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arsenic (ärˈsənĭk), a semimetallic chemical element; symbol As; at. no. 33; at. wt. 74.92160; m.p. 817℃ (at 28 atmospheres pressure); sublimation point 613℃; sp. gr. (stable form) 5.73; valence −3, 0, +3, or +5. Arsenic appears in several allotropic forms (see allotropy); the stable form is a silver-gray, brittle crystalline solid that tarnishes rapidly in air, and at high temperatures burns to form a white cloud of arsenic trioxide. A yellow crystalline form and a black amorphous form are also known. Arsenic is a member of Group 5 of the periodic table. It combines readily with many elements: with hydrogen to form arsine, an extremely poisonous gas; with oxygen to form a pentoxide and the above-mentioned trioxide (As2O3 or As4O6), a deadly poison also called arsenic (III) oxide, arsenious oxide, white arsenic, or, simply, arsenic; with the halogens; and with sulfur.
The element is used with other metals to make hard, strong, corrosion-resistant alloys. Its compounds are used in pigments, animal poisons, insecticides (e.g., Paris green), and poison gases (such as lewisite) for chemical warfare. They are also used in glassmaking, in calico and indigo printing, in tanning and taxidermy (as preservatives), and in pyrotechnics. Small quantities of arsenic added to lead in the manufacture of shot assure perfectly spherical pellets by delaying the solidification of the molten lead, and thereby allowing it to flow more readily; the arsenic also contributes hardness. A small amount of arsenic is added to germanium in the production of semiconductor devices such as transistors and integrated circuits.
A number of organic compounds of arsenic are used in medicine; the best known is Salvarsan, formerly used extensively in the treatment of syphilis and yaws. On the other hand, many arsenic compounds are strong poisons. Even in dilute concentrations that are not poisonous, as are found in some water supplies, arsenic may be carcinogenic. One delicate test for the presence of even minute quantities of arsenic in compounds is the Marsh test.
Arsenic occurs in many ores, including realgar, orpiment, and arsenopyrite, the chief commercial source. When it is prepared commercially from sulfide ores, e.g., arsenical pyrites, the ores are roasted (heated in the absence of air); the arsenic sublimes (passes directly from the solid to the gaseous state) and is condensed. In another method, white arsenic is reduced with carbon.
Although realgar, orpiment, and other arsenic minerals were known to the Greeks of Aristotle's time, the element itself was not. The “arsenic” so called by them and by the later alchemists was not true arsenic, but probably arsenic trioxide. The element was first described by Albertus Magnus in the 13th cent.
(Latin, arsenicum), As, a chemical element in Group V of the Mendeleev periodic system. Atomic number, 33; atomic weight, 74.9216; steel-gray crystals. Arsenic has one known stable isotope, 75As.
History. Ancient civilizations were familiar with the natural compounds of arsenic with sulfur (orpiment, As2S3, and realgar, AS4S4) and used them as medicines and paints. Arsenic trioxide, As2O3 (white arsenic), produced by roasting arsenic sulfides, was also known. The term arsenikon appeared as early as the writings of Aristotle; it is derived from the Greek arsen (“strong” or “courageous”), which was used to describe arsenic compounds (with reference to their powerful effect on the human body). The Russian name for arsenic, mysh’iak, is presumably derived from mysh’ (“mouse”), since arsenic preparations were commonly used to exterminate mice and rats. The production of arsenic in free form is attributed to Albertus Magnus (c. 1250). In 1789, A. Lavoisier included arsenic in the list of chemical elements.
Occurrence in nature. The average arsenic content (clarke) in the earth’s crust is 1.7 × 10-4 percent by weight. Arsenic is found in such quantities in most varieties of igneous rock. Since arsenic compounds are volatile at high temperatures, the element does not accumulate during magmatic processes but rather is concentrated upon precipitation from hot abyssal waters in association with sulfur, selenium, antimony, iron, cobalt, nickel, and copper. Volatile arsenic compounds are ejected into the atmosphere during volcanic eruptions. Since arsenic is multivalent, an oxidation-reduction medium exhibits a substantial effect on its migration. Oxidative conditions in the earth’s crust stimulate the formation of arsenates (As5+) and arsenites (As3+), rare minerals that occur only in certain areas of arsenic deposits. Native arsenic and As2+ minerals are even less common. Of the approximately 180 known arsenic minerals, only arsenopyrite, FeAsS, has become widely used in various sectors of industry.
Small quantities of arsenic are necessary for the viability of living organisms. However, in areas of arsenic deposits and recent volcanic activity, the arsenic content in the soil is up to 1 percent, which causes diseases of livestock and destruction of plant life. The accumulation of arsenic is particularly characteristic of soil in steppe and desert regions, where its migration is restricted. Arsenic is readily leached out of the soil under humid climatic conditions.
The average arsenic content in living matter is 3 × 10−5 percent; in rivers it is 3 × 10−7 percent. The arsenic that is carried into the ocean by rivers settles quickly to the bottom. Ocean waters contain only 1 × 10−7 percent arsenic, whereas clay and shale contain 6.6 × 10−4 percent. Sedimentary iron ores and ferromanganese concretions are often enriched with arsenic.
Physical and chemical properties. Arsenic is known in several allotropic forms. Gray, or metallic, arsenic (α-As), a brittle, steel-gray crystalline mass, is the most stable modification under ordinary conditions. Newly formed fractures have a metallic luster and tarnish rapidly upon exposure to air (a thin film of As2O3 forms on the surface). Gray arsenic has a rhombohedral laminar lattice (a —4.123 angstroms [Å]; angle α = 54°10’; ξ= 0.226). Density, 5.72 g/cm3 at 20°C; specific electrical resistivity, 35 × 10−8 ohm • m, or 35 × 10−6 ohm • cm; temperature coefficient of electrical resistivity, 3.9 × 10−3 (0°-100°C); Brinell hardness, 1, 470 meganewtons per sq m, or 147 kilograms-force per sq mm (3–4 on Mohs’ scale). It is diamagnetic. It sublimes without melting at 615°C under atmospheric pressure, since the triple point of α-As is fixed at 816°C and 36 atmospheres. Arsenic vapor is composed of AS4 molecules up to 800°C and of As2 molecules at temperatures above 1700°C. The condensation of arsenic vapor on a surface cooled by liquid air produces yellow arsenic, which occurs in the form of soft (wax like), transparent crystals with a density of 1.97 g/cm3 and is similar in properties to white phosphorus. Yellow arsenic is converted into the gray modification upon exposure to light or upon warming. Two glassy amorphous modifications are also known—black arsenic and brown arsenic, which are converted into the gray modification upon heating above 270°C.
The outer electron configuration of an arsenic atom is 3d104s24p3. Arsenic compounds have the oxidation numbers + 5, + 3, and —3. Gray arsenic is considerably less active chemically than phosphorus. Upon exposure to air and heating above 400°C, arsenic burns and forms As2O3. It combines directly with halogens. Under ordinary conditions AsF5 exists in the form of a gas; AsF3, AsCl3, and AsBr3 are colorless, highly volatile liquids; and AsI3 and As2I4 are red crystals. Two sulfides, orangered AS4S4 and lemon yellow As2S3, are produced upon heating of arsenic with sulfur. Pale yellow As2S5 is precipitated upon passage of H2S through an ice-cooled solution of arsenic acid (or its salts) in fuming hydrochloric acid; 2H3AsO4 + 5H2S = As2S5 + 8H2O. The precipitate decomposes into As2S3 and S at about 500°C. All arsenic sulfides are insoluble in water and dilute acids; they are converted into a mixture of H3AsO4 and H2SO4 upon action of strong oxidants (for example, the mixtures HNO3 + HC1, HC1 + KCIO3). The sulfide As2S3 undergoes rapid dissolution in the sulfides and polysulfides of ammonia and alkali metals to form salts of thioarsenous acid, H3AsS3,!, and thioarsenic acid, H3AsS4.
Arsenic combines with oxygen to produce arsenic (III) oxide, As2O3 (arsenous anhydride), and arsenic (V) oxide, As20S (arsenic anhydride). The former is obtained by the interaction of oxygen with arsenic or arsenic sulfides—for example, 2As2S3 + 9O2 = 2As2O3 + 6SO2. Arsenic (III) oxide vapors condense to form a colorless, vitreous mass that becomes opaque with time because of the appearance of minute cubic crystals with a density of 3.865 g/cm3. The vapor density corresponds to the formula of As4O6 whereas heating above 1800°C alters the vapor composition to As2O3; 2.1 g of As2O3 dissolve in 100 g of H2O at 25°C. Arsenic (III) oxide is an amphoteric compound with predominantly acidic properties. The corresponding salts (arsenites) of orthoarsenous acid, H3AsO3, and metaarsenous acid, HAsO2, are known, but the acids have not been isolated. Only the arsenites of alkali metals and ammonium are soluble in water. Arsenic (III) oxide and arsenites are usually reducing agents (for example, As2O3 + 2I2 + 5H2O = 4HI + 2H3As04), but they can also be oxidants in certain cases (for example, As2O3 + 3C = 2As + 3CO).
Arsenic (V) oxide is obtained by heating arsenic acid H3AsO4 to a temperature of approximately 200°C. It is colorless and decomposes into As2O3 and O2 at 500°C. Arsenic acid can be prepared by treating As or As2O3 with concentrated HNO3. Arsenic acid salts (arsenates) are insoluble in water, except for the salts of alkali metals and ammonium. Salts are known that correspond to orthoarsenic acid H3AsO4; metaarsenic acid, HAsO3; and pyroarsenic acid, H4As2O7. The last two acids cannot be produced in free form. Arsenic generally forms compounds (arsenides) upon fusion with metals.
Preparation and use. Arsenic is prepared commercially by heating arsenopyrite:
FeAsS = FeS + As
or, less frequently, by reducing As2O3 with carbon. Both processes are carried out in retorts made of refractory clay, which are connected to an arsenic vapor condenser. Arsenous anhydride is prepared by oxidation roasting of arsenic ores or as a by-product when roasting complex ores containing arsenic. Oxidation roasting produces As2O3 vapors, which are condensed in collection chambers. Crude AS2O3 is purified by distillation at 500°-600°C and then used in the manufacture of arsenic and various arsenic preparations.
Small quantities of arsenic (0.2–1.0 percent by weight) are added to lead for use in the manufacture of shot (it increases the surface tension of molten lead, thereby resulting in the formation of nearly spherical shot; it also increases the hardness of lead). Arsenic serves as a partial substitute for antimony in the preparation of certain types of babbitt and type metal.
Pure arsenic is nontoxic, but all arsenic compounds that are water-soluble or pass into solution under the action of gastric juice are extremely toxic, particularly hydrogen arsenide (arsine). Arsenous acid anhydride has the highest toxicity of all arsenic compounds used in industry. Nearly all nonferrous metal sulfide ores, as well as iron pyrites, contain arsenic as an impurity. Therefore, As2O3 always forms in addition to sulfur dioxide, SO2, upon oxidation roasting of such substances. Most of the As2O3 is condensed in chimney flues; however, if no purification system is provided or if the system is inefficient, substantial quantities of As2O3 are carried off in the waste gases from ore-roasting furnaces. Although pure arsenic is not a toxic substance, prolonged exposure to air during storage always results in the formation of a toxic film of As2O3 on its surface. Sufficient ventilation is essential when pickling metals (iron or zinc) with commercial sulfuric or hydrochloric acid containing arsenic as an impurity, since this produces highly toxic hydrogen arsenide.
S. A. POGODIN
Arsenic in the organism. Arsenic is distributed throughout the living world as a trace element. The average arsenic content in soil is 4 × 10−4 percent; in plant ash, 3 × 10−5 percent. Marine organisms have a higher arsenic content than terrestrial forms (fish contain 0.6–4.7 mg of arsenic per kilogram of raw matter, stored in the liver). The average arsenic content in the human body is 0.08–0.20 mg/kg. Arsenic is concentrated in erythrocytes, where it is bonded to a molecule of hemoglobin (therefore, the globin fraction contains twice the arsenic found in the heme). The greatest quantity of arsenic per gram of tissue is found in the kidneys and liver. The lungs, spleen, skin, and hair contain a substantial amount of arsenic, whereas relatively little is found in the cerebrospinal fluid, the cerebrum (mainly the pituitary body), and the genitalia. Arsenic is present in the basic protein fraction of tissue, and considerably smaller quantities are found in the acid-soluble fraction; the lipid fraction contains a negligible quantity of arsenic. Arsenic participates in various oxidation-reduction (redox) reactions, including the oxidization decomposition of complex carbohydrates, fermentation, and glycolysis. In biochemistry, arsenic compounds are used as enzyme inhibitors in the study of metabolic reactions.
Arsenic in medicine. Organic arsenic compounds (Aminarsone, Sulfarsphenamine, Neosalvarsan, and Stovarsol) are primarily used in the treatment of syphilis and various types of protozoiasis. Inorganic preparations (sodium arsenate, potassium arsenite, and arsenous anhydride) are classified as general restoratives and tonics. Local application of inorganic arsenic preparations may induce necrosis without prior irritation, which makes the process painless. This property, which is most strongly exhibited by As2O3, is used in stomatology to destroy dental pulp. Inorganic arsenic preparations are also used in the treatment of psoriasis. Artificially prepared radioactive isotopes of arsenic, 74 As (half-life T½ = 17.5 days) and 76 As (T1/2 = 26.8 hr) are used in diagnosis and therapy. The use of the isotopes makes possible accurate localization of brain tumors and determination of the type of surgery necessary for their complete removal. Radioactive arsenic is sometimes used in treating diseases of the blood.
The International Commission on Radiological Protection has recommended that the maximum tolerance level for 76 As in the organism be fixed at 11 microcuries. According to the health standards adopted by the USSR, the maximum permissible 76 As concentration is 1 × 10−7 curie per liter in water and open reservoirs and 5 × 10−11 curie per liter in the air surrounding work areas.
All arsenic preparations are highly toxic. Acute poisoning by such substances induces sharp abdominal pain, diarrhea, and kidney damage and may cause convulsions and collapse. The most common symptoms of chronic poisoning are gastrointestinal disorders, catarrhal infections of the respiratory tract (pharyngitis, laryngitis, and bronchitis), skin infections (exanthema, melanosis, and hyperkeratosis), and disruption of sensitivity. Aplastic anemia may develop. Unithiol (Dimercaprol) is the most effective known pharmaceutical used in the treatment of arsenic poisoning.
Preventive measures against occupational poisoning in industry should be primarily directed at the mechanization, hermetization, and dedusting of technological processes, the creation of an effective ventilation system, and the protection of workers from the harmful effects of dust by providing individual safety gear. Regular medical examinations are necessary for workers; new recruits are examined before being hired, and each staff member is given a complete checkup once every six months.
REFERENCESRemy, G. Kurs neorganicheskoi khimii, vol. 1. Moscow, 1963. Pages 700–12. (Translated from German.)
Pogodin, S. A. “Mysh’iak.” In Kratkaia khimicheskaia entsiklopediia, vol. 3. Moscow, 1964.
Vrednye veshchestva v promyshlennosti, 6th ed., part 2. Edited by N. V. Lazarev. Leningrad, 1971.