What is arsenic? Characteristics, properties and applications

Arsenic is a chemical element of the nitrogen group (group 15 of the periodic table). This is a gray, metallic glitter brittle substance (α-arsenic) with a rhombohedral crystal lattice. When heated to 600 ° C, As sublimes. When the vapor is cooled, a new modification arises - yellow arsenic. Above 270 ° C all As forms pass into black arsenic.

History of the discovery

About what arsenic was known long before its recognition as a chemical element. In the IV century. BC. E. Aristotle mentioned a substance called "sandarak", which is now believed to be a real or arsenic sulphide. And in the I century AD. E. The writers Pliny the Elder and Pedanius Dioscorides described the auripigment, the As ₂ S ₃ dye. In the XI century. N. E. Three types of "arsenic" were distinguished: white (As ₄ O ₆ ), yellow (As ₂ S ₃ ) and red (As ₄ S ₄ ). The element itself probably was first isolated in the 13th century by Albert the Great, who noted the appearance of a metal-like substance, when the arsenicum, another name As ₂ S ₃ , was heated with soap. But there is no certainty that this natural scientist has received pure arsenic. The first true certificate of the isolation of a pure chemical element is dated 1649. German pharmacist Johann Schroeder prepared arsenic by heating its oxide in the presence of coal. Later Nikola Lemery, a French physician and chemist, observed the formation of this chemical element by heating a mixture of its oxide, soap and potash. By the beginning of the 18th century, arsenic was already known as a unique semimetal.

Prevalence

In the earth's crust, the concentration of arsenic is small and is 1.5 ppm. It is found in soil and minerals and can get into the air, water and soil due to wind and water erosion. In addition, the element enters the atmosphere from other sources. As a result of volcanic eruptions, about 3,000 tons of arsenic are released into the air per year, microorganisms form 20,000 tons of volatile methylarsine per year, and as a result of fossil fuel combustion 80,000 tons are released during the same period.

Despite the fact that As is a deadly poison, it is an important component of the nutrition of some animals and, possibly, a person, although the required dose does not exceed 0.01 mg / day.

Arsenic is extremely difficult to translate into a water-soluble or volatile state. The fact that it is quite mobile means that large concentrations of matter in one place can not appear. On the one hand, it's good, but on the other - the ease with which it spreads, is the reason that arsenic contamination is becoming an increasing problem. Because of human activity, mainly due to mining and smelting, usually the immobile chemical element migrates, and now it can be found not only in places of its natural concentration.

The amount of arsenic in the earth's crust is about 5 g per ton. In space, its concentration is estimated as 4 atoms per million silicon atoms. This element is widely distributed. A small amount of it is present in the native state. As a rule, arsenic formations with the purity of 90-98% are found together with such metals as antimony and silver. Most of it, however, is part of more than 150 different minerals - sulphides, arsenides, sulphoarsenides and arsenites. Arsenopyrite FeAsS is one of the most common As-containing minerals. Other common arsenic compounds are the real As ₄ S ₄ minerals _{, the} As ₂ S ₃ auripigment _{, the} lelligite of FeAs ₂ and the enargite of Cu ₃ AsS ₄ . Arsenic oxide is also common. Most of this substance is a by-product of the smelting of copper, lead, cobalt and gold ores.

In nature, there is only one stable isotope of arsenic - ⁷⁵ As. Among the artificial radioactive isotopes, ⁷⁶ As is discerned with a half-life of 26.4 hours. Arsenic-72, -74 and -76 are used in medical diagnostics.

Industrial production and application

Metallic arsenic is produced by heating arsenopyrite to 650-700 ° C without air access. If arsenopyrite and other metallic ores are heated with oxygen, As easily enters into the compound with it, forming an easily sublimated As ₄ O ₆ , also known as "white arsenic." The oxide vapors are collected and condensed, and later purified by re-distillation. Most of As is produced by its reduction with carbon from the white arsenic obtained in this way.

The global consumption of metallic arsenic is relatively small - only a few hundred tons per year. Most of what is consumed comes from Sweden. It is used in metallurgy because of its metalloid properties. About 1% of arsenic is used in the production of lead shot, as it improves the roundness of the molten drop. Properties of lead-based bearing alloys improve both in thermal and mechanical properties, when they contain about 3% of arsenic. The presence of a small amount of this chemical element in lead alloys hardens them for use in rechargeable batteries and cable armor. Small admixtures of arsenic increase the corrosion resistance and thermal properties of copper and brass. In its pure form, the chemical elemental As is used for coating bronze and pyrotechnics. Highly purified arsenic is used in semiconductor technology, where it is used with silicon and germanium, as well as in the form of gallium arsenide (GaAs) in diodes, lasers and transistors.

Compounds As

Since the valence of arsenic is 3 and 5, and it has a number of degrees of oxidation from -3 to +5, the element can form various kinds of compounds. The most important commercial value is its oxides, the main forms of which are As ₄ O ₆ and As ₂ O ₅ . Arsenious oxide, commonly known as white arsenic, is a by-product of roasting copper, lead and some other metals, as well as arsenopyrite and sulphide ores. It is the starting material for most other compounds. In addition, it is used in pesticides, serves as a decolorant in the production of glass and a preservative for the skin. Arsenic pentoxide is formed by the action of an oxidizing agent (eg nitric acid) on white arsenic. It is the main ingredient of insecticides, herbicides and glue for metal.

Arsin (AsH ₃ ), a colorless poisonous gas consisting of arsenic and hydrogen, is another known substance. A substance, also called arsenic, is produced by hydrolysis of metal arsenides and reduction of metals from arsenic compounds in acid solutions. It has found application as a doping additive in semiconductors and a warfare poison gas. In agriculture, arsenic acid (H ₃ AsO ₄ ), lead arsenate (PbHAsO ₄ ) and calcium arsenate [Ca ₃ (AsO ₄ ) ₂ ], which are used for soil sterilization and pest control, are of great importance.

Arsenic is a chemical element that forms a variety of organic compounds. Kakodin (CH ₃ ) ₂ As-As (CH ₃ ) ₂ , for example, is used in the preparation of a widely used desiccant (drying agent) - cacodyl acid. Complex organic compounds of the element are used in the treatment of certain diseases, for example, amoebic dysentery caused by microorganisms.

Physical properties

What is arsenic in terms of its physical properties? In the most stable state, it is a brittle solid of steel gray with low thermal and electrical conductivity. Although some forms of As are metal-like, referring it to non-metals is a more accurate characteristic of arsenic. There are other types of arsenic, but they are not very well studied, especially the yellow metastable form, consisting of As ₄ molecules, like the white phosphorus P ₄ . Arsenic is sublimated at a temperature of 613 ° C, and as a vapor it exists as As ₄ molecules, which do not dissociate to a temperature of about 800 ° C. Complete dissociation into As ₂ molecules occurs at 1700 ° C.

The structure of the atom and the ability to form bonds

The electronic arsenic formula - 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 3d ¹⁰ 4s ² 4p ^{3 -} resembles nitrogen and phosphorus in that there are five electrons in the outer shell, but it differs from them by the presence of 18 electrons in the penultimate shell instead of two or Eight. Adding 10 positive charges in the core during the filling of five 3d orbitals often causes a general decrease in the electron cloud and an increase in the electronegativity of the elements. Arsenic in the periodic table can be compared with other groups that clearly demonstrate this pattern. For example, it is generally accepted that zinc is more electronegative than magnesium, and gallium is more than aluminum. However, in the subsequent groups this difference decreases, and many disagree that germanium is electronegative than silicon, despite the abundance of chemical evidence. Such a transition from an 8- to 18-element membrane from phosphorus to arsenic may increase electronegativity, but this remains controversial.

The similarity of the outer shell As and P suggests that they can form 3 covalent bonds per atom in the presence of an additional unconnected electron pair. The degree of oxidation must therefore be +3 or -3, depending on the relative mutual electronegativity. The arsenic structure also indicates the possibility of using an external d-orbitals to expand the octet, which allows the element to form 5 bonds. It is realized only when reacting with fluorine. The presence of a free electron pair for the formation of complex compounds (via electron donation) in the As atom manifests itself much less than in phosphorus and nitrogen.

Arsenic is stable in dry air, but in wet it is covered with black oxide. Its vapor is easily burned, forming As ₂ O ₃ . What is arsenic in a free state? It is practically not affected by water, alkalis and non-oxidizing acids, but oxidized by nitric acid to the state of +5. Arsenic reacts halogens, sulfur, and many metals form arsenides.

Analytical chemistry

The arsenic substance can be detected qualitatively in the form of a yellow auripigment, precipitated by a 25% solution of hydrochloric acid. Traces of As, as a rule, are determined by converting it into arsine, which can be detected using the Marsh test. Arsin is thermally decomposed, forming a black mirror from arsenic inside a narrow tube. According to the Gutzait method, a probe impregnated with mercury chloride under the action of Arsin darkens due to the release of mercury.

Toxicological characteristics of arsenic

The toxicity of the element and its derivatives varies widely, ranging from the extremely poisonous arsine and its organic derivatives to just As, which is relatively inert. What is arsenic is said to be the use of its organic compounds as war poisonous substances (lewisite), vesicant and defoliant ("Agent blu" based on an aqueous mixture of 5% cacodyl acid 26% of its sodium salt).

In general, the derivatives of this chemical element irritate the skin and cause dermatitis. Protection against inhalation of arsenic dust is also recommended, but most of the poisoning occurs when it is ingested. The maximum permissible concentration of As in the dust for an eight-hour working day is 0.5 mg / m ³ . For arsine, the dose is reduced to 0.05 parts per million. In addition to using compounds of this chemical element as herbicides and pesticides, the use of arsenic in pharmacology made it possible to obtain salvarsan, the first successful drug against syphilis.

Impact on health

Arsenic is one of the most toxic elements. Inorganic compounds of this chemical in natural conditions occur in small quantities. People can be exposed to arsenic through food, water and air. Exposure can also occur when skin contacts with contaminated soil or water.

The content of arsenic in food is quite low. However, its levels in fish and seafood can be very high, as they absorb this chemical element from the water in which they live. A significant amount of inorganic arsenic in fish can be dangerous for human health.

The impact of the substance is also affected by people who work with it, live in houses built from wood processed by them, and on agricultural land, where pesticides were used in the past.

Inorganic arsenic can cause various effects on human health, such as irritation of the stomach and intestines, reduced production of red and white blood cells, skin changes and lung irritation. It is believed that the absorption of a significant amount of this substance can increase the chances of developing cancer, especially cancer of the skin, lungs, liver and lymphatic system.

Very high concentrations of inorganic arsenic are the cause of infertility and miscarriages in women, dermatitis, reducing the body's resistance to infections, heart problems and brain damage. In addition, this chemical element can damage DNA.

The lethal dose of white arsenic is 100 mg.

Organic compounds of the element neither cancer nor damage to the genetic code cause, but high doses can harm human health, for example, cause nerve disorders or abdominal pain.

Properties of As

The main chemical-physical properties of arsenic are as follows:

• The atomic number is 33.
• The atomic weight is 74.9216.
• The melting point of the gray form is 814 ° C at a pressure of 36 atmospheres.
• The density of the gray form is 5.73 g / cm ³ At 14 ° C.
• The density of the yellow form is 2.03 g / cm ³ at 18 ° C.
• The electronic formula of arsenic is 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 3d ¹⁰ 4s ² 4p ³ _.
• The oxidation state is -3, +3, +5.
• Valence of arsenic - 3, 5.