Hexafluorosilicic acid (systematically named oxonium hexafluorosilanediuide and oxonium hexafluoridosilicate(2−)) is an inorganic compound with the chemical formula (H 3O) 2SiF 6 (also written as (H 3O) 2[SiF 6] or SiH 6O 2F 6). In aqueous solution, the oxonium cation is traditionally equated with a solvated proton, and as such, the formula is often written as H 2SiF 6. Extending that metaphor, the pure compound is then written as H 2SiF 6•2H 2O. It is a colorless liquid rarely encountered undiluted. Hexafluorosilicic acid has a distinctive sour taste and pungent smell. It is mainly produced as a precursor to aluminum trifluoride and synthetic cryolite. It is commonly used as a source of fluoride for water fluoridation. Concentrated hexafluorosilicic acid is corrosive and can attack the skin.