Hexafluorosilicic acid
Names | |
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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
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100.037.289 |
EC Number |
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PubChem CID
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RTECS number
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UNII | |
UN number | 1778 |
CompTox Dashboard (EPA)
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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 | c. 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)
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1.3465 |
Structure | |
Octahedral SiF62− | |
Hazards | |
GHS labelling: | |
Danger | |
H314 | |
P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501 | |
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 anions
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Hexafluorotitanic acid Hexafluorozirconic acid |
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
2SiF
6. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.
Hexafluorosilicic acid is produced naturally on a large scale in volcanoes.
Structure
Hexafluorosilicic acid has been crystallized as various hydrates. These include (H5O2)2SiF6, the more complicated (H5O2)2SiF6·2H2O, and (H5O2)(H7O3)SiF6·4.5H2O. In all of these salts, the octahedral hexafluorosilicate anion is hydrogen bonded to the cations.[3]
Aqueous solutions of hexafluorosilicic acid are often described as H
2SiF
6.
Production and principal reactions
Hexafluorosilicic acid is produced commercially from fluoride-containing minerals that also contain silicates. Specifically, apatite and fluorapatite are treated with sulfuric acid to give phosphoric acid, a precursor to several water-soluble fertilizers. This is called the wet phosphoric acid process.[4] As a by-product, approximately 50 kg of hexafluorosilicic acid is produced per tonne of HF owing to reactions involving silica-containing mineral impurities.[5]: 3
Some of the hydrogen fluoride (HF) produced during this process in turn reacts with silicon dioxide (SiO2) impurities, which are unavoidable constituents of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF.[citation needed] The net process can be described as:[6][page needed]
- 6 HF + SiO2 → SiF2−6 + 2 H3O+
Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.[6]
Reactions
Hexafluorosilic acid is only stable in hydrogen fluoride or acidic aqueous solutions. In any other circumstance, it acts as a source of hydrofluoric acid. Thus, for example, hexafluorosilicic acid pure or in oleum solution evolves silicon tetrafluoride until the residual hydrogen fluoride re-establishes equilibrium:[6]
- H2SiF6 ⇌ 2 HF(l) + SiF4(g)
In alkaline-to-neutral aqueous solutions, hexafluorosilicic acid readily hydrolyzes to fluoride anions and amorphous,
- SiF2−
6 + 2 H2O → 6 F− + SiO2 + 4 H+
Alkali and alkaline earth salts
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. At room temperature 15-30% concentrated hexafluorosilicic acid undergoes similar reactions with chlorides, hydroxides, and carbonates of alkali and alkaline earth metals.[8]
- 2NaCl + H2SiF6 Na2SiF6↓ + 2 HCl
- BaCl2 + H2SiF6 BaSiF6↓ + 2 HCl
Heating
- Na2SiF6 SiF4 + 2 NaF
Uses
The majority of the hexafluorosilicic acid is converted to aluminium fluoride and synthetic cryolite. These materials are central to the conversion of aluminium ore into aluminium metal. The conversion to aluminium trifluoride is described as:[6]
- H2SiF6 + Al2O3 → 2 AlF3 + SiO2 + H2O
Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt, Potassium fluorosilicate, 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 and the salts are used as wood preservation agents.[10]
Lead refining
Hexafluorosilicic acid is also used as an electrolyte in the Betts electrolytic process for refining lead.
Rust removers
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, which are essentially varieties of laundry sour.
Niche applications
H2SiF6 is a specialized
Treating concrete
The application of hexafluorosilica acid to a calcium rich surface such as concrete will give that surface some resistance to acid attack.[12]
- CaCO3 + H2O → Ca2+ + 2 OH− + CO2
- H2SiF6 → 2 H+ + SiF2−
6 - SiF2−
6 + 2 H2O → 6 F− + SiO2 + 4 H+ - Ca2+ + 2 F− → CaF2
Calcium fluoride (CaF2) is an insoluble solid that is acid resistant.
Natural salts
Some rare minerals, encountered either within volcanic or coal-fire fumaroles, are salts of the hexafluorosilicic acid. Examples include ammonium hexafluorosilicate that naturally occurs as two polymorphs: cryptohalite and bararite.[13][14][15]
Safety
Hexafluorosilicic acid can release
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.
- ^ .
- ^ USGS. Fluorspar.
- ^ a b c d "Sodium Hexafluorosilicate [CASRN 16893-85-9] and Fluorosilicic Acid [CASRN 16961-83-4] Review of Toxicological Literature" (PDF). National Toxicology Program (U.S.). Archived (PDF) from the original on 22 October 2012. Retrieved 13 July 2017.
- ^ ISBN 978-3527306732.
- PMID 16683594.
- .
- ^ a b Us Granted A345458, Keith, C. Hansen & L. Yaws, Carl, "Patent Silicon tetrafluoride generation", published January 3, 1982, issued 1982
- S2CID 9672269.
- ISBN 0471936235.
- ^ Properties of Concrete by A M Neville
- ^ "Cryptohalite".
- ^ "Bararite".
- S2CID 202563638.
- ^ "Fluorosilicic Acid – International Chemical Safety Cards". NIOSH. Retrieved 2015-03-10.