Formula H2SiF6 | Density 1.22 g/cm³ | |
 | ||
Related compounds Appearance transparent, colorless, fuming liquid | ||
Hexafluorosilicic acid is an inorganic compound with the chemical formula (H
3O)
2SiF
6 (also written as (H
3O)
2[SiF
6]). It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is produced naturally on large scale in volcanoes. It is manufactured as a precursor to aluminum trifluoride and synthetic cryolite. It is commonly used as a source of fluoride for water fluoridation. Salts derived from hexafluorosilicic acid are called hexafluorosilicates.
Contents
In aqueous solution, the hydronium cation (H3O+) is traditionally equated with a solvated proton, and as such, the formula for this compound is often written as H
2SiF
6. Extending that metaphor, the isolated compound is then written as H
2SiF
6·2H
2O.
Structure
Hexafluorosilicic acid is generally assumed to consist of oxonium ions charge balanced by hexafluorosilicate dianions as well as water. The situation is similar to that for chloroplatinic acid, fluoroboric acid, and hexafluorophosphoric acid. Hexafluorosilicate is an octahedral anion, the Si–F bond distances are 1.71 Å. Hexafluorosilicic acid is only available commercially as solution.
Production and principal reactions
The commodity chemical hydrogen fluoride is produced from fluorspar by treatment with sulfuric acid. As a by product, approximately 50 kg of (H3O)2SiF6 is produced per tonne of HF owing to reactions involving silica-containing mineral impurities. (H3O)2SiF6 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:
SiO2 + 6 HF → SiF2−
6 + 2 H
3O+
Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.
Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts:
(H3O)2SiF6 + 2 NaOH → Na2SiF6 + 4 H2OThe resulting salt Na2SiF6 is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications.
Near neutral pH, hexafluorosilicate salts hydrolyze rapidly according to this equation:
SiF2−6 + 2 H2O → 6 F− + SiO2 + 4 H+
Uses
The majority of the hexafluorosilicic acid is converted to aluminium fluoride and 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 + H2OHexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt 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 is also commonly used for water fluoridation in several countries including the United States, the United Kingdom, and the Republic of Ireland. In the U.S., about 40,000 tons of fluorosilicic acid is recovered from phosphoric acid plants, and then used primarily in water fluoridation, sometimes after being processed into sodium silicofluoride. In this application, the hexafluorosilicic acid decomposes to the fluoride ion (F−) – which is the active agent for the protection of teeth – and inert silicon dioxide (SiO2).
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.
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.
Hexafluorosilicic acid and the salts are used as wood preservation agents.
Safety
Hexafluorosilicic acid can release hydrogen fluoride when evaporated, so it has similar risks. Inhalation of the vapors may cause lung edema. Like hydrogen fluoride, it attacks glass and stoneware. The LD50 value of hexafluorosilicic acid is 430 mg/kg.