Silica

Silica
Silica
Are common
Systematic ; name	silicon oxide (IV)
Traditional names	silica
Chem. formula	SiO₂
Thermal properties
T. melt.	1600 ° C
T. Kip.	2950 ° C
Vapor pressure
Classification
Reg. CAS number
Pubchem
Reg. EINECS number
SMILES
Inchi
Codex Alimentarius
RTECS
CHEBI
Chemspider
Security
NFPA 704	0 0 0

Silicon dioxide ( silica , SiO ₂ ; lat. Silica ) - silicon oxide (IV). Colorless crystals with a melting point of + 1713 ... + 1728 ° C, possessing high hardness and strength .

Silicon dioxide is the main component of almost all terrestrial rocks , in particular, kieselguhr . Silica and silicates consist of 87% of the mass of the lithosphere . Silica concentration in human blood and plasma is 0.001% by weight ^[2] .

Properties

It belongs to the group of acid oxides .
When heated, interacts with basic oxides and alkalis.
Molar mass: 60.084 g / mol
Reacts with hydrofluoric acid .
SiO ₂ belongs to the group of glass-forming oxides, that is, it tends to form a supercooled melt - glass .
Dielectric ( electric current does not conduct, if it has no impurities and does not heat up).

Polymorphism

Silicon dioxide has several polymorphic modifications .

The most common of them on the surface of the earth - α- quartz - crystallizes in a trigonal syngony . Under normal conditions, silicon dioxide is most often found in the polymorphic modification of α-quartz, which at a temperature above +573 ° C reversibly turns into β-quartz. With further increase in temperature, quartz turns into tridymite and cristobalite . These polymorphic modifications are stable at high temperatures and low pressures.

In nature, forms are also found - opal , chalcedony , quartzin , lutecyte , authigenic quartz , which belong to the group of silica. Opal (SiO ₂ * nH ₂ O) in thin section is colorless, isotropic , has a negative relief, it is deposited in sea water bodies, it is a part of many siliceous rocks. Chalcedony, quartzin, lutecite - SiO ₂ - are cryptocrystalline varieties of quartz. Form fibrous aggregates, sockets, spherulites, colorless, bluish, yellowish. Some properties differ from each other - in chalcedony and quartzin - direct extinction, in lutecite - oblique, in chalcedony - negative elongation.

At high temperatures and pressures , silicon dioxide is first transformed into coesite (which was synthesized in 1953 by the American chemist Loring Coes), and then into stishovite (which was synthesized in 1961 by S. M. Stishov , and in 1962 was found in meteorite crater) . According to some studies, stishovit composes a significant part of the mantle , so the question of which type of SiO _{2 is} most prevalent on Earth does not yet have a definite answer.

Also has an amorphous modification - quartz glass .

Chemical Properties

Silica SiO ₂ is an acidic oxide that does not react with water.

Chemically resistant to acids, but reacts with gaseous hydrogen fluoride :

{\mathsf {SiO_{2}+4HF\rightarrow SiF_{4}+2H_{2}O}}

{\ displaystyle {\ mathsf {SiO_ {2} + 4HF \ rightarrow SiF_ {4} + 2H_ {2} O}}}

and hydrofluoric acid :

{\mathsf {SiO_{2}+6HF\rightarrow H_{2}[SiF_{6}]+2H_{2}O}}

{\ displaystyle {\ mathsf {SiO_ {2} + 6HF \ rightarrow H_ {2} [SiF_ {6}] + 2H_ {2} O}}}

These two reactions are widely used to melt glass.

When fusing SiO ₂ with alkalis and basic oxides, as well as with carbonates of active metals, silicates are formed - salts that do not have a constant composition of very weak, water-insoluble silicic acids of the general formula xH ₂ O · ySiO ₂ (quite often non-silicic acids are mentioned in the literature, and silicic acid, although in fact we are talking about the same substance).

For example, sodium orthosilicate can be obtained:

{\mathsf {SiO_{2}+4NaOH\rightarrow Na_{4}SiO_{4}+2H_{2}O}}

{\ displaystyle {\ mathsf {SiO_ {2} + 4NaOH \ rightarrow Na_ {4} SiO_ {4} + 2H_ {2} O}}}

Calcium Metasilicate :

{\mathsf {SiO_{2}+CaO\rightarrow CaSiO_{3}}}

{\ displaystyle {\ mathsf {SiO_ {2} + CaO \ rightarrow CaSiO_ {3}}}}

or mixed calcium and sodium silicate:

{\mathsf {Na_{2}CO_{3}+CaCO_{3}+6SiO_{2}\rightarrow Na_{2}CaSi_{6}O_{14}+2CO_{2}}}

{\ displaystyle {\ mathsf {Na_ {2} CO_ {3} + CaCO_ {3} + 6SiO_ {2} \ rightarrow Na_ {2} CaSi_ {6} O_ {14} + 2CO_ {2}}}}

Silicate Na ₂ CaSi ₆ O ₁₄ (Na ₂ O · CaO · 6SiO ₂ ) produce window glass .

Most silicates do not have a constant composition. Of all silicates, only sodium and potassium silicates are soluble in water. Solutions of these silicates in water are called liquid glass. Due to hydrolysis, these solutions are characterized by a strongly alkaline medium. For hydrolyzed silicates, the formation of colloidal solutions is not true. When acidifying solutions of sodium or potassium silicates, a gelatinous white precipitate of hydrated silicic acid precipitates out.

The main structural element of both solid silicon dioxide and all silicates is the [SiO _4/2 ] group, in which the silicon atom Si is surrounded by a tetrahedron of four oxygen atoms O. At the same time, each oxygen atom is connected to two silicon atoms. The [SiO _4/2 ] fragments can be interconnected in different ways. According to the nature of the bond in them, the [SiO _4/2 ] fragments distinguish island, chain, ribbon, layered, frame, and others.

Getting

Synthetic silicon dioxide is obtained by heating silicon to a temperature of + 400 ... + 500 ° C in an oxygen atmosphere, while silicon is oxidized to SiO ₂ dioxide. As well as thermal oxidation at high temperatures.

Under laboratory conditions, synthetic silicon dioxide can be obtained by the action of acids, even weak acetic acid , on soluble silicates. For example:

{\mathsf {Na_{2}SiO_{3}+2CH_{3}COOH\rightarrow 2CH_{3}COONa+H_{2}SiO_{3}\downarrow }}

{\ displaystyle {\ mathsf {Na_ {2} SiO_ {3} + 2CH_ {3} COOH \ rightarrow 2CH_ {3} COONa + H_ {2} SiO_ {3} \ downarrow}}}

silicic acid immediately decomposes into water and SiO ₂ that precipitates .

Natural silica in the form of sand is used where high purity of the material is not required.

Application

Amorphous non-porous silicon dioxide is used in the food industry as an excipient E551 , preventing caking and clumping, in parapharmaceuticals ( toothpastes ), in the pharmaceutical industry as an auxiliary substance (introduced in most pharmacopoeias ), to stabilize suspensions and liniments, as a thickener ointment bases, filler tablets and suppositories. It is part of the composition of filling materials, reduces the hygroscopicity of dry extracts, slows down the release of BAS from various dosage forms; as food additives and sorbent, as well as matrices for creating dosage forms with desired properties - since there is no crystalline structure (amorphous) - safe ^[3] , as well as a food additive or medicine as enterosorbent Polysorb MP with a wide range of applications taking into account the high specific surface sorption (in the range of 300-400 m²) per 1 g of the basic substance.

Silicon dioxide is used in the production of glass , ceramics , abrasives , concrete products , for the production of silicon , as a filler in the production of rubber , in the production of silica- refractory materials, in chromatography, and others.
Quartz crystals have piezoelectric properties and therefore are used in radio engineering , ultrasonic installations, in cigarette lighters .

Artificially produced silica films are used as an insulator in the manufacture of microchips and other electronic components.

Also used to produce fiber optic cables . Pure fused silicon dioxide is used with the addition of some special ingredients.

Silica filament is also used in the heating elements of electronic cigarettes, as it is well absorbed by the liquid and is not destroyed by the heating of the spiral.

Also, silicon dioxide has found the widest application in the tire industry, the production of rubber goods and plastics, the chemical industry, mechanical engineering, and in a number of specific operations:

as a carrier of catalysts and plant protection chemicals;
as sorbents and filtering powders for the regeneration of petroleum products;
as high-quality flux in non-ferrous metallurgy processes;
as a raw material for the production of environmentally friendly glass, glass, and crystal;
as a filler in paper and cardboard to obtain hygienically clean packaging materials for the food industry;
filter powders for beer, oils, juices, matting additives in varnishes and paints;
for the production of silicon carbide in engineering - ceramic engines, parts for aircraft manufacturing complex;
for obtaining crystalline silicon in the electronic and electrical industries, ceramic electrical insulators, fiberglass, fiber optics, superthin fiber;
for the synthesis of artificial zeolites in petrochemistry - oil cracking and so on.

Large transparent quartz crystals are used as semi-precious stones ; colorless crystals are called rock crystal , violet - amethysts , yellow - citrine .

In microelectronics , silicon dioxide is one of the main materials. It is used as an insulating layer, as well as a protective coating. It is obtained in the form of thin films by thermal oxidation of silicon, chemical vapor deposition , magnetron sputtering .

Porous Silica

Porous silica is obtained by various methods.

Silochrome is produced by aggregation of aerosil , which, in turn, is obtained by burning silane ( Si H ₄ ). Silochrome is characterized by high purity, low mechanical strength. The characteristic size of the specific surface is 60–120 m² / g. Used as a sorbent in chromatography , rubber filler, catalysis .

Silica gel is obtained by drying silica gel. In comparison with silochrome, it has a lower purity, but it can have an extremely developed surface: usually from 300 m² / g to 700 m² / g.

Silicon airgel consists of approximately 99.8% of air and can have a density of up to 1.9 kg / m³ (only 1.5 times the density of air).

Toxicity

Although silica itself is considered non-toxic, its dust is toxic and carcinogenic if inhaled (as is asbestos dust). Inhalation of dust leads to bronchitis, inflammation of the lungs ( silicosis ), and cancer.

Notes

↑ http://www.cdc.gov/niosh/npg/npgd0552.html
↑ Silicon dioxide at XuMuK.ru
↑ ed. A. A. Chuiko. Medical chemistry and clinical application of silicon dioxide. - Naukova Dumka,. - Kiev, 2003. - p. 415.

Literature

Knunyants I.L. and others. D. 2 Duffa-Medi // Chemical Encyclopedia. - M .: Soviet Encyclopedia, 1990. - 671 p. - 100 000 copies - ISBN 5-85270-035-5 .
I.Ye. Neymark // Silica gel, its preparation, properties and application. 1973 - Kiev - 200 p.

[_20204154a610b7fc-1] ttp://www.cdc.gov/niosh/npg/npgd0552.html

[2] Silicon dioxide at XuMuK.ru

[3] ed. A. A. Chuiko. Medical chemistry and clinical application of silicon dioxide. - Naukova Dumka,. - Kiev, 2003. - p. 415.

Silica


Are common
Systematic name	silicon oxide (IV)
Traditional names	silica
Chem. formula	SiO₂
Thermal properties
T. melt.	1600 ° C
T. Kip.	2950 ° C
Vapor pressure
Classification
Reg. CAS number
Pubchem
Reg. EINECS number
SMILES
Inchi
Codex Alimentarius
RTECS
CHEBI
Chemspider
Security
NFPA 704	0 0 0