Hexafluorosilicic acid

Hexafluorosilicic acid
Preferred IUPAC name Hexafluorosilicic acid
Systematic name Dihydrogen hexafluorosilicate
Other names Fluorosilicic acid, fluosilicic acid, hydrofluorosilic acid, silicofluoride, silicofluoric acid
Identifiers
CAS number	16961-83-4 Y
PubChem	21863527
ChemSpider	17215660 Y
EC number	241-034-8
UN number	1778
RTECS number	VV8225000
Jmol-3D images	Image 1 Image 2
SMILES [H+].[H+].F[Si-2](F)(F)(F)(F)F [H+].[H+].F[Si--](F)(F)(F)(F)F
InChI InChI=1S/F6Si/c1-7(2,3,4,5)6/q-2/p+2 Y Key: OHORFAFFMDIQRR-UHFFFAOYSA-P Y InChI=1/F6Si/c1-7(2,3,4,5)6/q-2/p+2 Key: OHORFAFFMDIQRR-SKRXCDHZAM
Properties
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.)
Structure
Molecular shape	Octahedral SiF62−
Hazards
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
Y acid (verify) (what is: Y/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

Uses

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]

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.^[7]

See also

• Ammonium fluorosilicate
• Sodium fluorosilicate

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. ^ ^{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