arsenic


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properties
General
Name , symbol , atomic number Arsenic, As, 33
Element category Semi-metals
Group , period , block 15 , 4 , p
Appearance metallic gray, yellow or black
CAS number 7440-38-2
EC number 231-148-6
ECHA InfoCard 100.028.316
Mass fraction of the earth's envelope 5.5 ppm
Atomic
Atomic mass 74.921595 (6) and
Atomic radius (calculated) 115 (114) pm
Covalent radius 119 pm
Van der Waals radius 185 pm
Electron configuration [ Ar ] 3 d 10 4 s 2 4 p 3
1. Ionization energy 9.78855 (25) eV944.45 kJ / mol
2. Ionization energy 18th.5892 (10) eV1 793.58 kJ / mol
3. Ionization energy 28.349 (12) eV2 735.3 kJ / mol
4. Ionization energy 50.15 (6) eV4 839 kJ / mol
5. Ionization energy 62.77 (6) eV6 056 kJ / mol
Physically
Physical state firmly
Modifications 3
Crystal structure trigonal (gray)
density 5.73 g / cm 3 (gray)
1.97 g / cm 3 (yellow)
4.7-5.1 g / cm 3 (black)
Mohs hardness gray ace: 3.5
magnetism diamagnetic (gray: Χ m = −2.2 10 −5
yellow: = −1.8 10 −6 black: = −1.9 10 −5 )
Sublimation point 886 K (613 ° C)
Molar volume 12.95 10 −6 m 3 mol −1
Heat of evaporation Sublimation: 32.4 kJ / mol
Heat of fusion 27.7 kJ mol −1
Specific heat capacity 328.5 J kg −1 K −1
Electric conductivity 3.03 · 10 6 A · V −1 · m −1
Thermal conductivity 50 W m −1 K −1
Chemically
Oxidation states −3, 3 , 5
Normal potential 0.240 V (½ As 2 O 3 + 3 H + + 3 e -
→ As + 1½ H 2 O)
Electronegativity 2.18 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
72 As {syn.} 26 h ε 4,356 72 Ge
73 As {syn.} 80.3 d ε 0.341 73 Ge
74 As {syn.} 17.77 d ε 2.562 74 Ge
β - 1.353 74 Se
75 As 100  % Stable
76 As {syn.} 1.0778 d β - 2,962 76 Se
77 As {syn.} 39 h β - 0.683 77 Se
For other isotopes see list of isotopes
safety instructions
GHS hazard labeling from  Regulation (EC) No. 1272/2008 (CLP) , expanded if necessary
06 - Toxic or very toxic 09 - Dangerous for the environment

danger

H and P phrases H: 301 + 331-410
P: 261-273-301 + 310-311-501
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Arsenic [ aʁˈzeːn ] is a chemical element with the element symbol As and the atomic number 33. In the periodic table of the elements it is in the 4th period and the 5th main group , or 15th  IUPAC group or nitrogen group . Arsenic seldom occurs naturally , mostly in the form of sulfides . It belongs to the semimetallic group because it shows metallic or non-metallic properties depending on the modification .

Colloquially, the arsenic known as murder poison is usually simply called “arsenic”. Arsenic compounds have been known since ancient times . As a mutagenic clastogen , arsenic compounds can act as a poison, which can cause chromosome aberrations and thus have a carcinogenic effect.

Arsenic is used for doping of semiconductors and as part of III-V semiconductors such as gallium arsenide used. The organic arsenic compound arsphenamine ( salvarsan ) was considered a breakthrough in the treatment of syphilis at the beginning of the 20th century, despite severe and extremely serious side effects . Today arsenic trioxide is used as the last treatment option in the therapy of promyelocytic leukemia .

history

Albertus Magnus is considered to be the first manufacturer of pure arsenic

The name arsenic goes back to ancient Greek ἀρσενικόν arsenikón , the ancient name of the arsenic mineral auripigment . It can already be found in Dioscurides in the 1st century. The Greek name for its part seems to have its origin in Old Persian (al-) zarnik (gold-colored, auripigment, “arsenic”) and probably got into Greek through Semitic mediation. In terms of folk etymology , the name was erroneously derived from the identical (old and new) Greek word αρσενικός arsenikós , which can be translated as masculine / strong. The name arsenic has only been in use since the 19th century. The element symbol was proposed by Jöns Jakob Berzelius in 1814 .

The first contact of humans with arsenic can be traced back to the 3rd millennium BC. In the hair of the mummy of the alpine inhabitant popularly known as Ötzi , which was preserved in the glacier ice , large amounts of arsenic could be detected, which is archaeologically interpreted as an indication that the man concerned worked in copper processing - copper ores are often contaminated with arsenic. In classical antiquity, arsenic was known in the form of the arsenic sulfides auripigment (As 2 S 3 ) and realgar (As 4 S 4 ), which were described by the Greek Theophrastus , the successor to Aristotle . The Greek philosopher Democritus also had in the 5th century BC Proven knowledge of arsenic compounds. The Leiden Papyrus X from the 3rd century AD suggests that they were used to color silver gold-like and copper white. The Roman emperor Caligula supposedly commissioned a project for the production of gold from the (golden yellow) auripigment as early as the 1st century AD. The alchemists , who knew arsenic compounds verifiably as mentioned in the ancient standard work Physica et Mystica , suspected a relationship with sulfur and mercury . Arsenic (III) sulfide was used as a painter's paint and depilatory, as well as for the external and internal treatment of lung diseases .

Alchemists' symbol for arsenic

In the Middle Ages , arsenic (arsenic (III) oxide) was found in smelter smoke (dust-laden exhaust gas from metallurgical furnaces). Albertus Magnus first described the production of arsenic by reducing arsenic with coal around 1250 . He is therefore considered the discoverer of the element, even if there are indications that the elementary metal was produced earlier. Paracelsus introduced it to medicine in the 16th century . Around the same time, arsenic preparations were described in the Chinese encyclopedia Pen-ts'ao Kang-mu by the pharmacist Li Shi-zhen. This author particularly highlights its use as a pesticide in rice fields .

In the 17th century, the yellow aura pigment became popular with Dutch painters as the royal yellow . Since the pigment turns into arsenic (III) oxide over long periods of time and crumbles from the canvas, difficulties arise during restoration . From 1740 onwards, arsenic preparations were used successfully in Europe as a dressing agent in crop protection. However, this use was banned again in 1808 because of its high toxicity . The use of arsenic additives for lead casting is based on the greater hardness of such lead alloys, typical applications are shotguns . Although its toxicity and use as a murderous poison was known, arsenic was one of the most important asthma drugs in the early 19th century . It appears to be based on reports that the Chinese were said to smoke arsenic in combination with tobacco to get lungs that were as strong as bellows. Also until the 19th century, arsenic compounds were used externally and internally in malignant tumors, skin diseases and (for example in the form of Fowler's drops ) in fever.

Arsenic was used in the form of copper arsenates in colorants such as Parisian green to print wallpaper. When the humidity was high, these pigments were transformed into toxic, volatile arsenic compounds by mold attack, which often led to chronic arsenic poisoning.

But arsenic was also used in wars: In the First World War , arsenic compounds were used in chemical warfare agents ( blue cross ) or lewisite . They attacked the victims' skin and lungs, causing excruciating pain and severe physical damage.

Occurrence

Silvery shining arsenic crystals (magnification, total size of the step : 4.2 cm × 3.2 cm × 3 cm)

Arsenic occurs virtually everywhere in soil in low concentrations of up to 10 ppm. It is about as common in the earth's crust as uranium or germanium . In the continental crust arsenic occurs with an average of 1.7 ppm, whereby it is enriched by its lithophilic character (= silicate loving) in the upper crust (2 ppm compared to 1.3 ppm in the lower crust); arsenic is thus in 53rd place in the table of the most common elements .

Arsenic ( broken cobalt ) occurs naturally, i.e. in elemental form, and is therefore recognized by the International Mineralogical Association (IMA) as an independent mineral . According to the Strunz system of minerals (9th edition) , arsenic is classified under system no. 1.CA.05 (elements - semimetals (metalloids) and non-metals - arsenic group elements) ( 8th edition : I / B.01-10 ). The systematics of minerals according to Dana , which is also common in English-speaking countries , leads the element mineral under the system no. 01.03.01.01.

Around the world (as of 2011) around 330 sites for native arsenic are currently known. In Germany it was found at several sites in the Black Forest (Baden-Württemberg), in the Bavarian Spessart and Upper Palatinate Forest , in the Hessian Odenwald , in the silver deposits of the Western Ore Mountains (Saxony), in the Hunsrück (Rhineland-Palatinate) and in the Thuringian Forest . In Austria, arsenic was found at several sites in Carinthia , Salzburg and Styria . In Switzerland, arsenic was found in the cantons of Aargau and Valais .

Other locations are in Australia , Belgium , Bolivia , Bulgaria , Chile , China , Finland , France , Greece , Ireland , Italy , Japan , Canada , Kazakhstan , Kyrgyzstan , Madagascar , Malaysia , Morocco , Mexico , Mongolia , New Zealand , Norway , Austria , Peru , Poland , Romania , Russia , Sweden , Slovakia , Spain , Czech Republic , Ukraine , Hungary , United Kingdom (Great Britain) and United States (USA) are known.

The element occurs much more frequently, however, in various intermetallic compounds with antimony ( allemontite ) and copper ( whitneyite ) as well as in various minerals that predominantly belong to the class of sulfides and sulfosalts . A total of 565 arsenic minerals are known to date (as of 2011). The highest concentrations of arsenic contain the minerals duranusite (approx. 90%), skutterudite and arsenolite (each approx. 76%), which are, however, rarely found. Arsenopyrite ( arsenic gravel ), lollingite , realgar ( rushing red ) and auripigment ( orpiment , rushing yellow ) are widespread . Other well-known minerals are cobaltite ( cobalt luster ), domeykite ( arsenic copper ), enargite , gersdorffite ( nickel arsenic ), prostite ( light red gold ore , ruby blende ), rammelsbergite as well as safflorite and sperrylite .

Arsenates are often found in phosphate- containing rocks because they have a comparable solubility and the most common sulfide mineral, pyrite, can incorporate up to a few percent by mass of arsenic.

Nowadays, arsenic is extracted as a by-product of the smelting of gold, silver, tin, copper, cobalt and other non-ferrous metal ores, as well as in the processing of phosphate raw materials. The largest producers in 2009 were China, Chile, Morocco and Peru. Arsenic is only sparingly soluble in water and is therefore only found in small traces, around 1.6 ppb (billionths of a mass fraction) in seas and oceans.

Arsenic is found in the air in the form of particulate arsenic (III) oxide . Volcanic eruptions have been identified as the natural cause of this, bringing an estimated 3000 tons into the earth's atmosphere every year . Bacteria release another 20,000 tons in the form of organic arsenic compounds such as trimethylarsine . A large part of the arsenic released comes from burning fossil fuels such as coal or petroleum . The estimated emissions caused by road traffic and stationary sources amounted to 120 tons in 1990 in the Federal Republic of Germany (20 tons in the old, 100 tons in the new federal states). The outdoor air concentration of arsenic is between 0.5 and 15 nanograms per cubic meter.

Refinery production by country (2009, estimated)
country metric tons
People's Republic of China 25,000
Chile 11,500
Morocco 07,000
Peru 04,000
Kazakhstan 01,500
Russia 01,500
Mexico 01,500
Belgium 01,000
other 00.500
total 53,500

Extraction and presentation

Former arsenic mines below the Rotgüldensee in Lungau

Arsenic occurs in large quantities as a by-product in the extraction of copper , lead , cobalt and gold . This is the main source of commercial exploitation for the item.

It can be prepared by thermal reduction of arsenic (III) oxide with coke or iron , and heating of arsenopyrite (FeAsS) or Arsenikalkies (FeAs 2 are obtained) in the absence of air lying Tonröhren. Here sublimates elemental arsenic, which returns to the solid state on cold surfaces.

Arsenic gravel decomposes into iron sulfide and elemental arsenic.
Arsenic lime decomposes into iron arsenide and elemental arsenic.

For semiconductor technology , arsenic, the purity of which must be over 99.99999 percent, is produced by reducing arsenic (III) chloride that has been distilled several times in a hydrogen stream :

Arsenic trichloride reacts with hydrogen to form hydrogen chloride and elemental arsenic.

It used to be created by sublimation from solutions in liquid lead . The sulfur in the arsenic ores is bound by the lead in the form of lead (II) sulfide . The purities of over 99.999 percent achieved in this way were not sufficient for semiconductor applications. Another possibility is to crystallize out of molten arsenic at high temperatures or to convert it into monoarsane , followed by cleaning and decomposition at 600  ° C into arsenic and hydrogen .

properties

Arsenic ions in chemical complexes (R denotes the ligands)
metallic shimmering gray arsenic

With nitrogen , phosphorus , antimony and bismuth, arsenic forms the 5th main group of the periodic table and, because of its physical and chemical properties, occupies the middle place in this group of elements. Arsenic has a relative atomic mass of 74.92159. The radius of the arsenic atom is 124.5 picometers . In covalently bound state, it is slightly smaller (121 picometers). Because of the release of the outer electrons ( valence electrons ) during ionization , the radius is reduced considerably to 34 picometers (As 5+ ; the outermost p- and the outermost s- atomic orbital remain unoccupied) or 58 picometers (As 3+ ; only the p- Orbital is unoccupied). In chemical complexes , the ace is 5+ - cation of four binding partners ( ligands ), As 3+ six surrounded. However, arsenic occurs only very rarely in a clearly ionic form.

According to Pauling , the value for electronegativity is 2.18 on a scale from 0 ( metals ) to 4 ( non-metal ) and is thus comparable with the value of the group neighbor phosphorus. The semi-metal character of arsenic is also shown in the fact that the required dissociation energy of 302.7  kJ / mol , i.e. the energy that has to be applied to separate a single arsenic atom from an arsenic solid , is between that of the non-metal nitrogen (473.02 kJ / mol; covalent bond ) and the metal bismuth (207.2 kJ / mol; metallic bond ). Under normal pressure, arsenic sublimes at a temperature of 613 ° C, i.e. it passes from the solid state of aggregation directly into the gas phase. Arsenic vapor is lemon-yellow and is composed of As to about 800 ° C 4 - molecules together. As 2 molecules are present above 1700 ° C.

Depending on the compound partner, arsenic shows oxidation states between −3 and +5. It forms compounds with electropositive elements such as hydrogen or metals in which it has an oxidation state of −3. Examples are monoarsane (AsH 3 ) and arsenic copper (Cu 3 As). In compounds with electronegative elements such as the non-metals oxygen , sulfur and chlorine , it has the +3 or +5 oxidation state; The former tends to be preferred over the elements nitrogen and phosphorus, which are in the same main group.

Modifications

Like other elements of the nitrogen group, arsenic occurs in various allotropic modifications. In contrast to nitrogen, which occurs in the form of diatomic molecules with a covalent triple bond, the corresponding As 2 molecules are unstable and arsenic forms covalent networks instead.

Gray arsenic

Layer structure of gray arsenic

Gray or metallic arsenic is the most stable form. It has a density of 5.73 g / cm 3 . Its crystals are steel gray, have a metallic sheen and conduct electricity .

If one looks at the structural structure of the gray arsenic, one recognizes layers of corrugated arsenic six-membered rings that assume the chair conformation . The arsenic atoms in it form a double layer if you look at the structure of the layer in cross section. The superposition of these double layers is very compact. Certain atoms of the next layer above or below are almost at the same distance from a reference atom as within the double layer under consideration. This structure means that the gray arsenic modification, like the homologous elements antimony and bismuth, is very brittle. This is why these three elements are often referred to as brittle metals .

Yellow arsenic

Is arsenic vapor, usually in the arsenic as As 4 - tetrahedra present, cooled rapidly, then the forms metastable yellow arsenic having a density of 1.97 g / cm 3 . It also consists of tetrahedral As 4 - molecules . Yellow arsenic is a non-metal and as a result does not conduct electricity. It crystallizes from carbon disulfide and forms cubic, highly refractive crystals that smell of garlic . At room temperature and particularly quickly when exposed to light , yellow arsenic converts into gray arsenic.

Black arsenic

Black arsenic itself can come in two different forms. Amorphous black arsenic is created by cooling arsenic vapor on surfaces with a temperature of 100 to 200 ° C. It does not have an ordered structure, but is in an amorphous, glass-like form, analogous to red phosphorus . The density is 4.7 to 5.1 g / cm 3 . Above 270 ° C, the black arsenic changes into the gray modification. If vitreous, amorphous black arsenic is heated to 100 to 175 ° C in the presence of metallic mercury , the metastable orthorhombic black arsenic is formed , which is comparable to black phosphorus.

Naturally formed orthorhombic black arsenic is known in nature as the rare mineral arsenolamprite .

Brown arsenic

When arsenic compounds are reduced in aqueous solution , mixed polymers are formed, similar to phosphorus . In these, a part of the free valences of the arsenic binds hydroxyl groups  (-OH). This form of arsenic is called brown arsenic .

Reactions

Arsenic reacts violently with oxidizing agents and halogens . In the air, arsenic burns with a bluish flame to form a white smoke of poisonous arsenic (III) oxide .

Arsenic reacts with oxygen to form arsenic (III) oxide.

Without external heat, the reaction with chlorine takes place with the appearance of fire to form arsenic (III) chloride .

Arsenic reacts with chlorine to form arsenic trichloride.

Further oxidation is possible.

Arsenic trichloride reacts with chlorine to form arsenic pentachloride.

Analogous reaction equations apply to the corresponding reactions with fluorine . Strongly oxidizing acids , such as concentrated nitric acid or aqua regia , convert arsenic into arsenic acid .

Arsenic reacts with nitric acid to form nitrogen dioxide, water and arsenic acid.

If the oxidation strength is lower - for example when using dilute nitric acid or sulfuric acid - arsenic acid is formed .

Arsenic reacts with sulfuric acid to form sulfur dioxide and arsenic acid.

Under acidic conditions and in the presence of non-passivated base metals , especially zinc , arsenic reacts with the hydrogen formed to form monoarsane .

Zinc reacts with hydrogen ions to form zinc ions and neutral hydrogen.
Arsenic reacts with hydrogen to form monoarsane.

The corresponding arsenite salt is formed with basic sodium hydroxide .

Arsenic reacts with sodium hydroxide to form sodium arsenite and elemental hydrogen.

Isotopes

Artificially produced radioactive isotopes with mass numbers between 65 and 87 are known of arsenic . The half-lives are between 96 milliseconds ( 66 As) and 80.3 days ( 73 As). Naturally occurring arsenic consists 100 percent of the isotope 75 As, so it is an anisotopic element. The corresponding arsenic nucleus therefore consists of exactly 33 protons and 42 neutrons . Physically, it is therefore one of the ug kernels (u stands for odd, g for even). Its nuclear spin is 3/2.

use

Arsenic is added to lead alloys to improve their strength and make the lead castable. Especially the finely structured plates of accumulators could not be cast without arsenic. Historically, arsenic was an important ingredient in copper alloys , which made it easier to process. In the past, metallic arsenic was occasionally used to create matt gray surfaces on metal parts in order to simulate aging.

Different size
wafers

In electronics , it does pure element as at least 99.9999 percent gallium arsenide - semiconductors , so-called III-V semiconductors (due to the combination of elements from the third and fifth main group of the periodic table ), as well as epitaxial layers on wafers in The form of indium arsenide phosphide and gallium arsenide phosphide play an important role in the production of high-frequency components such as integrated circuits (ICs), light-emitting diodes (LEDs) and laser diodes (LDs). At the beginning of 2004 there were only three producers of high-purity arsenic worldwide, two in Germany and one in Japan .

In the form of its compounds, arsenic is used in some countries as a pesticide in viticulture , as a fungicide (antifungal agent) in the timber industry , as a wood preservative , as a rat poison and as a decolorizing agent in glass production. Its use is controversial because the arsenic compounds used (mainly arsenic (III) oxide) are toxic.

Arsenic in medicines

The use of arsenic-containing minerals as a medicinal product is already attested in antiquity by Hippocrates and Pliny . They were used as a febrifuge, as a tonic and for the therapy of migraines , rheumatism , malaria , tuberculosis and diabetes . In the 18th century, a mixture of potassium arsenite and lavender water became known as Fowler's solution , which was long considered a medicinal miracle cure and was used as an antipyretic, medicinal water and even as an aphrodisiac . Potassium arsenite, as a component of Fowler's solution, was used in Germany as a treatment for psoriasis until the 1960s .

Constantinus Africanus (1017–1087) recommended an arsenic application to combat toothache. The use of arsenic to treat a sore tooth is said to have been described in Chinese medicine as early as 2700 BC. In the mid-10th century published work " Liber Regius ", the Arab doctor Haly Abbas (ʿAli ibn al-ʿAbbās; † 944) also recommended the use of arsenic to devitalize the pulp. Arsenic (III) oxide was used until modern times to devitalize the tooth pulp and disappeared from the range of therapies in the 1970s because of its carcinogenic effects, inflammation of the gums , the loss of one or more teeth including necrosis of the surrounding alveolar bone , allergies and symptoms of poisoning.

Medicines containing arsenic or arsenic compounds experienced an upswing at the beginning of the 20th century. In 1905, Harold Wolferstan Thomas and Anton Breinl observed that the arsenic-containing preparation atoxyl kills trypanosomes , which are the pathogens causing sleeping sickness . In 1920 a further development, tryparsamide , was used in tropical Africa from 1922 to 1970 for the treatment of sleeping sickness. It was important in containing this epidemic in the first half of the last century, but it could lead to blindness. Melarsoprol , developed in the 1950s, was the drug of choice for the treatment of sleeping sickness for several decades and is still used today because there are no effective follow-up products available.

Also stimulated by the trypanosome-toxic effect of Atoxyl, Paul Ehrlich developed the arsenic-containing arsphenamine (Salvarsan). The remedy introduced in 1910 in the treatment of syphilis was the first systematically developed, specifically acting chemotherapeutic agent based on preliminary theoretical considerations and was the model for the development of the sulfonamides used to this day . It has also long been used in the treatment of dysentery .

In 2000, a preparation containing arsenic was approved in the USA under the name Trisenox for the treatment of acute promyelocytic leukemia (APL). Trisenox has been approved for the treatment of APL in Europe since 2002 (distribution in the EU and USA: Cephalon ). Its effectiveness in cancer therapy is also attributed to its antiangioneogenic effect.

The various arsenic sulfides are components of medicinal products used in Chinese medicine .

Arsenic as an insecticide in taxidermy

Because of the toxic properties of arsenic compounds has been previously mainly arsenic for the preservation of vertebrates ( Taxidermie ) as the insecticide used. Many other substances, including lindane , were used for the same purpose as described in the taxidermy literature from 1868 to 1996. However, such substances are also poisonous for humans and nowadays place special demands on taxidermists, as they come into contact with such contaminated preparations.

Biological importance

The biological importance of arsenic for humans is not fully understood. It is considered a trace element in humans, but deficiency symptoms have so far only been demonstrated in animals. The necessary requirement, if it should exist, is between 5 and 50 µg per day. A daily arsenic intake of - depending on the choice of food - up to one milligram is considered harmless. In a new study, an increased arsenic load due to high arsenic levels in the groundwater of rice-growing areas could be linked to the development of cancer. However, the promotion of cancer development is dose-dependent and is only given if contaminated rice is consumed as a daily staple food. With regular consumption of arsenic compounds, especially arsenic trioxide, there is a habit that is even accompanied by withdrawal symptoms when the dose is discontinued.

People who used to consume arsenic frequently because of its general stimulating effect (especially in Styria), with frequent habituation and addiction, are called arsenic eaters .

Marine animals such as mussels and shrimp contain a particularly high amount of arsenic, the latter up to 175  ppm . Presumably, it acts as an inhibitor by binding to free thiol groups in enzymes , thus preventing their effect.

Arsenic is an essential trace element for many animals. For example, chickens or rats with an arsenic-free diet show significant growth disorders; this is likely related to the element's influence on the metabolism of the amino acid arginine . Numerous algae and crustaceans contain organic arsenic compounds such as the arsenobetaine already mentioned. Arsenic leads to an increased formation of the oxygen-transporting red blood cells . For this reason, it was previously added to the feed of poultry and pigs to enable faster fattening. Trainers of racehorses used it for illegal doping of their animals - today, however, the addition of arsenic to food can easily be detected in urine.

Soluble arsenic compounds are easily absorbed through the gastrointestinal tract and rapidly distributed throughout the body within 24 hours. Most of the ingested arsenic is found in the muscles , bones , kidneys and lungs . In humans, it has been found together with thallium in almost every organ. Blood contains up to 8  ppb arsenic, in the other organs of the body such as the bones it has a proportion of between 0.1 and 1.5 ppm, in hair the proportion is around 1 ppm. The total level of arsenic in an adult's body averages around 7 milligrams.

Organic arsenic compounds such as those from fish and seafood originating dimethylarsinic , Trimethylarsenoxid , trimethyl and Arsenobetaine leave the human body almost unchanged within two to three days by the kidneys. Inorganic arsenic compounds are converted into monomethylarsonic acid (MMAA) and dimethylarsinic acid (DMAA) in the liver and then also excreted via the kidneys.

In plants, the element increases the carbohydrate turnover. The banded border fern (Pteris vittata) prefers to take up the semimetal from the ground and can absorb up to five percent of its dry weight in arsenic. For this reason, the fast-growing plant is used for the biological cleaning of arsenic-contaminated soils.

The stimulating effect of arsenic is probably also the cause of the arsenic eating , which was previously widespread in some Alpine regions . In the 17th century, some of the local residents consumed up to 250 milligrams of arsenic twice a week for life - for men because it helped work at high altitudes, for women because it supposedly contributed to a strong complexion. Long dismissed as a fairy tale in science, a farmer from the Styrian Alps took a dose of 400 milligrams of arsenic trioxide in front of the German experts gathered in Graz in 1875, which could later also be detected in his urine. The dose was well over twice the amount of arsenic, which is lethal for normal people, but did not show any negative effects on the farmer. Something similar has been reported by residents of a settlement in the high Chilean Atacama Desert , whose drinking water is highly contaminated with arsenic, but who show no symptoms of poisoning. Today it is assumed that it is physiologically possible to get used to the poison slowly with gradually increasing doses.

In 2010 it was reported about the bacterial strain GFAJ-1 that under certain conditions in arsenate- containing culture media it was able to incorporate arsenate instead of phosphate into biomolecules such as DNA without dying, which was previously considered impossible. However, the finding seems to be based on unclean working methods and the findings could not be replicated.

safety instructions

Arsenic dusts are highly flammable.

Classification according to ICD-10
T57.0 Toxic effect: arsenic and its compounds
ICD-10 online (WHO version 2019)

toxicity

Trivalent soluble compounds of arsenic are highly toxic because they disrupt biochemical processes such as DNA repair , cellular energy metabolism, receptor-mediated transport processes and signal transduction. This presumably does not result in a direct effect on the DNA, but rather a displacement of the zinc ion from its binding to metallothioneins and thus inactivation of tumor suppressor proteins (see also zinc finger protein ). Arsenic (III) and zinc (II) ions have comparable ionic radii and thus a similar affinity to these zinc finger proteins, but arsenic then does not activate the tumor suppressor proteins.

Acute arsenic poisoning leads to cramps, nausea, vomiting, internal bleeding, diarrhea and colic , up to and including kidney and circulatory failure. In severe cases of poisoning, the skin feels damp and cold and the person concerned can fall into a coma . Ingestion of 60 to 170 milligrams of arsenic is considered a lethal dose for humans ( LD 50  = 1.4 mg / kg body weight); usually death occurs within several hours to a few days due to kidney and cardiovascular failure. Chronic exposure to arsenic can cause skin diseases and damage to blood vessels, leading to death of the affected regions ( black foot disease ) and malignant tumors of the skin, lungs, liver and bladder. These symptoms were as Reichensteiner designated disease, for a place in Silesia whose drinking water by the arsenic contained cleardown up to 0.6 mg of arsenic per liter.

The chronic arsenic poisoning via binding to sulfhydryl groups of enzymes in blood formation (for example, Delta-amino-Laevulin acid synthetase) to an initial drop in hemoglobin in the blood, resulting in a reactive polycythemia leads. Furthermore, chronic consumption of arsenic results in the substitution of the phosphorus atoms in adenosine triphosphate (ATP) and thus a decoupling of the respiratory chain, which leads to further reactive polyglobules. Clinically, after years of exposure to As, drumstick fingers , watch glass nails , Mees nail bands and acrocyanosis ( Raynaud's syndrome ), with the consequence of black foot disease, are found here .

Metallic arsenic, on the other hand, is only slightly toxic due to its insolubility, as it is hardly absorbed by the body (LD 50 = 763 mg / kg rat, oral). However, since it can easily become coated with its very poisonous oxides such as arsenic in air, it should always be treated with the greatest care. The situation is different with arsenic, which was used in earlier times as a stimulant by arsenic eaters to prevent arsenic poisoning. The mechanism of this immunization against arsenic is not known.

Limit values

Anionic arsenic occurs as arsenite ([AsO 3 ] 3− ) and arsenate ([AsO 4 ] 3− ) in high concentrations in groundwater in many countries . Over 100 million people around the world drink contaminated water through leaching from ores containing arsenic in the form of trivalent and pentavalent ions . Especially in India , Bangladesh and Thailand , where numerous wells were dug with international support in the 20th century in order to be able to escape from surface water contaminated with pathogens to groundwater, this undetected pollution of the drinking water led to chronic arsenic poisoning in large parts of the affected population. The problem can, where known, be remedied chemically by oxidation of the arsenic compounds and subsequent precipitation with iron ions. From Rice University an inexpensive filter with possibility was Nano - magnetite developed.

Arsenic in groundwater: risk areas worldwide

The World Health Organization (WHO) has recommended a limit value for arsenic in drinking water of 10 micrograms per liter since 1992. The value is still exceeded in many European countries and in the USA. In Germany, however, it has been complied with since 1996. A directive of the European Union (EU) from 1999 prescribes a maximum value of 10 micrograms per liter of drinking water throughout the EU. The USA committed itself in 2001 to complying with this limit value from 2006 onwards.

The arsenic found in groundwater accumulates ten times as much in rice as in other cereals. The varieties offered on the world market contain between 20 and 900 micrograms of arsenic per kilogram. In 2005, the Chinese government reduced the permissible level of inorganic arsenic compounds from 700 to 150 micrograms per kilogram of food, and in July 2014 the Codex Alimentarius Commission decided for the first time a maximum value of 200 micrograms for polished rice. The EU Commission responsible for food safety is discussing a 15 percent higher limit for products made from puffed rice and only half as high for special products for small children (i.e. 100 micrograms per kg).

There are still no limit values for other contaminated foods such as beer or fruit juices, although they may contain more arsenic than is permitted for drinking water. Consumer organizations demand a limit value of 3, but no more than 4.4 ppb (corresponds to micrograms per kg) for apple juice .

Fish and seafood have high levels of arsenic, but almost exclusively in the organically bound form that is considered harmless. There are no limit values ​​such as those for mercury or cadmium .

The new EU Chemicals Act, implemented in Germany's Hazardous Substances Ordinance of 2005, prohibits in Annex 4 the “commercial” (non-private) processing of arsenic-containing agents and preparations that contain more than 0.3 percent by weight of arsenic. Such limit value regulations exist because arsenic is added to the zinc melt in the galvanizing industry worldwide in order to improve the adhesive properties of the zinc on the iron surface of the metal piece to be galvanized. Due to the temperature in the zinc weld pool of 460 ° C to 480 ° C, arsenic, cadmium and other highly volatile metals evaporate and accumulate in the workplace air. In this way, permissible limit values ​​can be exceeded by a factor of a thousand for a short time, with the result that the body is absorbed into the body by air and alveolar. Measurements showed that arsenic (and cadmium) in the high-purity zinc (99.995 degree of purity , DIN-1179 degree of purity) were shown to be less than 0.0004% by weight and that after adding 450 grams of this high-purity zinc to the zinc melt, the Cd increased - / As concentration from 3 to 7 µg / m 3 air to over 3000 µg / m 3 air. For arsenic, this fact was surprisingly established in a galvanizing plant by measuring the arsenic concentration in zinc melt, blood and urine (unpublished). In electroplating workers, the urine arsenic concentration is measured at 25 to 68 µg / l urine, compared to 0.1 µg arsenic / l urine in the unpolluted population.

Depletion

There are methods for removing ionic arsenic from drinking water that are based on adsorption on activated carbon , activated aluminum oxide or iron hydroxide granules . The latter is used as standard in fixed bed reactors in drinking water treatment in Germany and internationally. In addition to ion exchangers used. It is possible to remove arsenic from the soil using genetically modified plants that store it in leaves. The thick-stemmed water hyacinth , which stores arsenic especially in its root tissue, and thus causes a depletion of the contaminated water, is suitable for phytoregulation of drinking water. Organic arsenic compounds in polluted soils can be broken down enzymatically with the help of fungi.

In Bangladesh, a process by the Swiss research institute EAWAG is attempting to reduce arsenic with the help of transparent PET bottles and lemon juice. With this method, called SORAS (Solar Oxidation and Removal of Arsenic) , sunlight oxidizes the arsenic; the ingredients in lemon juice help with precipitation. This cost-effective method can reduce the arsenic content by 75 to 90 percent.

In waters of Yellowstone National Park , resulting from geysers and other thermal springs of volcanic dine origin, were eukaryotic algae of the genus Cyanidioschyzon found to tolerate high arsenic concentrations of waters and to less biologically available organic compounds oxidize can. Work was carried out in 2009 on a use for depletion in drinking water.

Antidotes

The sulfur-containing complexing agents dimercaptopropane sulfonic acid (DMPS), dimercaptosuccinic acid and the older, less well-tolerated dimercaprol are available as antidotes for acute arsenic poisoning . They are still effective at high doses of arsenic if the poisoning is diagnosed in a timely manner. Their role in the treatment of chronic arsenic poisoning, however, is controversial. Activated charcoal one to several hours after ingestion can also bind the metal and cause it to be excreted.

prophylaxis

Indian researchers have found in animal experiments that the consumption of garlic can lower the arsenic levels in the blood and increase the arsenic levels in the urine. This is explained by the precipitation of arsenic when it reacts with sulfur-containing substances such as allicin , which is a component of garlic. Two to three cloves of garlic per day are recommended for prophylaxis.

proof

Inorganic detection reactions

Arsenic compounds show an uncharacteristic pale blue flame color when burned . In the case of the glow tube sample , arsenic compounds are heated, some of which sublime and are reflected on cold surfaces in the form of black arsenic, white arsenic (III) oxide or yellow arsenic trisulfide .

  • The so-called Marsh test is the classic detection reaction in chemistry and forensic medicine for arsenic:
  • In the Bettendorf sample, arsenic oxidizes divalent tin ions in concentrated hydrochloric acid regardless of the oxidation state . Elemental arsenic precipitates:
  • If magnesium ions are added to an ammoniacal , ammonium chloride-containing solution of arsenate , a crystalline precipitate of magnesium ammonium arsenate hexahydrate is obtained: Arsenate reacts with magnesium ions, ammonium ions and water to form magnesium ammonium arsenate hexahydrate.

  • Another detection reaction of arsenic (at) in aqueous solution is the precipitation with ammonium heptamolybdate . The yellow precipitate is sparingly soluble in acids , but readily soluble in bases : Dihydrogen arsenate reacts with hydrogen ions, ammonium ions and molybdate ions to form ammonium arsenomolybdate and water.

Instrumental determination methods for arsenic

Atomic Absorption Spectrometry (AAS)

In flame AAS, the arsenic compounds are ionized in a reducing air-acetylene flame. An atomic absorption measurement is then carried out at 189.0 nm and 193.8 nm, respectively. Detection limits up to 1 µg / ml have been described. The arsenic is often converted into gaseous arsine (AsH 3 ) with the help of NaBH 4 (hydride technology). In quartz tube technology, AsH 3 is first thermally decomposed into its atomic constituents at around 1000 ° C in an electrically heated quartz tube . Determine wavelengths. The detection limit for this technique is 0.01 µg / l. Another method is the so-called graphite furnace technique, in which the arsenic in a solid sample is volatilized at 1700 ° C and higher and the extinction is then measured at 193.8 nm.

Atomic emission spectrometry

The coupling of hydride technology with the inductively coupled plasma / laser-induced fluorescence measurement is a very powerful method for determining arsenic. AsH 3 released by means of hydride generation is atomized in the plasma and stimulated to emit with a laser. With this method, detection limits of 0.04 ng / mL were achieved.

Mass spectrometry (MS)

In mass spectrometry , the arsenic species is first thermally ionized by an inductively coupled argon plasma (ICP-MS). The plasma is then passed into the mass spectrometer. A detection limit of 0.2 µg / l has been described for arsenite.

Photometry

The photometric detection of As as arsenomolybdenum blue is widespread. As (V) initially reacts with (NH 4 ) 2 MoO 4 . This is followed by a reduction with SnCl 2 or hydrazine to a blue complex. The photometry takes place at 730 nm and is therefore almost free of interference. The detection limits can be improved by using basic dyes as complexing agents.

Neutron activation analysis

A very sensitive determination of arsenic in the ppt range is possible using neutron activation analysis . It is used in particular when the sample has a complex composition or is difficult to digest. However, this method gives no indication of the chemical compound in which the arsenic is present. When neutrons interact with the sample, which contains the natural isotope arsenic-75, the heavier isotope arsenic-76 is formed, which, however, is unstable and converts to selenium-76 with β-decay . The β-rays , which can be used to determine the amount of arsenic, are measured .

Biosensors

With biosensors , the bioluminescence is detected when arsenic dissolved in water comes into contact with genetically modified bacteria (e.g. Escherichia coli K12) and a light meter ( luminometer ). The arsenic concentration present correlates directly with the amount of light emitted.

links

Arsine

Chemical compounds of arsenic and hydrogen (→ arsanes ) are not very numerous and very unstable compared to the corresponding compounds of the main group neighbors nitrogen and phosphorus . There are currently three known arsans.

Halogen compounds

Arsenic forms binary compounds of the types AsX 3 , AsX 5 and As 2 X 4 with halogens (X denotes the corresponding halogen).

Oxygen compounds

Important oxygen acids are:

  • Arsenic acid (2 H 3 AsO 4 · H 2 O), the salts of which are called arsenates or arsenates (V) and are similar to phosphates. Examples are calcium arsenate (Ca 3 (AsO 4 ) 2 · 3H 2 O) and lead hydrogen arsenate (PbHAsO 4 ), which were used as crop protection agents
  • Arsenic acid (H 3 AsO 3 ), the salts of which are known as arsenites or arsenates (III).

The most important arsenic oxide is arsenic (III) oxide (arsenic trioxide also arsenic or white arsenic, As 2 O 3 , the anhydride of arsenic acid), which is present in the gas phase in the form of double molecules with the formula As 4 O 6 . It is amphoteric and thus indicates the semi-metallic character of arsenic. In addition to As 2 O 3 , As 2 O 5 (arsenic pentoxide, the anhydride of arsenic acid) and the mixed anhydride of arsenic acid and arsenic acid As 2 O 4 (arsenic tetraoxide) are known

A historically important dye and pesticide is a copper arsenic oxide with the common name Schweinfurter Grün (Cu (AsO 2 ) 2 · Cu (CH 3 COO) 2 ).

Sulfur compounds

There are two important arsenic sulfides, both of which occur naturally as minerals.

Arsenic metal compounds

There are important compounds of arsenic with metals

Organic compounds

In analogy to the amines and phosphines , corresponding compounds are found with arsenic instead of nitrogen or phosphorus . They are known as arsines .

The arsoranes , compounds of the R 5 As type, where R 5 stands for five - possibly different - organic groups, include, for example, pentaphenyl arsenic or pentamethyl arsenic . If one of the five groups is missing, a single positively charged ion remains (R stands for - possibly different - organic groups), which is called the arsonium ion (AsR 4 ) + .

Structural formula of a polycyclic molecule with arsenic backbone (R = tert.butyl group )

Analogous to the carboxylic acids , two classes of arseno-organic acids can be formed:

In addition, heteroaromatics with arsenic as a heteroatom are known, such as arsabenzene , which consists of a benzene ring in which one carbon atom has been replaced by arsenic and which is therefore structured analogously to pyridine .

Extract of the structure of polyarsine

Homocyclic arsenic compounds also exist. examples are

whose molecules have a five- or six-membered ring of arsenic atoms as the backbone, to which one methyl group per arsenic atom is bound to the outside . The molecule opposite is a polycyclic variant, the backbone of which is composed of one six-membered ring and two attached five-membered rings (R stands for a tert - butyl group in each case ).

Finally, arsenic let polymeric represent long chain molecules as Polyarsine be called. They consist of a central “rope ladder” of arsenic atoms to which a methyl group is attached to each “rung” on the outside of each side, resulting in the chemical formula (AsCH 3 ) 2n , where the natural number n can be well over 100. Polyarsines show clear semiconductor properties.

Bio-organic compounds

In bioorganic play Arsenolipide , Arsenosaccharide and arsenic glycolipids a significant role. Important representatives of these substance classes are, for example, arsenobetaine , arsenocholine and variously substituted arsenoriboses. Above all, they occur cumulatively in marine organisms and can get into the human food chain in this way . Arsenic-containing biomolecules could be detected in algae, sea ​​sponges and in fish tissue after extraction by means of HPLC - ICP-MS . The analysis of organo-arsenic compounds (including their speciation ) is very complex.

Arsenic in crime history, literature, and film

The element arsenic achieved dubious fame as a murder poison, as evidenced by historical records as well as its instrumentalization in literature and film. The murder poison was never elemental arsenic, but its compounds.

In Italy and France dukes, kings and popes died of deliberately induced arsenic poisoning. In 17th century France, the Marquise de Brinvilliers , who poisoned her father and two brothers with a mixture of arsenic , is at the center of a poisonous scandal. In Germany, the serial killer Gesche Gottfried from Bremen killed 15 people. The case of serial killer Anna Margaretha Zwanziger at the beginning of the 19th century also caused a stir . However, the perpetrators of the murders mostly remained undetected, as arsenic could not be detected in small quantities until 1836. Only the Marsh sample developed by James Marsh and named after him made it possible to identify traces of the element and thus prove an unnatural cause of death. In the 19th and 20th centuries, intentional poisoning with arsenic-containing agents continued - on the one hand because they were easily available as herbicides, on the other hand, chronic administration of small doses could simulate death caused by illness. In September 1840, the first judgment was made in the trial of Marie Lafarge , which was based solely on the results of Marsh's test. In the case of Marie Besnard , who allegedly was responsible for several deaths in her environment in Loudun between 1927 and 1949 , clear evidence could not be produced because the results of the investigation were contradicting the results, and she had to be acquitted in 1954.

Emperor Guangxu was poisoned with arsenic
Napoléon I Bonaparte is believed to have died from arsenic poisoning

For years, experts believed that the death of the former French emperor Napoleon Bonaparte at the age of 51 on the island of St. Helena must be ascribed to a poison attack with arsenic. At least highly concentrated traces of the poison had been found in his hair. Various other theses exist today to explain the factual findings. One possibility is that the arsenic was added to the hair after his death in order to preserve it, a method that was quite common at the time. An excessive use of the arsenic-containing Fowler's solution is possible , which at the time was considered by many of his contemporaries as a medical miracle drug. The third and most likely possibility today is that Napoleon poisoned himself with organic arsenic compounds that molds constantly released from his wallpaper made with green arsenic pigments. Their high arsenic content is conclusively proven by a material sample found in a notebook in 1980.

The famous philosopher René Descartes died in 1650 a few months after his arrival at the court of the Swedish Queen Christine . The suspicion that he had been poisoned with arsenic by one of the Jesuits who were at the court of the Protestant queen for religious-political reasons increased when Christine later actually converted to Catholicism , but could not be substantiated, so that the official cause of death, Pneumonia , established itself in the biographies. Only recently was the old suspicion corroborated on the basis of newly found and reinterpreted documents and claims that the "poisoning of Descartes seems very likely, not to say almost certain".

In 1900, mass poisoning occurred in Manchester, UK , affecting several thousand people. It turned out that they all drank beer from the same brewery. In the preliminary stages of beer production, sulfuric acid was apparently used, which in turn was produced from sulfur obtained from sulphide minerals contaminated with arsenopyrite. About 70 people died from poisoning.

Two men died of arsenic poisoning in Austria in 2010 and 2011. On April 11, 2013, a 52-year-old Polish woman was found guilty of the murder of both of them at the Krems Regional Court and was not finally sentenced to life imprisonment by the jury. In the 1950s, at the height of the Cold War, the American ambassador, Clare Booth Luce, fell ill in Rome from poisoning with the arsenic released from wallpaper. The fact that the disease could be traced back to the mold-infested wallpaper and not to enemy secret agents not only contributed to the ambassador's recovery in this case, but also to the preservation of the peace.

In Friedrich Schiller's bourgeois tragedy Kabale und Liebe , the young Major Ferdinand von Walter first poisoned his lover Luise Millerin and then himself. However, in Kabale und Liebe, death occurs unrealistically within minutes.

The protagonist of the famous novel Madame Bovary by Gustave Flaubert , the unhappily married country doctor wife Emma Bovary, dies at the end of the novel by suicide with arsenic in the form of a white powder. The scion of a family of doctors, Flaubert, describes the symptoms of poisoning and the extremely painful death of the Bovary in great detail.

In the novel, Strong Poison (Strong Poison) by Dorothy L. Sayers the victim was poisoned with arsenic. The suspect, crime writer Harriet Vane, dealt intensively with arsenic murders at the time in question and even sought advice from the pharmacist.

The famous detective " Kalle Blomquist " from Astrid Lindgren 's children's book of the same name used the Marsh test to examine a piece of chocolate poisoned with arsenic.

In the play by Joseph Kesselring Arsenic and Old Lace (English: Arsenic and Old Lace ) poison two old ladies in well-meaning intention elderly lonely men with arsenic, strychnine - and cyanide mixture. The play became known through the film of the same name by Frank Capra with Cary Grant , Peter Lorre and Priscilla Lane in the leading roles.

literature

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  • Erwin Riedel: Inorganic Chemistry . de Gruyter, Berlin 2002, ISBN 3-11-017439-1 .
  • Dietmar Ritter: Characterization and use of alternative arsenic and phosphorus sources for the organometallic molecular beam epitaxy of InP and GaInAs. Shaker, Aachen 1998, ISBN 3-8265-4489-7 .
  • Giulio Morteani, Lorenz Eichinger: Arsenic in drinking water and dearsenation. Statutory regulations, toxicology, hydrochemistry. In: water, air, soil. United Fachverlag, Mainz 48. 6, 2004, pp. 24-26.
  • Nicholas C. Norman: Chemistry of Arsenic, Antimony and Bismuth. Blackie, London 1998, ISBN 0-7514-0389-X .
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Web links

Wiktionary: Arsenic  - explanations of meanings, word origins, synonyms, translations
Commons : Arsenic  - collection of pictures, videos and audio files

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This version was added to the list of excellent articles on January 1st, 2005 .