|Boiling Point: 876°K, 603°C, 1117°F
Melting Point: 1081°K, 808°C, 1486°F
Electrons Energy Level: 2, 8, 18, 5
Isotopes: 28 + 1 Stable + 2 meta states
Heat of Vaporization: 34.76 kJ/mol
Heat of Fusion: 369.9 kJ/mol
Density: 5.72 g/cm3 @ 300°K
Specific Heat: 0.33 J/g°K
Atomic Radius: 1.33Å
Ionic Radius: 0.58Å
Electronegativity: 2.18 (Pauling); 2.2 (Allrod Rochow)
|The word arsenic
is borrowed from a Persian word 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 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 (mostly as an impurity), which made the alloy harder.
Albert Magnus (Albert the Great, 1193-1280) is believed to have been the first to isolate the element in 1250. In 1649 Johann Schroder published two ways of preparing arsenic.
1s2 2s2p6 3s2p6d10 4s2p3
Alchemical Symbol for Arsenic, As
In the Victorian Era, "Arsenic" (colorless, crystalline, soluble 'White Arsenic') 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'.
Albert Magnus (Albert the Great, 1193-1280)
Arsenic has been known for a very long time and the person who may have first isolated it is not known but credit generally is given to Albertus Magnus in about the year 1250. The element, which is classified as a metalloid, is named from the Latin arsenicum and Greek arsenikon which are both names for a pigment, yellow orpiment.
Tiny amounts of arsenic are used today in the semiconductor industry to create LEDs. Some compounds are used in the manufacture of everything from wallpaper to ceramics. Of course, arsenic is most often thought of in terms of its toxicity and it is used as a weed killer and rat poison. Intentional arsenic poisoning is now pretty much the stuff of old murder mysteries. Originally arsenious oxide was administered in small doses over a period of time, precipitating death accompanied by symptoms reminiscent of pneumonia. Today the arsenic remaining in the body can be detected during autopsy so this is not (happily) a very effective way to dispatch people undetected.
Commercially, arsenic is produced by heating the pigment orpiment (As2S3) in air and then reacting the oxide product with carbon.
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 strictu and the much rarer Arsenolamprite and Pararsenolamprite), but it is 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 As43- ions in the Arsenide Skutterudite. In the +3 oxidation state, the stereochemistry of Arsenic is affected by possession of a lone pair of electrons.
Arsenic is very similar chemically to its predecessor in the 'pnictide' column of the Periodic Table, Phosphorus. Similar to Phosphorus, it forms colorless crystalline oxides As2O3 and As2O5 which are hygroscopic and readily soluble in water to form acidic solutions. Arsenic (V) Acid, like Phosphoric Acid, is a strong acid. Like phosphorus, Arsenic forms an unstable, gaseous hydride: Arsine (AsH3). 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. Arsenic compounds were also once used to dope racehorses.
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 can also sublime upon heating, converting directly to a gaseous form without an intervening liquid state. Elemental Arsenic is found in many solid forms: the yellow form is soft, waxy and unstable, and is made of tetrahedral As4 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.
Arsenopyrite also unofficially called Mispickel (FeAsS) is the most common Arsenic-bearing mineral. On roasting in air, the Arsenic sublimes as Arsenic (III) Oxide leaving Iron Oxides.
The most important compounds of Arsenic are Arsenic (III) Oxide, As2O3, ("White Arsenic"), the yellow Sulfide Orpiment (As2S3) and Red Realgar (As4S4), Paris Green, Calcium Arsenatem, and Lead Hydrogen Arsenate. The latter three have been used as agricultural insecticides and poisons. Orpiment and realgar were formerly used as painting pigments, though they have fallen out of use due to their toxicity and reactivity. Although arsenic is sometimes found native in nature, its main economic source is the mineral Arsenopyrite mentioned above; it is also found in arsenides of metals such as Silver, Cobalt (Cobaltite: CoAsS and Skutterudite: CoAs3) and Nickel as Sulfides, and when oxidised as Arsenate minerals such as Mimetite, Pb5(AsO4)3Cl and Erythrite, Co3(AsO4)2. 8H2O, and more rarely Arsenites ('Arsenite' = Arsenate (III), AsO33- as opposed to Arsenate (V), AsO43-). 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.
Nickernuts are said to contain arsenic.
For many years, the greatest use of arsenic was for poisons, and in particular, insecticides. Lead arsenate, Pb3(AsO4)2 was a common insecticide; later, the calcium salt replaced it. Paris Green, copper acetoarsenite, Cu(C2H3O2)2 · 3CuOAsO3, and copper arsenite (Scheele's Green) CuHAsO3 are also useful. These are all insoluble compounds. The last is used as a wood preservative, giving a characteristic green tint. There was a so-called Fliegenstein, a condensed white arsenic which killed flies that landed on it, and an arsenic soap for preventing the growth of larvae in untreated animal hides.
The boll weevil, Anthonomus grandis, moving northward, attacked Texas cotton crops as early as 1894, and by 1909 had spread to all the Gulf states. One fairly effective method of control was dusting the cotton, preferably at night, with calcium arsenate. Arsenic is not an essential plant nutrient. 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 first syphilis plague in Europe occurred in 1495, not long after Columbus and his sailors had returned from America, where they had traded smallpox for this great pox. When it broke out while the French were sacking Naples, the Italians called it the French Disease, while the French called it the Naples Disease. It was named after a shepherd Syphilis in a play, whom it pleased God to infect, but was soon known in polite company by euphemisms. It is caused by the spirochete treponema pallidum, and attacks over many years in three stages, each worse than the last, though sometimes missing the tertiary stage. There was no effective treatment, though many were tried.
Paul Ehrlich (1854-1915), the bacteriologist, received the Nobel Prize in Medicine for 1908. In 1909 he announced the arsenic compound that was known as Salvarsan, or as "606" after its number in a list of preparations. This was phenol to which an arsenic atom had been added opposite to the OH, and an amine group, NH2, next to the OH, whose systematic name was arsphenamine. It was usually accompanied by HCl to make it more soluble. This molecule is small enough to get past the intestines and into the blood stream, where it played havoc with treponema. Because it was also quite toxic, being administered in small doses over a considerable period, making the therapy inconvenient. It was, however, quite effective and relieved great amounts of suffering. Salvarsan was the first synthetic chemotherapeutic agent. Around 1943 it was finally superseded by penicillin. Penicillin was just as effective, and could be administered in large doses, shortening the time of treatment.
Arsenic Trioxide (by Thomas Fowler) 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.
White arsenic can be added to glass melts in a concentration of about 0.5% to remove the objectionable green tint produced by iron impurities. Metallic arsenic also has a few uses as an alloying element. Arsenic hardens lead, and is used in alloys for making lead shot and in lead-acid battery plates. It is also said to make lead shot more spherical, when made the old way in a shot tower. Arsenic has long been known to decolorize copper, to the delight of counterfeiters. Smaller amounts of arsenic raise the annealing temperature of copper, so that copper intended to be toughened by work hardening does not become soft when exposed to heat. This was important in the fabrication of copper locomotive fireboxes, long used in Britain from a distrust of steel in this application. Arsenical copper is strong and very tough. Arsenic does destroy the conductivity of copper, so if a tough and still highly conductive copper is required, cadmium is used in place of arsenic.
Arsenic sulphide, As2S2 is used in pyrotechnic mixtures to produce a brilliant white light. It can be produced in a transparent form called ruby sulphur. Paris green makes a blue light, but that is due to the copper, not the arsenic.
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. Protocols for safe disposal of CCA timber are still in place only patchily; there is concern in some quarters about the widespread landfill disposal of such timber.
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 Minerals & Compounds of Arsenic|
|Arsenopyrite, Mispickel, FeAsS||Red Realgar, As4S4|
|Arsenic Acid, H3AsO4||Arsenite, Arsenous Acid, H3AsO3|
|Niccolite, Kupfernickel, NiAs||Arsine, Arsenic Trihydride, AsH3|
|Cobaltite, CoAsS||Ruby Silver, Proustite, Ag3AsS3|
|Cadmium Arsenide, Cd3As2||Gallium Arsenide, GaAs|
|Butter of Arsenic, Arsenious Chloride, AsCl3|
|Black Enargite, Cuprous Thioarsenate, Cu3AsS4|
|Smaltite, Skutterudite, (Co,Ni,Fe)As3|
|Scorodite, Ferric Arsenate, FeAsO4·2H2O|
|Mimetite, Pb5(AsO4)3Cl or Mimetite, Pb5Cl(AsO3)3|
|Erythrite, Cobaltous Arsenate, Co3(AsO4)2·8H2O|
|White Arsenic, Arsenious Oxide, Arsenic (III) Oxide, As2O3|
|Lead Hydrogen Arsenate, PbHAsO4|
|Yellow Orpiment, Arsenic Trisulfide, As2S3|
Additional Representations of Alchemical Symbols for Arsenic
Alchemical Substances of Arsenic
Aqua Tofani: Arsenious Oxide. Extremely poisonous. Used by Paracelsus.
King's Yellow: A mixture of Orpiment (As2S3) withWhite Arsenic (As2O3).
Orpiment: auripigmentum. Yellow ore of arsenic. Arsenic Trisulfide, As2S3.
Red Realgar: Red ore of Arsenic (As4S4).
White Arsenic: Arsenious Oxide (As2O3). Made from arsenical soot from the roasting ovens, purified by sublimation.
The ancient world knew only two arsenic compounds. These were the sulphides realgar, As2S2, and orpiment, As2S3. Realgar was then called sandarach, Greek sandarakh, sandaRAke. It was a soft, light, translucent substance of a deep red-orange colour that was ground and used as a pigment. The other was a soft, light, translucent substance of a fine yellow color, also used as a pigment. In Greek, it was also called sandarach or arsenikon, "arsenikon." The Greek word arsen, "arsen," means "male," or by extension "strong." In Latin, it was called auripigmentum, from which the French or-piment comes. Gaius Caesar (Caligula) engaged in alchemy in order apply the yellow color onto heavy metal to produce gold, but unsuccessfully. This was long before mystical alchemy was introduced from the Arabs. The toxic nature of arsenic was unknown to the ancients.
Realgar and orpiment are rare and rather unusual arsenic minerals. They are secondary alteration products found in association with native metallic arsenic. In them, arsenic acts like a metal, but the compounds are mainly covalent. The formula of realgar is As4S4, though the simple formula AsS gives the relative amounts of As and S. Phosphorus, likewise, forms a number of sulphides. They became known due to their bright colors; the obscure, grey metallic arsenic would have been overlooked, perhaps considered an impure lead. Native arsenic is rather rare, but is found in the Harz and Erzgebirge mountains.
Additionally, the Egyptians used arsenical copper, probably for mirrors, which was called pseudargyros. Zosimos of Panopolis, an Alexandrian alchemist of the 2nd century BC, is said to have made metallic arsenic from realgar. The property of mixtures with slaked lime causing hair to fall out was also noted, and has been practiced in the Orient rather than using razors for shaving.
When arsenic compounds are heated under oxidizing conditions (as in air) the oxide As2O3, arsenious oxide, is produced. Arsenious oxide is a volatile substance appearing as a white vapor with the odor of garlic and condensing as a white powder, or forming small octahedral crystals. The name arsenic applies to this compound, not to the metal. In German, the metal is called Arsen, the oxide Arsenik, but in English "arsenic" is used for both. This is the most common in which arsenic appears. It is the most common method of arsenic production and is usually called "white arsenic." White arsenic was described by the Arabic alchemist Geber (Jabir ibn-Hayyan, 760-815) in the 8th century, believing it to be a form of sulphur driven out when metallic arsenic was heated. Later, the Arab iatrochemist Avicenna (Ibn-Sina, 979-1037) recognized the toxic properties of white arsenic. Kunkel later showed that white arsenic did not contain sulphur. When arsenic is added to copper, the copper is decolorized and becomes white. In ancient times, this was called pseudargyros, and was taken to be silver by later alchemists. This alloy was popular with counterfeiters. Albertus Magnus, the Renaissance alchemist, showed that by further heating arsenic could be driven out and copper restored. He believed, correctly, that white copper was a mixture of copper and arsenic, and arsenic was a metal common to several important compounds. There are claims that he heated white arsenic with soot and obtained metallic arsenic around 1250. Others suggest the pure metal was first isolated by Johann Schroeder in 1694. Arsenic metal is easily made by heating the oxide with carbon (coke or charcoal) in a retort, and condensing the vapors. It is approximately ten times more expensive than white arsenic.
In the trioxide As2O3, As has a valence of +3, while in the pentoxide As2O5 the valence is +5. When oxides such as these are dissolved in water, they attract H+ and OH- ions. Adding water to Na2O, the oxide of sodium metal, produces:
Na2O + 2H2O 2NaOH 2Na+ + 2OH-
The solution becomes basic due to the presence of OH- ions. In the case of As2O5, we find that
As2O5 + 3H2O 2H3AsO4 6H+ + AsO43-
The molecule formed is Arsenic Acid, which dissociates to give H+ ions, and an acidic solution. Recall the similar Phosphoric Acid, H3PO4. Here, arsenic behaves as a nonmetal, like phosphorus, and can form salts with metals called arsenates. Similarly, As2O3 hydrolyzes to H3AsO3, arsenious acid, whose salts are the arsenites. The arsenic acids cannot be isolated in stable dry form (arsenic acid seems to form a hydrated crystal); on evaporation of the solutions, the oxides are reformed. The arsenate ion, AsO43-, is very similar to the phosphate ion, PO43-.
A few arsenate minerals are found, mostly in altered surface deposits and hot springs. Mimetite, Pb5Cl(AsO3)3, is a soft, heavy mineral found in supergene-enriched zones. Scorodite, FeAsO4·2H2O (ferric arsenate) is soft and light, often in hot springs. Erythrite, Co3(AsO4)2·8H2O (cobaltous arsenate) is a bright red mineral called cobalt bloom. There is a series of such minerals in which Ni replaces Co, ending with annabergite or nickel bloom. The water of crystallization should be noted. These are all rare. A hot spring with abundant arsenic would, of course, be poisonous, and such springs have been known.
In most arsenic minerals, the arsenic replaces sulphur in more familiar minerals. The hard and heavy mineral NiAs, niccolite or kupfernickel, is an example. Silver white cobaltite, CoAsS, is hard and heavy and occurs in pyritohedrons, but is distinguished from pyrite by its color. FeAsS, arsenopyrite, is perhaps the commonest arsenic mineral, and is also called mispickel. It is found in striated prisms. The mineral once known as smaltite, now as skutterudite, is (Co,Ni,Fe)As3; it contains varying quantites of the iron group metals. Proustite, Ag3AsS3, or "ruby silver," is soft and of medium weight. Cu3AsS4 is black enargite, the source of arsenic in the mines of Butte, Montana. Note that in the last two compounds, S has replaced O in the arsenate and arsenite. Enargite is cuprous thioarsenate.
Arsenic forms a number of halogen compounds with valence +3 and +5. For example, AsCl3, arsenious chloride or "butter of arsenic," an oily liquid that decomposes when added to water; and AsF5, arsenic fluoride, a colorless gas. The "ic" denoting the higher valence state conflicts somewhat with the attributive "ic" in the name. There is also As2S5, arsenic (arsenicic) sulphide. These compounds are not found in nature.
The gas AsH3 or arsine is analogous to NH3, ammmonia, and PH3, phosphine. This compound was discovered by Scheele in 1775. It is highly toxic, which was brought out forcefully soon after its discovery by the death of the prominent chemist Gehlen of Munich during experiments with it. In arsine, the conventional valence of arsenic is -3. If arsine is produced, strong heating will decompose it into arsenic and hydrogen. The arsenic vapor will condense as a shiny film of metallic arsenic, forming a "arsenic mirror,". This behaviour is used in the Marsh test for arsenic. The material to be tested is placed in a flask with granulated zinc and hydrochloric acid added. Any arsenic present is converted to arsine as hydrogen is evolved and passes off in the evolved gas. The tube carrying the hydrogen is strongly heated at a certain point. If arsenic is present, an arsenic mirror is formed on the cooler part of the tube. The hydrogen can also be lighted at the end of the tube. A cold surface held above the flame will also condense any arsenic that is formed. Arsine generally oxidizes readily in the atmosphere.
Arsenic had little application in warfare. Some derivatives of arsine, such as methyldichloroarsine, have been used but found to be ineffective due to its short life in air. The frightening French Vincennite, one of the first poison gases used in World War I, contained not only hydrogen cyanide, but also arsenic trichloride. It was determined to be ineffective.
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 1.0942 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.
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, H2O2, production is also increased, which might form reactive oxygen species and oxidative stress. These metabolic interferences lead to death from multi-system organ failure probably from necrotic cell death, not apoptosis. A post mortom reveals brick red colored mucosa, due to severe hemorrhage. Although Arsenic causes toxicity, it can also play a protective role.
|Elemental Arsenic and Arsenic compounds are classified as "toxic" and "dangerous for the environment" in the European Union.|
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 [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 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 unfortunately failed to test for Arsenic in the groundwater. 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 highsurface 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.
Atomic Radius (Å): 1.33Å
Electrochemical Equivalents: 0.93177 g/amp-hr
Atomic Mass Average: 74.92159