Everything about Arsenic totally explained
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Critical temperature || 1673
K
Arsenic is a
chemical element that has the symbol
As and
atomic number 33. Arsenic was first written about by
Albertus Magnus (
Germany) in 1250. Its Atomic Mass is 74.92. Its Ionic Charge is (3-) Its position in the periodic table is shown at right. This is a notoriously poisonous
metalloid that has many
allotropic forms: yellow (molecular non-metallic) and several black and gray forms (metalloids) are a few that are seen. Three metalloidal forms of arsenic with different crystal structures are found free in nature (the minerals arsenic
sensu stricto and the much rarer arsenolamprite and pararsenolamprite), but it's more commonly found as arsenide and arsenate compounds. Several hundred such mineral species are known. Arsenic and its compounds are used as
pesticides,
herbicides,
insecticides and various
alloys.
The most common
oxidation states for arsenic are -3 (arsenides: usually alloy-like intermetallic compounds), +3 (arsenates(III) or arsenites, and most organoarsenic compounds), and +5 (arsenates(V): the most stable inorganic arsenic oxycompounds). Arsenic also bonds readily to itself, forming, for instance, As-As pairs in the red sulfide
realgar and square As
43- ions in the arsenide skutterudite. In the +3 oxidation state, the stereochemistry of arsenic is affected by possession of a
lone pair of
electrons.
Notable characteristics
Arsenic is very similar chemically to its predecessor,
phosphorus. Like phosphorus, it forms colourless, odourless, crystalline oxides
As2O3 and
As2O5 which are
hygroscopic and readily soluble in water to form acidic solutions.
Arsenic (V) acid, like phosphorous acid, is a weak acid. Like phosphorus, arsenic forms an unstable, gaseous hydride:
arsine (AsH
3). The similarity is so great that arsenic will partly substitute for phosphorus in biochemical reactions and is thus
poisonous. However, in subtoxic doses, soluble arsenic compounds act as
stimulants, and were once popular in small doses as medicinals by people in the mid 18th century.
When heated in air it
oxidizes to
arsenic trioxide; the fumes from this reaction have an odor resembling
garlic. This odor 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. The liquid state appears at 20 atmospheres and above, which explains why the melting point is higher than the boiling point . Elemental arsenic is found in many solid forms: the yellow form is soft, waxy and unstable, and is made of tetrahedral As
4 molecules similar to the molecules of white phosphorus. The gray, black or 'metallic' forms have somewhat layered crystal structures with bonds extending throughout the crystal. They are brittle
semiconductors with a metallic luster. The
density of the yellow form is 1.97 g/cm³; rhombohedral 'gray arsenic' is much denser with a density of 5.73 g/cm³; the other metalloidal forms are similarly dense.
Applications
Lead hydrogen arsenate was used well into the 20th century as an
insecticide on
fruit trees. Its use sometimes resulted in
brain damage to those working the sprayers. In the last half century,
monosodium methyl arsenate (MSMA), a less toxic organic form of arsenic, has replaced lead arsenate's role in agriculture.
Scheele's Green, a copper arsenate, was used in the 19th century as a
coloring agent in
sweets.
The application of most concern to the general public is probably that of
wood treated with
chromated copper arsenate, also known as CCA or
Tanalith. 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. It was commonly used in situations where
rot or
insect infestation was a possibility. 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 don't exist evenly throughout the world; there's also 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 US
Food and Drug Administration in 2000 approved this compound for the treatment of patients with
acute promyelocytic leukemia that's 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;
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 iodine tends to transport iodine to the thyroid gland producing a lot of noise.(see the journal of clinical cancer - 2008,14,1377 (DOI:10.1158/1078-0432.CCR-07-1516)
Occupational Exposures
Exposure to higher-than-average levels of arsenic can occur in some occupations placing workers at risk. Industries that use inorganic arsenic and its compounds include 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.
History
The word
arsenic is borrowed from the
Persian word زرنيخ
Zarnikh meaning "yellow
orpiment".
Zarnikh was borrowed by
Greek as
arsenikon, which means masculine or potent. 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 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
bronze, which made the alloy harder (so-called "
arsenical bronze").
Albertus Magnus (Albert the Great, 1193-1280) is believed to have been the first to isolate the
element in 1250.
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 [HAsO
42-; As(V)] and arsenite [H
3AsO
3; As(III)]”. The ability of arsenic to undergo redox conversion 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.
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 failed to test for arsenic in the groundwater.(2) Many other countries and districts in
South East Asia, such as
Vietnam,
Cambodia, and
Tibet,
China, are thought to have geological environments similarly 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 hasn't been a public health problem due to the use of bottled water.
The northern United States, including parts of
Michigan,
Wisconsin,
Minnesota and the Dakotas are known to have significant concentrations of arsenic in ground water. Increased levels of skin cancer has been associated with arsenic exposure in Wisconsin, even at levels below the 10 part per billion drinking water standard.
Epidemiological evidence from
Chile shows a dose dependent connection between chronic arsenic exposure and various forms of cancer, particularly when other risk factors, such as cigarette smoking, are present. These effects have been demonstrated to persist below 50 parts per billion.
A study of cancer rates in Taiwan suggested that significant increases in cancer mortality appear only at levels above 150 parts per billion.
Analyzing multiple epidemiological studies on inorganic arsenic exposure suggests a small but measurable risk increase for bladder cancer at 10 parts per billion. According to Peter Ravenscroft, of the Department of Geography at the University of Cambridge 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 doesn't 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 remmediation.
Arsenic can be removed from drinking water through
coprecipitation 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 (Fe
3O
4)
nanocrystals. Using the high specific surface area of Fe
3O
4 nanocrystals the mass of waste associated with arsenic removal from water has been dramatically reduced.
Compounds
Arsenic acid (H3AsO4)
Arsenous acid (H3AsO3)
Arsenic trioxide (As2O3)
Arsine (Arsenic Trihydride AsH3)
Cadmium arsenide (Cd3As2)
Gallium arsenide (GaAs)
Lead hydrogen arsenate (PbHAsO4)
Arsenic also occurs in the II oxidation state, but only in the As24+ cation, As(II) is never found otherwise.
See also .
Isotopes
Arsenic has been proposed as a "salting" material for nuclear weapons (cobalt is another, better-known salting material). A jacket of 75As, irradiated by the intense high-energy neutron flux from an exploding thermonuclear weapon, would transmute into the radioactive isotope 76As 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 isn't known to have ever been built, tested, or used.
Further Information
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