Hexafluorosilicic acid

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Dihydrogen hexafluorosilicate
IUPAC name	Dihydrogen hexafluorosilicate
Other names	Hexafluorosilicic acid fluorosilicic acid fluosilicic acid
Identifiers
CAS number	[16961-83-4]
RTECS number	VV8225000
Properties
Molecular formula	H2F6Si
Molar mass	144.0922 g/mol
Appearance	colourless solution
Density	1.22 g/cm3 25% aqueous soln
Structure
Molecular shape	Octahedral SiF62−
Hazards
MSDS	External MSDS
Main hazards	Toxic, corrosive.
R-phrases	R20, R21, R22, R34, R41
S-phrases	S26, S27
Flash point	Non-flammable.
Related compounds
Related compounds	HF HPF6 HBF4
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Hexafluorosilicic acid is the chemical compound with the formula H₂SiF₆. The free acid is encountered as an equilibrium mixture with hexaflurorosilicate anion (SiF₆²⁻) only in solution in solvents that are proton donors^[1] at low pH. In aqueous solution, evaporation of H₂SiF₆ results in loss of HF and SiF₄.

H₂SiF₆ is a by-product from the reaction of fluoroapatite with sulfuric acid, which produces HF, which in turn reacts with silicate minerals:

SiO₂ + 6 HF → H₂SiF₆ + 2 H₂O

Aqueous solutions of H₂SiF₆ contain the hexafluorosilicate anion, SiF₆²⁻. In this octahedral anion, the Si-F bond distances are 1.71 Å.^[2]

[edit] Uses

H₂SiF₆ is commonly used for water fluoridation in several countries including the United States, Great Britian, and Ireland.

H₂SiF₆ is a 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.^[3]

Hexafluorosilicic acid and the salts are used as wood preservation agents. The aluminium and magnesium hexafluorosilicate are two used compounds. ^[4]

[edit] Safety

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.^[5]

[edit] References

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. ^ Pilcher, A. S.; DeShong, P. “Fluorosilicic Acid” in Encyclopedia of Reagents for Organic Synthesis, Copyright © 2001 John Wiley & Sons. DOI: 10.1002/047084289X.rf013
4. ^ 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). doi:10.1007/s00226-003-0205-5. 
5. ^ Hexafluorosilicic acid Chemical Safety Card http://www.cdc.gov/niosh/ipcsneng/neng1233.html