Hexafluorosilicic acid: Difference between revisions
Updated with neurological study about the toxicity of fluoridated water. |
Not really a "dilute solution" at all, as the hexafluorosilicate rapidly and completely decomposes to ordinary fluoride at low concentrations. |
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'''Hexafluorosilicic acid''' is an [[inorganic compound]] with the [[chemical formula]] {{Chem|(H|3|O)|2|SiF|6}} (also written as {{Chem|(H|3|O)|2|[SiF|6|]}}). It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is produced naturally on large scale in volcanoes.<ref name="Palache">Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.</ref><ref name="Anthony">Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson. |
'''Hexafluorosilicic acid''' is an [[inorganic compound]] with the [[chemical formula]] {{Chem|(H|3|O)|2|SiF|6}} (also written as {{Chem|(H|3|O)|2|[SiF|6|]}}). It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is produced naturally on large scale in volcanoes.<ref name="Palache">Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.</ref><ref name="Anthony">Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson. |
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*[http://www.handbookofmineralogy.org/pdfs/BARARITE.pdf link to bararite] |
*[http://www.handbookofmineralogy.org/pdfs/BARARITE.pdf link to bararite] |
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*[http://www.handbookofmineralogy.org/pdfs/CRYPTOHALITE.pdf link to cryptohalite]</ref> It is manufactured as a precursor to [[aluminum trifluoride]] and [[sodium hexafluoroaluminate|synthetic cryolite]]. It is commonly used as a source of [[fluoride]] for [[water fluoridation]].<ref name=www.cdc.gov>{{cite web|url=http://www.cdc.gov/fluoridation/factsheets/engineering/wfadditives.htm |title=CDC - Water Fluoridation Additives - Engineering Fact Sheet - Community Water Fluoridation - Oral Health |publisher=Cdc.gov |date= |accessdate=2015-03-10}}</ref><ref name="The New Zealand Institute of Chemistry (NZIC)">The New Zealand Institute of Chemistry (NZIC) - Hydrofluorosilic acid and water fluoridation [http://nzic.org.nz/ChemProcesses/production/1C.pdf hydrofluorosilic acid].</ref> |
*[http://www.handbookofmineralogy.org/pdfs/CRYPTOHALITE.pdf link to cryptohalite]</ref> It is manufactured as a precursor to [[aluminum trifluoride]] and [[sodium hexafluoroaluminate|synthetic cryolite]]. It is commonly used as a source of [[fluoride]] for [[water fluoridation]].<ref name=www.cdc.gov>{{cite web|url=http://www.cdc.gov/fluoridation/factsheets/engineering/wfadditives.htm |title=CDC - Water Fluoridation Additives - Engineering Fact Sheet - Community Water Fluoridation - Oral Health |publisher=Cdc.gov |date= |accessdate=2015-03-10}}</ref><ref name="The New Zealand Institute of Chemistry (NZIC)">The New Zealand Institute of Chemistry (NZIC) - Hydrofluorosilic acid and water fluoridation [http://nzic.org.nz/ChemProcesses/production/1C.pdf hydrofluorosilic acid].</ref> Salts derived from hexafluorosilicic acid are called '''hexafluorosilicates'''. |
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In aqueous solution, the [[hydronium]] cation (H<sub>3</sub>O<sup>+</sup>) is traditionally equated with a solvated proton, and as such, the formula for this compound is often written as {{Chem|H|2|SiF|6}}. Extending that metaphor, the isolated compound is then written as {{Chem|H|2|SiF|6|·2H|2|O}}. |
In aqueous solution, the [[hydronium]] cation (H<sub>3</sub>O<sup>+</sup>) is traditionally equated with a solvated proton, and as such, the formula for this compound is often written as {{Chem|H|2|SiF|6}}. Extending that metaphor, the isolated compound is then written as {{Chem|H|2|SiF|6|·2H|2|O}}. |
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Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt is used in the production of porcelains, the magnesium salt for hardened concretes and as an insecticide, and the barium salts for phosphors. |
Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt is used in the production of porcelains, the magnesium salt for hardened concretes and as an insecticide, and the barium salts for phosphors. |
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Hexafluorosilicic acid is also commonly used for [[water fluoridation]] in several countries including the United States, the United Kingdom, and the Republic of Ireland.<!-- is this peculiar to the United States? If not, needs to be generalized--><!-- I did not locate a comprehensive list --> In the U.S., about 40,000 tons of fluorosilicic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride.<ref name=USGS/> In this application, the hexafluorosilicic acid |
Hexafluorosilicic acid is also commonly used for [[water fluoridation]] in several countries including the United States, the United Kingdom, and the Republic of Ireland.<!-- is this peculiar to the United States? If not, needs to be generalized--><!-- I did not locate a comprehensive list --> In the U.S., about 40,000 tons of fluorosilicic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride.<ref name=USGS/> In this application, the hexafluorosilicic acid decomposes to the [[fluoride]] ion (F<sup>−</sup>) – which is the active agent for the protection of teeth – and inert [[silicon dioxide]] (SiO<sub>2</sub>). |
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Hexafluorosilicic acid is also used as an electrolyte in the [[Betts electrolytic process]] for refining lead. |
Hexafluorosilicic acid is also used as an electrolyte in the [[Betts electrolytic process]] for refining lead. |
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Revision as of 06:34, 5 January 2017
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| Names | |
|---|---|
| Preferred IUPAC name
Hexafluorosilicic acid | |
| Systematic IUPAC name
Dihydrogen hexafluorosilicate | |
| Other names
Fluorosilicic acid, fluosilic acid, hydrofluorosilicic acid, silicofluoride, silicofluoric acid, oxonium hexafluorosilanediuide, oxonium hexafluoridosilicate(2−)
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| Identifiers | |
3D model (JSmol)
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| ChemSpider | |
| ECHA InfoCard | 100.037.289 |
| EC Number |
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PubChem CID
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| RTECS number |
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| UN number | 1778 |
CompTox Dashboard (EPA)
|
|
| |
| |
| Properties | |
| F6H2Si | |
| Molar mass | 144.091 g·mol−1 |
| Appearance | transparent, colorless, fuming liquid |
| Odor | sour, pungent |
| Density | 1.22 g/cm3 (25% soln.) 1.38 g/cm3 (35% soln.) 1.46 g/cm3 (61% soln.) |
| Melting point | ca. 19 °C (66 °F; 292 K) (60–70% solution) < −30 °C (−22 °F; 243 K) (35% solution) |
| Boiling point | 108.5 °C (227.3 °F; 381.6 K) (decomposes) |
| miscible | |
Refractive index (nD)
|
1.3465 |
| Structure | |
| Octahedral SiF62− | |
| Hazards | |
| NFPA 704 (fire diamond) | |
| Flash point | Non-flammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose)
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430 mg/kg (oral, rat) |
| Safety data sheet (SDS) | External MSDS |
| Related compounds | |
Other cations
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Ammonium hexafluorosilicate |
Related compounds
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Hexafluorophosphoric acid Fluoroboric acid |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Hexafluorosilicic acid is an inorganic compound with the chemical formula (H
3O)
2SiF
6 (also written as (H
3O)
2[SiF
6]). It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is produced naturally on large scale in volcanoes.[1][2] It is manufactured as a precursor to aluminum trifluoride and synthetic cryolite. It is commonly used as a source of fluoride for water fluoridation.[3][4] Salts derived from hexafluorosilicic acid are called hexafluorosilicates.
In aqueous solution, the hydronium cation (H3O+) is traditionally equated with a solvated proton, and as such, the formula for this compound is often written as H
2SiF
6. Extending that metaphor, the isolated compound is then written as H
2SiF
6·2H
2O.
Structure
Hexafluorosilicic acid is generally assumed to consist of oxonium ions charge balanced by hexafluorosilicate dianions as well as water. The situation is similar to that for chloroplatinic acid, fluoroboric acid, and hexafluorophosphoric acid. Hexafluorosilicate is an octahedral anion, the Si–F bond distances are 1.71 Å.[5] Hexafluorosilicic acid is only available commercially as solution.[6]
Production and principal reactions
The commodity chemical hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid.[7] As a by product, approximately 50 kg of (H3O)2SiF6 is produced per tonne of HF owing to reactions involving silica-containing mineral impurities. (H3O)2SiF6 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:[8]
- SiO
2 + 6 HF → SiF2−
6 + 2 H
3O+
Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.
Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:
- (H3O)2SiF6 + 2 NaOH → Na2SiF6 + 4 H2O
The resulting salt Na2SiF6 is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications.
Near neutral pH, hexafluorosilicate salts hydrolyze rapidly according to this equation:[9]
- SiF2−
6 + 2 H2O → 6 F− + SiO2 + 4 H+
Uses
The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite.[8] These materials are central to the conversion of aluminium ore into aluminium 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 porcelains, 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, the United Kingdom, and the Republic of Ireland. In the U.S., about 40,000 tons of fluorosilicic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride.[7] In this application, the hexafluorosilicic acid decomposes to the fluoride ion (F−) – which is the active agent for the protection of teeth – and inert silicon dioxide (SiO2).
Hexafluorosilicic acid is also used as an electrolyte in the Betts electrolytic process for refining lead.
Hexafluorosilicic acid (identified as hydrofluorosilicic acid on the label) along with oxalic acid are the active ingredients used in Iron Out rust-removing cleaning products.
Niche applications
H2SiF6 is a specialized reagent 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.[10]
Hexafluorosilicic acid and the salts are used as wood preservation agents.[11]
Safety
Hexafluorosilicic acid can release hydrogen fluoride when evaporated, so it has similar risks. Inhalation of the vapors may cause lung edema. Like hydrogen fluoride, it attacks glass and stoneware.[12] The LD50 value of hexafluorosilicic acid is 430 mg/kg.[13]
See also
References
- ^ Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.
- ^ Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson.
- ^ "CDC - Water Fluoridation Additives - Engineering Fact Sheet - Community Water Fluoridation - Oral Health". Cdc.gov. Retrieved 2015-03-10.
- ^ The New Zealand Institute of Chemistry (NZIC) - Hydrofluorosilic acid and water fluoridation hydrofluorosilic acid.
- ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5.
- ^ J. P. Nicholson (2005). "Electrodeposition of Silicon from Nonaqueous Solvents". J. Electrochem. Soc. 152 (12): C795–C802. doi:10.1149/1.2083227.
- ^ a b USGS. Fluorspar.
- ^ a b Aigueperse, J.; Mollard, P.; Devilliers, D.; Chemla, M.; Faron, R.; Romano, R.; Cuer, J. P. (2005). "Fluorine Compounds, Inorganic". Ullmann’s Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307.
- ^ Finney, William F.; Wilson, Erin; Callender, Andrew; Morris, Michael D.; Beck, Larry W. (2006). "Reexamination of Hexafluorosilicate Hydrolysis by 19F NMR and pH Measurement". Environ. Sci. Technol. 40 (8): 2572–2577. doi:10.1021/es052295s.
- ^ Pilcher, A. S.; DeShong, P. (2001). "Fluorosilicic Acid". Encyclopedia of Reagents for Organic Synthesis. John Wiley & Sons. doi:10.1002/047084289X.rf013.
- ^ Carsten Mai, Holger Militz (2004). "Modification of wood with silicon compounds. inorganic silicon compounds and sol-gel systems: a review". Wood Science and Technology. 37 (5): 339. doi:10.1007/s00226-003-0205-5.
- ^ "CDC – Fluorosilicic Acid – International Chemical Safety Cards - NIOSH". Cdc.gov. Retrieved 2015-03-10.
- ^ [1] Archived October 22, 2012, at the Wayback Machine