Hexafluorosilicic acid

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Hexafluorosilicic acid
Hexafluorosilicic acid molecular structure.png
CAS number 16961-83-4 YesY
PubChem 21863527
ChemSpider 17215660 YesY
EC number 241-034-8
UN number 1778
RTECS number VV8225000
Jmol-3D images Image 1
Image 2
Molecular formula H2F6Si
Molar mass 144.09 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)
Solubility in water miscible
Refractive index (nD) 1.3465
Molecular shape Octahedral SiF62−
MSDS External MSDS
EU Index 009-011-00-5
EU classification Toxic T - Toxic
Corrosive C - Corrosive
R-phrases R34, R25
S-phrases (S1/2), S26, S27, S45
NFPA 704
Flash point Non-flammable
LD50 430 mg/kg (oral, rat)
Related compounds
Other cations Ammonium hexafluorosilicate

Sodium fluorosilicate

Related compounds Hexafluorophosphoric acid
Fluoroboric acid
Infobox references

Hexafluorosilicic acid (systematically named oxonium hexafluorosilanediuide and oxonium hexafluoridosilicate(2−)) is an inorganic compound with the chemical formula (H
(also written as (H
or SiH
). In aqueous solution, the oxonium cation is traditionally equated with a solvated proton, and as such, the formula is often written as H
. Extending that metaphor, the pure compound is then written as H
. It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is mainly produced as a precursor to aluminum trifluoride and synthetic cryolite. It is commonly used as a source of fluoride for water fluoridation.[1][2] Concentrated hexafluorosilicic acid is corrosive and can attack the skin.


In solid hexafluorosilicic acid, the component ions form a network, being connected by ionic bonds. In the liquid phase, the oxonium ions react reversibly with the hexafluoridosilicate(1−) ions, producing water and various protonated silicon complexes. These complexes undergo decomposition reversibly, producing a small concentration of hydrogen fluoride. The result is a complex mixture containing water, hydrogen fluoride, tetrafluorosilane, and other related species, all in dynamic equilibrium. Therefore, unless the liquid phase is kept in a sealed container, the differing volatilities will cause the hexafluorosilicic acid to degrade rapidly. Hexafluorosilicic acid is only available commercially as an equilibrium mixture in an aqueous solution or other solvents that contain strong proton donors[3] at low pH (acids described similarly include chloroplatinic acid, fluoroboric acid, and hexafluorophosphoric acid, and, more commonly, carbonic acid). Purifying hexafluorosilicic acid by using distillation has not proven possible, all reported attempts has only yielded the decomposition products, which are HF, SiF
, and water. In this octahedral anion, the Si-F bond distances are 1.71 Å.[4]

Production and principal reactions[edit]

The commodity chemical hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid.[5] 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:[6]

+ 6 HF → SiF2−
+ 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:[7]

SiF62− + 2 H2O → 6 F + SiO2 + 4 H+


The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite.[6] 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.[5] 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.

Niche applications[edit]

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.[8]

Hexafluorosilicic acid and the salts are used as wood preservation agents.[9]


Hexafluorosilicic acid can release 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.[10] The LD50 value of hexafluorosilicic acid is 70 mg/kg.[11][12]

See also[edit]


  1. ^ http://www.cdc.gov - Water Fluoridation Additives - Engineering Fact Sheet - Community Water Fluoridation -Oral Health hydrofluorosilic acid.
  2. ^ The New Zealand Institute of Chemistry (NZIC) - Hydrofluorosilic acid and water fluoridation hydrofluorosilic acid.
  3. ^ J. P. Nicholson (2005). "Electrodeposition of Silicon from Nonaqueous Solvents". J. Electrochem. Soc. 152 (12): C795–C802. doi:10.1149/1.2083227. 
  5. ^ a b USGS. Fluorspar.
  6. ^ a b J. Aigueperse, P. Mollard, D. Devilliers, M. Chemla, R. Faron, R. Romano, J. P. Cuer, "Fluorine Compounds, Inorganic" in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a11_307
  7. ^ 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
  8. ^ Pilcher, A. S.; DeShong, P. "Fluorosilicic Acid" in Encyclopedia of Reagents for Organic Synthesis, Copyright © 2001 John Wiley & Sons. doi:10.1002/047084289X.rf013
  9. ^ 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. 
  10. ^ Hexafluorosilicic acid Chemical Safety Card http://www.cdc.gov/niosh/ipcsneng/neng1233.html
  11. ^ ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPDF/Fluorosilicates.pdf
  12. ^ http://www.sciencelab.com/msds.php?msdsId=9927475