semimetal, but becomes a semiconductor with a bandgap of 1.2–1.4 eV if amorphized.[10] Yellow arsenic is soft and waxy, and somewhat similar to tetraphosphorus (P4). Both have four atoms arranged in a tetrahedral structure in which each atom is bound to each of the other three atoms by a single bond. This unstable allotrope, being molecular, is the most volatile, least dense and most toxic. Solid yellow arsenic is produced by rapid cooling of arsenic vapour, As4. It is rapidly transformed into the grey arsenic by light. The yellow form has a density of 1.97 g/cm3.[9] Black arsenic is similar in structure to red phosphorus.[9]

Isotopes

Naturally occurring arsenic is composed of one stable isotope, 75As.[11] As of 2003, at least 33 radioisotopes have also been synthesized, ranging in atomic mass from 60 to 92. The most stable of these is 73As with a half-life of 80.3 days. Isotopes that are lighter than the stable 75As tend to decay by β+ decay, and those that are heavier tend to decay by β- decay, with some exceptions.

At least 10 nuclear isomers have been described, ranging in atomic mass from 66 to 84. The most stable of arsenic's isomers is 68mAs with a half-life of 111 seconds.[11]

Chemistry

When heated in air, arsenic oxidizes to arsenic trioxide; the fumes from this reaction have an odour resembling garlic. This odour can be detected on striking arsenide minerals such as arsenopyrite with a hammer. Arsenic (and some arsenic compounds) sublimes upon heating at atmospheric pressure, converting directly to a gaseous form without an intervening liquid state at 887 K (614 °C).[2] The triple point is 3.63 MPa and 1,090 K (820 °C).[9][2] Arsenic makes arsenic acid with concentrated nitric acid, arsenious acid with dilute nitric acid, and arsenic trioxide with concentrated sulfuric acid.[12]

Arsenic burning in the air

Compounds

Arsenic compounds resemble in some respects those of phosphorus, which occupies the same group (column) of the periodic table. Arsenic is less commonly observed in the pentavalent state, however. The most common oxidation states for arsenic are: −3 in the arsenides, such as alloy-like intermetallic compounds; and +3 in the arsenites, arsenates(III), and most organoarsenic compounds. Arsenic also bonds readily to itself as seen in the square As3−
4
ions in the mineral skutterudite. In the +3 oxidation state, arsenic is typically pyramidal, owing to the influence of the lone pair of electrons.

Inorganic

Arsenic forms colorless, odorless, crystalline oxides As2O3 ("white arsenic") and As2O5, which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic(V) acid is a weak acid. Its salts are called arsenates, which is the basis of arsenic contamination of groundwater, a problem that affects many people. Man-made arsenates include Paris Green (copper(II) acetoarsenite), calcium arsenate, and lead hydrogen arsenate. The latter three have been used as agricultural insecticides and poisons.

The protonation steps between the arsenate and arsenic acid are similar to those between phosphate and phosphoric acid. Unlike phosphorus acid, arsenous acid is genuinely tribasic, with the formula As(OH)3.

A broad variety of sulfur compounds of arsenic are known. Orpiment (As2S3) and realgar (As4S4) are somewhat abundant and were formerly used as painting pigments. In As4S10, arsenic has a formal oxidation state of +2 in As4S4, which features As-As bonds so that the total covalency of As is still in fact three.[13]

The trifluoride, trichloride, tribromide, and triiodide of arsenic(III) are well known, whereas only Arsenic pentafluoride (AsF5) is the only important pentahalide. Again reflecting the lower stability of the 5+ oxidation state, the pentachloride is stable only below −50 °C.[9]

Organoarsenic compounds

Trimethylarsine

A large variety of organoarsenic compounds are known. Several were developed as chemical warfare agents during World War I, including vesicants such as lewisite and vomiting agents such as adamsite.[14][15][16] Cacodylic acid, which is of historic and practical interest, arises from the methylation of arsenic trioxide, a reaction that has no analogy in phosphorus chemistry.

Alloys

Arsenic is used as the group 5 element in the III-V semiconductors gallium arsenide, indium arsenide, and aluminium arsenide. The valence electron count of GaAs is the same as a pair of Si atoms, but the band structure is completely different, which results distinct bulk properties. Other arsenic alloys include the II-IV semiconductor cadmium arsenide.

Occurrence and production

A large sample of native arsenic

Minerals with the formula MAsS and MAs2 (M = Fe, Ni, Co) are the dominant commercial sources of arsenic, together with realgar (an arsenic sulfide mineral) and native arsenic. An illustrative mineral is arsenopyrite (FeAsS), which is structurally related to iron pyrite. Many minor As-containing minerals are known. Arsenic also occurs in various organic forms in the environment.[17] 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.[18]

Other naturally occurring pathways of exposure include volcanic ash, weathering of arsenic-containing minerals and ores, and dissolved in groundwater. It is also found in food, water, soil, and air.[19] The most common pathway of exposure for humans is ingestion, and the predominant source of arsenic in our diet is through seafood. An additional route of exposure is through inhalation.[20]

Arsenic output in 2006[21]

In 2005, China was the top producer of white arsenic with almost 50% world share, followed by Chile, Peru, and Morocco, according to the British Geological Survey and the United States Geological Survey.[21] Most operations in the US and Europe have closed for environmental reasons. The arsenic is recovered mainly as a side product from the purification of copper. Arsenic is part of the smelter dust from copper, gold, and lead smelters.[22]

On roasting in air of arsenopyrite, arsenic sublimes as arsenic(III) oxide leaving iron oxides,[17] while roasting without air results in the production of metallic arsenic. Further purification from sulfur and other chalcogens is achieved by sublimation in vacuum or in a hydrogen atmosphere or by distillation from molten lead-arsenic mixture.[23]

History

Realgar
Alchemical symbol for arsenic

The word arsenic was borrowed from the Syriac word ܠܐ ܙܐܦܢܝܐ (al) zarniqa [24] and the Persian word زرنيخ Zarnikh, meaning "yellow orpiment", into Greek as arsenikon (Αρσενικόν). It is also related to the similar Greek word arsenikos (Αρσενικός), meaning "masculine" or "potent". The word was adopted in Latin arsenicum and Old French arsenic, from which the English word arsenic is derived.[24] Arsenic sulfides (orpiment, realgar) and oxides have been known and used since ancient times.[25] Zosimos (circa 300 AD) describes roasting sandarach (realgar) to obtain cloud of arsenic (arsenious oxide), which he then reduces to metallic arsenic.[26] 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.) Owing to its use by the ruling class to murder one another and its potency and discreetness, arsenic has been called the Poison of Kings and the King of Poisons.[27]

During the Bronze Age, arsenic was often included in bronze, which made the alloy harder (so-called "arsenical bronze"[28]). Albertus Magnus (Albert the Great, 1193–1280) is believed to have been the first to isolate the element in 1250 by heating soap together with arsenic trisulfide.[5] In 1649, Johann Schröder published two ways of preparing arsenic.

Cadet's fuming liquid (impure cacodyl), often claimed as the first synthetic organometallic compound, was synthesized in 1760 by Louis Claude Cadet de Gassicourt by the reaction of potassium acetate with arsenic trioxide.[29]

In the Victorian era, "arsenic" ("white arsenic" trioxide) was mixed with vinegar and chalk and eaten by women to improve the complexion of their faces, making their skin paler to show they did not work in the fields. Arsenic was also rubbed into the faces and arms of women to "improve their complexion". The accidental use of arsenic in the adulteration of foodstuffs led to the Bradford sweet poisoning in 1858, which resulted in approximately 20 deaths.[30]

Applications

Agricultural

Roxarsone is a controversial arsenic compound used as a nutritional supplement for chickens

The toxicity of arsenic to insects, bacteria and fungi led to its use as a wood preservative.[31] In the 1950s a process of treating wood with chromated copper arsenate (also known as CCA or Tanalith) was invented, and for decades this treatment was the most extensive industrial use of arsenic. An increased appreciation of the toxicity of arsenic resulted in a ban for the use of CCA in consumer products; the European Union and United States initiated this process in 2004.[32][33] CCA remains in heavy use in other countries however, e.g. Malaysian rubber plantations.[6]

Arsenic was also used in various agricultural insecticides, termination and poisons. For example, lead hydrogen arsenate was a common insecticide on fruit trees,[34] but contact with the compound sometimes resulted in brain damage among those working the sprayers. In the second half of the 20th century, monosodium methyl arsenate (MSMA) and disodium methyl arsenate (DSMA) – less toxic organic forms of arsenic – have replaced lead arsenate in agriculture.

Arsenic is still added to animal food, in particular in the U.S. as a method of disease prevention[35][36] and growth stimulation. One example is roxarsone, which is used as a broiler starter by about 70% of the broiler growers since 1995.[37] The Poison-Free Poultry Act of 2009 proposes to ban the use of roxarsone in industrial swine and poultry production.[38]

Medical use

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 500 years, but most commonly in the treatment of cancer. The US Food and Drug Administration in 2000 approved this compound for the treatment of patients with acute promyelocytic leukemia that is resistant to ATRA.[39] It was also used as Fowler's solution in psoriasis.[40] Recently new research has been done in locating tumours using arsenic-74 (a positron emitter). The advantages of using this isotope instead of the previously used iodine-124 is that the signal in the PET scan is clearer as the body tends to transport iodine to the thyroid gland producing a lot of noise.[41]

In subtoxic doses, soluble arsenic compounds act as stimulants, and were once popular in small doses as medicine by people in the mid-18th century.[9]

Alloys

The main use of metallic arsenic is for alloying with copper and especially lead. Lead components in car batteries are strengthened by the presence of a few percent of arsenic. Gallium arsenide is an important semiconductor material, used in integrated circuits. Circuits made from GaAs 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.[6]

Military

After World War I, the United States built up a stockpile of 20000tons of lewisite (ClCH=CHAsCl2), a chemical weapon that is a vesicant (blister agent) and lung irritant. The stockpile was neutralized with bleach and dumped into the Gulf of Mexico after the 1950s.[42] During the Vietnam War the United States used Agent Blue, a mixture of sodium cacodylate and its acid form, as one of the rainbow herbicides to deprive the Vietnamese of valuable crops.

Other uses

  • Copper acetoarsenite was used as a green pigment known under many names, including 'Paris Green' and 'Emerald Green'. It caused numerous arsenic poisonings. Scheele's Green, a copper arsenate, was used in the 19th century as a coloring agent in sweets.[43]
  • Also used in bronzing and pyrotechnics.
  • Up to 2% of arsenic is used in lead alloys for lead shots and bullets.[44]
  • Arsenic is added in small quantities to alpha-brass to make it dezincification resistant. This grade of brass is used to make plumbing fittings or other items that are in constant contact with water.[45]
  • Arsenic is also used for taxonomic sample preservation.
  • Until recently arsenic was used in optical glass. Modern glass manufacturers, under pressure from environmentalists, have removed it, along with lead.[46]

Biological role

Bacteria

Arsenobetaine

Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenate to arsenite. Under oxidative environmental conditions some bacteria use arsenite, which is oxidized to arsenate as fuel for their metabolism.[47] The enzymes involved are known as arsenate reductases (Arr).

In 2008, bacteria were discovered that employ a version of photosynthesis in the absence of oxygen with arsenites as electron donors, producing arsenates (just as ordinary photosynthesis uses water as electron donor, producing molecular oxygen). Researchers conjecture that, over the course of history, these photosynthesizing organisms produced the arsenates that allowed the arsenate-reducing bacteria to thrive. One strain PHS-1 has been isolated and is related to the γ-Proteobacterium Ectothiorhodospira shaposhnikovii. The mechanism is unknown, but an encoded Arr enzyme may function in reverse to its known homologues.[48]

Heredity

Arsenic has been linked to epigenetic changes that are heritable changes in gene expression that occur without changes in DNA sequence and include DNA methylation, histone modification, and RNA interference. Toxic levels of arsenic cause significant DNA hypermethylation of tumour suppressor genes p16 and p53, thus increasing risk of carcinogenesis. These epigenetic events have been observed in in vitro studies with human kidney cells and in vivo tests with rat liver cells and peripheral blood leukocytes in humans.[49] Inductive coupled plasma mass spectrometry (ICP-MS) is used to detect precise levels of intracellular arsenic and its other bases involved in epigenetic modification of DNA.[50] Studies investigating arsenic as an epigenetic factor will help in developing precise biomarkers of exposure and susceptibility.

The Chinese brake fern (Pteris vittata) hyperaccumulates arsenic present in the soil into its leaves and has a proposed use in phytoremediation.[51]

Arsenic reported substituting for phosphorus as a building block of life

A NASA-funded astrobiology research team claimed on December 2, 2010 that the microbe strain GFAJ-1 of the Gammaproteobacteria (designated Halomonadaceae) group has the ability to substitute arsenic for at least part of the phosphorus in the molecules of its cells, including DNA and ATP.[52][53] Bacteria from Mono Lake, a naturally arsenic-rich site in California, were cultured in an environment high in arsenic but low in phosphorus. This finding has faced strong criticism from the scientific community; many scientists have argued that there is no evidence that arsenic is actually incorporated into biomolecules.[54][55]

Biomethylation of arsenic

Inorganic arsenic and its compounds, upon entering the food chain, are progressively metabolised through a process of methylation.[56] For example, the mold Scopulariopsis brevicaulis produce significant amounts of trimethylarsine if inorganic arsenic is present.[57] The organic compound arsenobetaine is found in some marine foods such as fish and algae, and also in mushrooms in larger concentrations. The average person's intake is about 10–50 µg/day. Values about 1000 µg are not unusual following consumption of fish or mushrooms. But there is little danger in eating fish because this arsenic compound is nearly non-toxic.[58]

Environmental issues

Arsenic in drinking water

Widespread arsenic contamination of groundwater has led to a massive epidemic of arsenic poisoning in Bangladesh[59] and neighbouring countries. As of this writing, 42 major incidents around the world have been reported on groundwater arsenic contamination. 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. However, a study of cancer rates in Taiwan[60] suggested that significant increases in cancer mortality appear only at levels above 150 parts per billion. The arsenic in the groundwater is of natural origin, and is released from the sediment into the groundwater, owing to the anoxic conditions of the subsurface. This groundwater began to be used after local and western NGOs and the Bangladeshi government undertook a massive shallow tube well drinking-water program in the late twentieth century. This program was designed to prevent drinking of bacteria-contaminated surface waters, but failed to test for arsenic in the groundwater. Many other countries and districts in Southeast Asia, such as Vietnam and Cambodia have geological environments conducive to generation of high-arsenic groundwaters. Arsenicosis was reported in Nakhon Si Thammarat, Thailand in 1987, and the dissolved arsenic in the Chao Phraya River is suspected of containing high levels of naturally occurring arsenic, but has not been a public health problem owing to the use of bottled water.[61]

In the United States, arsenic is most commonly found in the ground waters of the southwest.[62] Parts of New England, Michigan, Wisconsin, Minnesota and the Dakotas are also known to have significant[clarification needed] concentrations of arsenic in ground water. Increased levels of skin cancer have been associated with arsenic exposure in Wisconsin, even at levels below the 10 part per billion drinking water standard.[63] According to a recent film funded by the US Superfund, millions of private wells have unknown arsenic levels, and in some areas of the US, over 20% of wells may contain levels that exceed established limits.[64]

Low-level exposure to arsenic at concentrations found commonly in US drinking water compromises the initial immune response to H1N1 or swine flu infection according to NIEHS-supported scientists. The study, conducted in laboratory mice, suggests that people exposed to arsenic in their drinking water may be at increased risk for more serious illness or death in response to infection from the virus.[65]

Epidemiological evidence from Chile shows a dose-dependent connection between chronic arsenic exposure and various forms of cancer, in particular when other risk factors, such as cigarette smoking, are present. These effects have been demonstrated to persist below 50 parts per billion.[66]

Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion.[67] According to Peter Ravenscroft of the Department of Geography at the University of Cambridge,[68] roughly 80 million people worldwide consume between 10 and 50 parts per billion arsenic in their drinking water. If they all consumed exactly 10 parts per billion arsenic in their drinking water, the previously cited multiple epidemiological study analysis would predict an additional 2,000 cases of bladder cancer alone. This represents a clear underestimate of the overall impact, since it does not include lung or skin cancer, and explicitly underestimates the exposure. Those exposed to levels of arsenic above the current WHO standard should weigh the costs and benefits of arsenic remediation.

Early (1973) evaluations of the removal of dissolved arsenic by drinking water treatment processes demonstrated that arsenic is very effectively removed by co-precipitation with either iron or aluminum oxides. The use of iron as a coagulant, in particular, was found to remove arsenic with efficiencies exceeding 90%.[69][70] 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). A team of European and Indian scientists and engineers have set up six arsenic treatment plants in West Bengal based on in-situ remediation method (SAR Technology). This technology does not use any chemicals and arsenic is left as an insoluble form (+5 state) in the subterranean zone by recharging aerated water into the aquifer and thus developing an oxidation zone to support arsenic oxidizing micro-organisms. This process does not produce any waste stream or sludge and is relatively cheap.[71]

Another effective and inexpensive method to remove arsenic from contaminated well water is to sink wells 500 feet or deeper to reach purer waters. A recent 2011 study funded by the U.S. National Institute of Environmental Health Sciences' Superfund Research Program shows that deep sediments can remove arsenic and take it out of circulation. Through this process called adsorption in which arsenic sticks to the surfaces of deep sediment articles, arsenic can be naturally removed from well water.[72]

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.[73]

Epidemiological studies have suggested a correlation between chronic consumption of drinking water contaminated with arsenic and the incidence of all leading causes of mortality. The literature provides reason to believe arsenic exposure is causative in the pathogenesis of diabetes.

Wood preservation in the US

As of 2002, US-based industries consumed 19,600 metric tons of arsenic. Ninety percent of this was used for treatment of wood with chromated copper arsenate (CCA). In 2007, 50% of the 5,280 metric tons of consumption was still used for this purpose.[22][74] In the United States, the use of arsenic in consumer products was discontinued for residential, and general consumer construction on December 31, 2003 and alternative chemicals are now used, such as Alkaline Copper Quaternary, borates, copper azole, cyproconazole, and propiconazole.[75]

Although discontinued, this application is also one of the most concern to the general public. The vast majority of older pressure-treated wood was treated with CCA. CCA lumber 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. Although the use of CCA lumber was banned in many areas after studies showed that arsenic could leach out of the wood into the surrounding soil (from playground equipment, for instance), a risk is also presented by the burning of older CCA timber. The direct or indirect ingestion of wood ash from burnt CCA lumber has caused fatalities in animals and serious poisonings in humans; the lethal human dose is approximately 20 grams of ash. Scrap CCA lumber from construction and demolition sites may be inadvertently used in commercial and domestic fires. Protocols for safe disposal of CCA lumber do not exist evenly throughout the world; there is also concern in some quarters about the widespread landfill disposal of such timber.

Mapping of industrial releases in the US

One tool that maps releases of arsenic to particular locations in the United States[76] and also provides additional information about such releases is TOXMAP. TOXMAP is a Geographic Information System (GIS) from the Division of Specialized Information Services of the United States National Library of Medicine (NLM) that uses maps of the United States to help users visually explore data from the United States Environmental Protection Agency's (EPA) Toxics Release Inventory and Superfund Basic Research Programs. TOXMAP is a resource funded by the US Federal Government. TOXMAP's chemical and environmental health information is taken from NLM's Toxicology Data Network (TOXNET)[77] and PubMed, and from other authoritative sources.

Toxicity and precautions

Skull and crossbones.svg

Arsenic and many of its compounds are especially potent poisons. Many water supplies close to mines are contaminated by these poisons. Arsenic disrupts ATP production through several mechanisms. At the level of the citric acid cycle, arsenic inhibits lipoic acid, which is a cofactor for pyruvate 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, it is speculated, has potential to form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure, it is presumed from necrotic cell death, not apoptosis. A post mortem reveals brick-red-coloured mucosa, owing to severe haemorrhage. Although arsenic causes toxicity, it can also play a protective role.[78]

Elemental arsenic and arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union under directive 67/548/EEC. The International Agency for Research on Cancer (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 owing to its manifestation in drinking water, “the most common species being arsenate [HAsO42-; As(V)] and arsenite [H3AsO3 ; As(III)]”. The ability of arsenic to undergo redox conversion between As(III) and As(V) makes its availability in the environment more abundant. 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.[79]

Treatment of chronic arsenic poisoning is easily accomplished. British anti-lewisite (dimercaprol) is prescribed in dosages of 5 mg/kg up to 300 mg each 4 hours for the first day. Then administer the same dosage each 6 hours for the second day. Then prescribe this dosage each 8 hours for eight additional days.[80] However the Agency for Toxic Substances and Disease Registry (ATSDR) states that the long-term effects of arsenic exposure cannot be predicted.[20] Blood, urine, hair, and nails may be tested for arsenic; however, these tests cannot foresee possible health outcomes from the exposure.[20] Excretion occurs in the urine and long-term exposure to arsenic has been linked to bladder and kidney cancer in addition to cancer of the liver, prostate, skin, lungs and nasal cavity.[81]

Occupational exposure and arsenic poisoning may occur in persons working in industries involving the use of inorganic arsenic and its compounds, such as wood preservation, glass production, nonferrous metal alloys, and electronic semiconductor manufacturing. Inorganic arsenic is also found in coke oven emissions associated with the smelter industry.[82]

Biochemical basis of arsenic toxicity

The high affinity of arsenic(III) oxides for thiols is usually assigned as the cause of the high toxicity. Thiols, in the form of cysteine residues, are situated at the active sites of many important enzymes.[6]

See also

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External links


Translations:

Arsenic

Top

Dansk (Danish)
n. - arsenik, arsentrioxid
adj. - arsenik-

Nederlands (Dutch)
arsenicum, rattenkruit

Français (French)
n. - arsenic
adj. - arsénique

Deutsch (German)
n. - Arsenik, Arsen
adj. - arsenhaltig

Ελληνική (Greek)
n. - (χημ.) αρσενικό

Italiano (Italian)
arsenico

Português (Portuguese)
n. - arsênico (m) (Quím.)

Русский (Russian)
мышьяк

Español (Spanish)
n. - arsénico
adj. - relativo al arsénico

Svenska (Swedish)
n. - arsenik

中文(简体)(Chinese (Simplified))
砷, 砒霜, 砷的, 含砷的

中文(繁體)(Chinese (Traditional))
n. - 砷, 砒霜
adj. - 砷的, 含砷的

한국어 (Korean)
n. - 비소(양성 금속 원소)
adj. - 비소의

日本語 (Japanese)
n. - 砒素
adj. - 砒素の

العربيه (Arabic)
‏(الاسم) زرنيخ, زرنيخي‏

עברית (Hebrew)
n. - ‮ארסן או זרניך (יסוד מתכתי-למחצה, SA, מס' אטומי 33)‬
adj. - ‮של ארסן (זרניך), מכיל זרניך בערכיות הכי גבוהה שלו של חמש (כימיה)‬


 
 
Related topics:
Arsenic Trioxide
arsenic oxide (inorganic chemistry)
arseniferous

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