Hexafluorosilicic acid

Hexafluorosilicic acid
Hexafluorosilicic acid
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
Exact mass 143.982995827 g mol-1
Density 1.22 g/cm3 (25% soln.)
1.38 g/cm3 (35% soln.)
1.46 g/cm3 (61% soln.)
Melting point

ca. 19 °C (60–70% soln.)
<−30 °C (35& soln.)

Molecular shape Octahedral SiF62−
MSDS External MSDS
EU Index 009-011-00-5
EU classification Corrosive (C)
R-phrases R34
S-phrases (S1/2), S26, S27, S45
Flash point Non-flammable
Related compounds
Other cations Ammonium hexafluorosilicate

Sodium fluorosilicate

Related compounds Hexafluorophosphoric acid
Fluoroboric acid
 YesY acid (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

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.


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 (SiF62−) in an aqueous solution or other solvents that contain strong proton donors[1] 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.[2]

Production and principal reactions

The commodity chemical Hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid.[3] 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:[4]

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


The majority of the hexafluorosilicic acid is converted to aluminium fluoride and cryolite.[4] 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, 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.[3]

Hexafluorosilicic acid is also used as an electrolyte in the Betts Electrowinning process for refining Lead.

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.[5]

Hexafluorosilicic acid and the salts are used as wood preservation agents.[6]


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.[7]

See also

  • Ammonium fluorosilicate
  • Sodium fluorosilicate


  1. ^ J. P. Nicholson (2005). "Electrodeposition of Silicon from Nonaqueous Solvents". J. Electrochem. Soc. 152 (12): C795–C802. doi:10.1149/1.2083227. 
  2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
  3. ^ a b USGS. Fluorspar.
  4. ^ 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
  5. ^ Pilcher, A. S.; DeShong, P. “Fluorosilicic Acid” in Encyclopedia of Reagents for Organic Synthesis, Copyright © 2001 John Wiley & Sons. DOI: 10.1002/047084289X.rf013
  6. ^ 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. 
  7. ^ Hexafluorosilicic acid Chemical Safety Card http://www.cdc.gov/niosh/ipcsneng/neng1233.html

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