|Jmol-3D images||Image 1
|Molar mass||144.09 g mol−1|
|Appearance||transparent, colorless, fuming liquid|
|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)|
|Solubility in water||miscible|
|Refractive index (nD)||1.3465|
|Molecular shape||Octahedral SiF62−|
|EU classification|| T - Toxic
C - Corrosive
|S-phrases||(S1/2), S26, S27, S45|
|LD50||430 mg/kg (oral, rat)|
|Other cations||Ammonium hexafluorosilicate
|Related compounds||Hexafluorophosphoric acid
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
Hexafluorosilicic acid (systematically named oxonium hexafluorosilanediuide and oxonium hexafluoridosilicate(2−)) is an inorganic compound with the chemical formula (H
6 (also written (H
6] or SiH
6). It can also be written as H
6 with the hydrogen rather than the oxonium cation. It is commonly used as a source of fluoride for water fluoridation.  
Like several related compounds, hexafluorosilicic acid has not been isolated. Hexafluorosilicic acid is only available as an equilibrium mixture with hexafluorosilicate anion (SiF62−) 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 (H3O)2SiF6; instead the vapor consists of HF, SiF4, and water. Aqueous solutions of (H3O)2SiF6 contain the hexafluorosilicate anion, SiF62− and protonated water. In this octahedral anion, the Si-F bond distances are 1.71 Å.
Production and principal reactions
The commodity chemical hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid. 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:
2 + 6 HF → SiF2−
6 + 2 H
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:
- SiF62− + 2 H2O → 6 F− + SiO2 + 4 H+
The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite. 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 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, 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. In this application, the hexafluorosilicic acid converts to the fluoride ion (F-), which is the active agent for the protection of teeth.
Hexafluorosilicic acid is also used as an electrolyte in the Betts electrolytic process for refining lead.
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.
Hexafluorosilicic acid and the salts are used as wood preservation agents.
Hexafluorosilicic acid releases hydrogen fluoride 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. The LD50 value of hexafluorosilicic acid is 70 mg/kg.
- http://www.cdc.gov - Water Fluoridation Additives - Engineering Fact Sheet - Community Water Fluoridation -Oral Health hydrofluorosilic acid.
- The New Zealand Institute of Chemistry (NZIC) - Hydrofluorosilic acid and water fluoridation hydrofluorosilic acid.
- J. P. Nicholson (2005). "Electrodeposition of Silicon from Nonaqueous Solvents". J. Electrochem. Soc. 152 (12): C795–C802. doi:10.1149/1.2083227.
- Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
- USGS. Fluorspar.
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- William F. Finney , Erin Wilson , Andrew Callender , Michael D. Morris, Larry W. Beck "Reexamination of Hexafluorosilicate Hydrolysis by 19F NMR and pH Measurement" Environ. Sci. Technol., 2006, 40 (8), pp 2572–2577. doi:10.1021/es052295s
- Pilcher, A. S.; DeShong, P. "Fluorosilicic Acid" in Encyclopedia of Reagents for Organic Synthesis, Copyright © 2001 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.
- Hexafluorosilicic acid Chemical Safety Card http://www.cdc.gov/niosh/ipcsneng/neng1233.html