Hexafluorosilicic acid is the chemical compound with the formula HM₂SiF₆. The free acid is encountered as an equilibrium mixture with hexaflurorosilicate anion (SiF₆²⁻) only in solution in solvents that are proton donors at low pH. In aqueous solution, evaporation of H₂SiF₆ results in loss of HF and SiF₄.

H₂SiF₆ is a by-product from the reaction of fluoroapatite with sulfuric acid, which produces HF, which in turn reacts with silicate minerals:

SiO₂ + 6 HF  H₂SiF₆ + 2 H₂O

Aqueous solutions of H₂SiF₆ contain the hexafluorosilicate anion,
SiF₆²⁻. In this octahedral anion, the Si-F bond distances are 1.71.

Uses

H₂SiF₆ is commonly used for water fluoridation in several countries including the United States, Great Britain, and Ireland.

H₂SiF₆ is a 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. The aluminium and magnesium hexafluorosilicate are two used compounds.

Safety

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.