Hexafluorosilicic acid

Hexafluorosilicic acid is a chemical compound that is an acid. It has the simplified chemical formula H2SiF6, but has a complicated chemistry that means no compound with this formula truly exists. A better formula is (H3O)2SiF6·(H2O)n, indicating a hydrated hydronium salt.[1]

It can be reacted with bases such as sodium hydroxide to produce a fluorosilicate salt such as sodium fluorosilicate. It is used to make artificial cryolite. It releases hydrogen fluoride when it is heated, so it is toxic like hydrofluoric acid. It is used in fluoridation of water.

Production

Most hexafluorosilicic acid is a byproduct from refining minerals that contain fluoride.[2]

For example, pure fluorite has the formula CaF2, and makes hydrofluoric acid in a chemical reaction with sulfuric acid:

CaF2 + H2SO4 → CaSO4 + 2 HF

Silica, SiO2, is a common impurity in fluorite. If silica is present during the above reaction, it will react with the HF product. This equation can be written as

SiO2 + 6 HF → (H3O)2SiF6

References

  1. ↑ Mootz, D.; Oellers, E.-J. (1988). "The Crystalline Hydrates of Hexafluorosilicic Acid: A Combined Phase-Analytical and Structural Study". Zeitschrift für Anorganische und Allgemeine Chemie. 559: 27–39. doi:10.1002/zaac.19885590103.
  2. ↑ Kounbach, Said; Ben Embarek, Mokhtar; Mahi, Lhassane; Boulif, Rachid; Beniazza, Redouane; Benhida, Rachid (2022). "Simultaneous determination of H2SiF6, HF and total fluoride in fluorosilicic acid recovered from wet phosphoric acid production by potentiometric titration". Microchemical Journal. 175. doi:10.1016/j.microc.2021.107152.
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