Hexafluorosilicic acid

Names
Preferred IUPAC name Hexafluorosilicic acid
Systematic IUPAC name Dihydrogen hexafluorosilicate
Other names Fluorosilicic acid, fluosilic acid, hydrofluorosilicic acid, silicofluoride, silicofluoric acid, oxonium hexafluorosilanediuide, oxonium hexafluoridosilicate(2−)
Identifiers
CAS Number	16961-83-4 Y
3D model (JSmol)	Interactive image Interactive image
ChemSpider	17215660 Y
ECHA InfoCard	100.037.289
EC Number	241-034-8
PubChem CID	21863527
RTECS number	VV8225000
UNII	53V4OQG6U1 Y
UN number	1778
CompTox Dashboard (EPA)	DTXSID2029741
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
SMILES [H+].[H+].F[Si-2](F)(F)(F)(F)F [H+].[H+].F[Si--](F)(F)(F)(F)F
Properties
Chemical formula	F6H2Si
Molar mass	144.091 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	c. 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
Acidity (pKa)	1.92[1]
Refractive index (nD)	1.3465
Structure
Molecular shape	Octahedral SiF62−
Hazards
GHS labelling:
Pictograms
Signal word	Danger
Hazard statements	H314
Precautionary statements	P260, P264, P280, P301+P330+P331, P303+P361+P353, P304+P340, P305+P351+P338, P310, P321, P363, P405, P501
NFPA 704 (fire diamond)	3 0 0
Flash point	Non-flammable
Lethal dose or concentration (LD, LC):
LD50 (median dose)	430 mg/kg (oral, rat)
Safety data sheet (SDS)	External MSDS
Related compounds
Other anions	Hexafluorotitanic acid Hexafluorozirconic acid
Other cations	Ammonium hexafluorosilicate Sodium fluorosilicate
Related compounds	Hexafluorophosphoric acid Fluoroboric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Y verify (what is YN ?) Infobox references

Hexafluorosilicic acid is an inorganic compound with the chemical formula H
₂SiF
₆. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.

Hexafluorosilicic acid is produced naturally on a large scale in volcanoes.^[2][3] It is manufactured as a coproduct in the production of phosphate fertilizers. The resulting hexafluorosilicic acid is almost exclusively consumed as a precursor to aluminum trifluoride and synthetic cryolite, which are used in aluminium processing. Salts derived from hexafluorosilicic acid are called hexafluorosilicates.

Hexafluorosilicic acid has been crystallized as various hydrates. These include (H₅O₂)₂SiF₆, the more complicated (H₅O₂)₂SiF₆·2H₂O, and (H₅O₂)(H₇O₃)SiF₆·4.5H₂O. In all of these salts, the octahedral hexafluorosilicate anion is hydrogen bonded to the cations.^[4]

Aqueous solutions of hexafluorosilicic acid are often described as H
₂SiF
₆.

Hexafluorosilicic acid is produced commercially from fluoride-containing minerals that also contain silicates. Specifically, apatite and fluorapatite are treated with sulfuric acid to give phosphoric acid, a precursor to several water-soluble fertilizers. This is called the wet phosphoric acid process.^[5] As a by-product, approximately 50 kg of hexafluorosilicic acid is produced per tonne of HF owing to reactions involving silica-containing mineral impurities.^[6]: 3

Some of the hydrogen fluoride (HF) produced during this process in turn reacts with silicon dioxide (SiO₂) impurities, which are unavoidable constituents of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF.^{[citation needed]} The net process can be described as:^{[7][page needed]}

6 HF + SiO₂ → SiF2−6 + 2 H₃O⁺

Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.^[7]

Hexafluorosilic acid is only stable in hydrogen fluoride or acidic aqueous solutions. In any other circumstance, it acts as a source of hydrofluoric acid. Thus, for example, hexafluorosilicic acid pure or in oleum solution evolves silicon tetrafluoride until the residual hydrogen fluoride re-establishes equilibrium:^[7]

H₂SiF₆ ⇌ 2 HF(l) + SiF₄(g)

In alkaline-to-neutral aqueous solutions, hexafluorosilicic acid readily hydrolyzes to fluoride anions and amorphous, hydrated silica ("SiO₂"). Strong bases give fluorosilicate salts at first, but any stoichiometric excess begins hydrolysis.^[7] At the concentrations usually used for water fluoridation, 99% hydrolysis occurs:^[6][8]

SiF²⁻
₆ + 2 H₂O → 6 F⁻ + SiO₂ + 4 H⁺

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. At room temperature 15-30% concentrated hexafluorosilicic acid undergoes similar reactions with chlorides, hydroxides, and carbonates of alkali and alkaline earth metals.^[9]

Sodium hexafluorosilicate for instance may be produced by treating sodium chloride (NaCl) by hexafluorosilicic acid:^{[6]: 3 [10]: 7}

2NaCl + H₂SiF₆ 27 °C→ Na₂SiF₆↓ + 2 HCl

BaCl₂ + H₂SiF₆ 27 °C→ BaSiF₆↓ + 2 HCl

Heating sodium hexafluorosilicate gives silicon tetrafluoride:^[10]: 8

Na₂SiF₆ >400 °C→ SiF₄ + 2 NaF

The majority of the hexafluorosilicic acid is converted to aluminium fluoride and synthetic cryolite. These materials are central to the conversion of aluminium ore into aluminium metal. The conversion to aluminium trifluoride is described as:^[7]

H₂SiF₆ + Al₂O₃ → 2 AlF₃ + SiO₂ + H₂O

Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt, Potassium fluorosilicate, 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 and the salts are used as wood preservation agents.^[11]

Hexafluorosilicic acid is also used as an electrolyte in the Betts electrolytic process for refining lead.

Hexafluorosilicic acid (identified as hydrofluorosilicic acid on the label) along with oxalic acid are the active ingredients used in Iron Out rust-removing cleaning products, which are essentially varieties of laundry sour.

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.^[12]

The application of hexafluorosilica acid to a calcium rich surface such as concrete will give that surface some resistance to acid attack.^[13]

CaCO₃ + H₂O →  Ca²⁺ + 2 OH⁻ + CO₂

H₂SiF₆ → 2 H⁺ + SiF²⁻
₆

SiF²⁻
₆ + 2 H₂O → 6 F⁻ + SiO₂ + 4 H⁺

 Ca²⁺ + 2 F⁻ → CaF₂

Calcium fluoride (CaF₂) is an insoluble solid that is acid resistant.

Some rare minerals, encountered either within volcanic or coal-fire fumaroles, are salts of the hexafluorosilicic acid. Examples include ammonium hexafluorosilicate that naturally occurs as two polymorphs: cryptohalite and bararite.^[14][15][16]

Hexafluorosilicic acid can release hydrogen fluoride (HF) when evaporated, so it has similar risks. Inhalation of the vapors may cause lung edema. Like hydrogen fluoride, it attacks glass and stoneware.^[17] The LD₅₀ value of hexafluorosilicic acid is 430 mg/kg.^[6]

• Ammonium fluorosilicate
• Sodium fluorosilicate
• Potassium fluorosilicate