Hexafluorosilicic acid

Hexafluorosilicic acid is the chemical compound with the formula H₂SiF₆. Hexafluorosilicic acid refers to an equilibrium mixture with hexafluorosilicate anion (SiF₆²⁻) in an aqueous solution or other solvents that contain strong proton donors at low pH.

 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. 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

H₂SiF₆ is mainly produced as a by-product from the production of phosphoric acid from apatite and fluorapatite. In the U.S. about 85% of fluorspar is used to produce hydrofluorosilic acid. The phosphoric and hydrofluoric acids are liberated from the mineral by the action of sulfuric acid. 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 → 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. 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.
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