Hexafluorosilicic acid is an

inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

with the chemical formula . 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.Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson.
link to bararitelink to cryptohalite
/ref> 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 Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It h ...

and synthetic cryolite, which are used in aluminium processing. Salts derived from hexafluorosilicic acid are called hexafluorosilicates.

Structure

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 bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a ...

ed to the cations. Aqueous solutions of hexafluorosilicic acid are often described as .

Production and principal reactions

Hexafluorosilicic acid is produced commercially from fluoride-containing minerals that also contain silicates. Specifically,

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

and

fluorapatite Fluorapatite, often with the alternate spelling of fluoroapatite, is a phosphate mineral with the formula Ca5(PO4)3F (calcium fluorophosphate). Fluorapatite is a hard crystalline solid. Although samples can have various color (green, brown, bl ...

are treated with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

to give phosphoric acid, a precursor to several water-soluble fertilizers. This is called the wet phosphoric acid process.USGS.
Fluorspar
As a by-product, approximately 50 kg of hexafluorosilicic acid is produced per tonne of HF owing to reactions involving silica-containing mineral impurities. Some of the

hydrogen fluoride Hydrogen fluoride (fluorane) is an inorganic compound with the chemical formula . This colorless gas or liquid is the principal industrial source of fluorine, often as an aqueous solution called hydrofluoric acid. It is an important feedstock i ...

(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. The net process can be described as: :HF + → SiF₆^2- + 2 H₃O⁺ Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.

Reactions

In water, hexafluorosilicic acid readily hydrolyzes to

hydrofluoric acid Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...

and various forms of amorphous and hydrated silica ("SiO₂"). At the concentration usually used for water fluoridation, 99% hydrolysis occurs and the pH drops. The rate of hydrolysis increases with pH. At the pH of drinking water, the degree of hydrolysis is essentially 100%. :H₂SiF₆ + 2 H₂O → 6 HF + "SiO₂" Near neutral pH, hexafluorosilicate salts hydrolyze rapidly according to this equation: : + 2 H₂O → 6 F⁻ + SiO₂ + 4 H⁺

Alkali and alkaline earth salts

Neutralization of solutions of hexafluorosilicic acid with

alkali metal The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

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. Sodium hexafluorosilicate for instance may be produced by treating

sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g ...

() by hexafluorosilicic acid: : : Heating sodium hexafluorosilicate gives silicon tetrafluoride: :

Uses

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: :H₂SiF₆ + Al₂O₃ → 2 AlF₃ + SiO₂ + H₂O Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt,

Potassium fluorosilicate Potassium fluorosilicate is a chemical compound with the chemical formula . When doped with Potassium hexafluoromanganate(IV) () it forms a narrow band red producing phosphor, (often abbreviated PSF or KSF), of economic interest due to its applic ...

, 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.

Lead refining

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

Rust removers

Hexafluorosilicic acid (identified as hydrofluorosilicic acid on the label) along with

oxalic acid Oxalic acid is an organic acid with the systematic name ethanedioic acid and formula . It is the simplest dicarboxylic acid. It is a white crystalline solid that forms a colorless solution in water. Its name comes from the fact that early inve ...

are the active ingredients used in ''Iron Out'' rust-removing cleaning products, which are essentially varieties of

laundry sour A laundry sour is a chemical added to clothing during the final rinse cycle of a washing machine to lower the pH of the water and to assist with the removal of detergents and rust stains. Most such sours are fluoride-based, including ammonium sil ...

Niche applications

H₂SiF₆ is a specialized

reagent In chemistry, a reagent ( ) or analytical reagent is a substance or compound added to a system to cause a chemical reaction, or test if one occurs. The terms ''reactant'' and ''reagent'' are often used interchangeably, but reactant specifies a ...

organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...

for cleaving Si–O bonds of

silyl ether Silyl ethers are a group of chemical compounds which contain a silicon atom covalently bonded to an alkoxy group. The general structure is R1R2R3Si−O−R4 where R4 is an alkyl group or an aryl group. Silyl ethers are usually used as protecting g ...

s. It is more reactive for this purpose than HF. It reacts faster with ''t''- butyldimethysilyl (

TBDMS Silyl ethers are a group of chemical compounds which contain a silicon atom covalently bonded to an alkoxy group. The general structure is R1R2R3Si−O−R4 where R4 is an alkyl group or an aryl group. Silyl ethers are usually used as protecting g ...

) ethers than triisopropylsilyl ( TIPS) ethers.

Treating concrete

The application of hexafluorosilica acid to a calcium rich surface such as concrete will give that surface some resistance to acid attack. :CaCO₃ + H₂O → Ca²⁺ + 2 OH⁻ + CO₂ :H₂SiF₆ → 2 H⁺ + : + 2 H₂O → 6 F⁻ + SiO₂ + 4 H⁺ : Ca²⁺ + 2 F⁻ → CaF₂ Calcium fluoride (CaF₂) is an insoluble solid that is acid resistant.

Natural salts

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.

Safety

Hexafluorosilicic acid can release

(HF) when evaporated, so it has similar risks. Inhalation of the vapors may cause

lung edema Pulmonary edema, also known as pulmonary congestion, is excessive liquid accumulation in the tissue and air spaces (usually alveoli) of the lungs. It leads to impaired gas exchange and may cause hypoxemia and respiratory failure. It is due to ...

. Like hydrogen fluoride, it attacks glass and stoneware. The LD₅₀ value of hexafluorosilicic acid is 430 mg/kg.

References

{{Hydrogen compounds Mineral acids Hydrogen compounds Nonmetal halides Hexafluorosilicates Fluoro complexes