Hexafluorosilicic acid
Encyclopedia
Hexafluorosilicic acid is the inorganic compound
with the formula H2SiF6. It is a product of the production of hydrogen fluoride and the production of phosphate fertilizers. The majority of the hexafluorosilicic acid is used for the production of aluminium
metal. Hexafluorosilicic acid is also commonly used for water fluoridation
.
− ) in an aqueous solution or other solvents that contain strong proton donors at low pH (acids described similarly include chloroplatinic acid, fluoroboric acid
, and hexafluorophosphoric acid
, and, more commonly, carbonic acid
). Distillation of hexafluorosilicic acid solutions produces no molecules of H2SiF6; instead the vapor consists of HF
, SiF4
, and water. Aqueous solutions of H2SiF6 contain the hexafluorosilicate anion, SiF62− and protonated water. In this octahedral anion, the Si-F bond distances are 1.71 Å
.
is produced from fluorspar by treatment with sulfuric acid. As a by product, approximately 50 kg of H2SiF6 is produced per tonne of HF owing to reactons involving silica-containing mineral impurities. H2SiF6 is also produced as a by-product from the production of phosphoric acid from apatite
and fluorapatite
. Again, some of the HF in turn reacts with silicate minerals, which are an unavoidable constituent of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF. The net process can be described as:
Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride and hydrofluoric acid.
Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:
The resulting salt Na2SiF6 is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications. With excess base, the hexafluorosilicate undergoes hydrolysis, so the neutralization of the hexafluorosilicic acid must guard against this easy hydrolysis reaction:
and cryolite
. These materials are central to the conversion of aluminium ore into aluminium
metal. The conversion to aluminium trifluoride is described as:
Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt is used in the production of porceleins, the magnesium salt for hardened concretes and as an insecticide, and the barium salts for phosphors.
Hexafluorosilicic acid is also commonly used for water fluoridation
in several countries including the United States, Great Britain, and Ireland. In the U.S., about 40,000 tons of fluorosilic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride.
Hexafluorosilicic acid is also used as an electrolyte in the Betts
Electrowinning process for refining Lead.
in organic synthesis
for cleaving Si-O bonds of silyl ethers. It is more reactive for this purpose than HF. It reacts faster with t-butyldimethysilyl (TBDMS) ethers than triisopropylsilyl (TIPS) ethers.
Hexafluorosilicic acid and the salts are used as wood preservation agents.
when evaporated, so it has similar risks. It is corrosive
and may cause fluoride poisoning
; inhalation of the vapors may cause lung edema. Like hydrogen fluoride
, it attacks glass and stoneware
.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula H2SiF6. It is a product of the production of hydrogen fluoride and the production of phosphate fertilizers. The majority of the hexafluorosilicic acid is used for the production of aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
metal. Hexafluorosilicic acid is also commonly used for water fluoridation
Water fluoridation
Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay. Fluoridated water has fluoride at a level that is effective for preventing cavities; this can occur naturally or by adding fluoride...
.
Nature of hexafluorosilicic acid
Like several related compounds, hexafluorosilicic acid does not exist as a discrete species; that is, a material with the formula H2SiF6 has not been isolated. Hexafluorosilicic acid refers to an equilibrium mixture with hexafluorosilicate anion (SiF62Fluoroboric acid
Fluoroboric acid is the chemical compound with the formula HBF4. It is the conjugate acid of tetrafluoroborate. It is available commercially as a solution in water and other solvents such as diethyl ether...
, and hexafluorophosphoric acid
Hexafluorophosphoric acid
Hexafluorophosphoric acid is the chemical compound with the chemical formula HPF6. This strong Brønsted acid features a non-coordinating anion, hexafluorophosphate . It is formed from the reaction of hydrogen fluoride with phosphorus pentafluoride....
, and, more commonly, carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...
). Distillation of hexafluorosilicic acid solutions produces no molecules of H2SiF6; instead the vapor consists of HF
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
, SiF4
Silicon tetrafluoride
Silicon tetrafluoride or Tetrafluorosilane is the chemical compound with the formula SiF4. This tetrahedral molecule is notable for having a remarkably narrow liquid range...
, and water. Aqueous solutions of H2SiF6 contain the hexafluorosilicate anion, SiF62
Å
Å represents various sounds in several languages. Å is part of the alphabets used for the Alemannic and the Bavarian-Austrian dialects of German...
.
Production and principal reactions
The commodity chemical Hydrogen fluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
is produced from fluorspar by treatment with sulfuric acid. As a by product, approximately 50 kg of H2SiF6 is produced per tonne of HF owing to reactons involving silica-containing mineral impurities. H2SiF6 is also produced as a by-product from the production of phosphoric acid from apatite
Apatite
Apatite is a group of phosphate minerals, usually referring to hydroxylapatite, fluorapatite, chlorapatite and bromapatite, named for high concentrations of OH−, F−, Cl− or Br− ions, respectively, in the crystal...
and fluorapatite
Fluorapatite
Fluorapatite, often with the alternate spelling of fluoroapatite, is a mineral with the formula Ca53F . Fluorapatite is a hard crystalline solid. Although samples can have various color , the pure mineral is colorless as expected for a material lacking transition metals...
. Again, some of the HF in turn reacts with silicate minerals, which are an unavoidable constituent of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF. The net process can be described as:
- SiO2 + 6 HF → H2SiF6 + 2 H2O
Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride and hydrofluoric acid.
Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:
- H2SiF6 + 2 NaOH → Na2SiF6 + 2 H2O
The resulting salt Na2SiF6 is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications. With excess base, the hexafluorosilicate undergoes hydrolysis, so the neutralization of the hexafluorosilicic acid must guard against this easy hydrolysis reaction:
- Na2SiF6 + 4 NaOH → 6 NaF + SiO2 + 2 H2O
Uses
The majority of the hexafluorosilicic acid is converted to aluminium fluorideAluminium fluoride
Aluminium fluoride is an inorganic compound used primarily in the production of aluminium. This colourless solid can be prepared synthetically but also occurs in nature.-Production and occurrence:...
and cryolite
Cryolite
Cryolite is an uncommon mineral identified with the once large deposit at Ivigtût on the west coast of Greenland, depleted by 1987....
. These materials are central to the conversion of aluminium ore into aluminium
Aluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
metal. The conversion to aluminium trifluoride is described as:
- H2SiF6 + Al2O3 → 2 AlF3 + SiO2 + H2O
Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt is used in the production of porceleins, the magnesium salt for hardened concretes and as an insecticide, and the barium salts for phosphors.
Hexafluorosilicic acid is also commonly used for water fluoridation
Water fluoridation
Water fluoridation is the controlled addition of fluoride to a public water supply to reduce tooth decay. Fluoridated water has fluoride at a level that is effective for preventing cavities; this can occur naturally or by adding fluoride...
in several countries including the United States, Great Britain, and Ireland. In the U.S., about 40,000 tons of fluorosilic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride.
Hexafluorosilicic acid is also used as an electrolyte in the Betts
Betts
Betts is an English surname, and may refer to* Blair Betts , Canadian ice hockey player* Clive Betts , British politician* Daisy Betts, Australian actress* Denis Betts , English rugby league player...
Electrowinning process for refining Lead.
Niche applications
H2SiF6 is a specialized reagentReagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...
in organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
for cleaving Si-O bonds of silyl ethers. It is more reactive for this purpose than HF. It reacts faster with t-butyldimethysilyl (TBDMS) ethers than triisopropylsilyl (TIPS) ethers.
Hexafluorosilicic acid and the salts are used as wood preservation agents.
Safety
Hexafluorosilicic acid releases hydrogen fluorideHydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
when evaporated, so it has similar risks. It is corrosive
Corrosive
A corrosive substance is one that will destroy or irreversibly damage another surface or substance with which it comes into contact. The main hazards to people include damage to the eyes, the skin, and the tissue under the skin; inhalation or ingestion of a corrosive substance can damage the...
and may cause fluoride poisoning
Fluoride poisoning
In high concentrations, soluble fluoride salts are toxic and skin or eye contact with high concentrations of many fluoride salts is dangerous. Referring to a common salt of fluoride, sodium fluoride , the lethal dose for most adult humans is estimated at 5 to 10 g...
; inhalation of the vapors may cause lung edema. Like hydrogen fluoride
Hydrogen fluoride
Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...
, it attacks glass and stoneware
Stoneware
Stoneware is a vitreous or semi-vitreous ceramic ware with a fine texture. Stoneware is made from clay that is then fired in a kiln, whether by an artisan to make homeware, or in an industrial kiln for mass-produced or specialty products...
.