Stability of fluoride complex with silica and its distribution in natural water systems

Fluoride reacts with silicic acid to form SiF²⁻₆. A fluoride electrode was used to obtain an equilibrium constant of 10^30.18 for the reaction:Si(OH)06+ 6F−+4H+=SiF2−6+ 4H2O">Si(OH)06+ 6F−+4H+=SiF2−6+ 4H2O at 25°C

Although there may be some experimental evidence for existence of traces of species containing less than six F⁻ ions per silicon (n = 6), the species SiF²⁻₆ predominates for n values from about 0.1 to 6.

Silicic-acid complexing with fluoride is important only in solutions which have rather low pH and low concentrations of other cations which compete with silicon for fluoride. Computations for cold volcanic condensates from Hawaii indicate that for some samples much of the silicon is complexed by fluoride as SiF²⁻₆. However, in most cooled acidic natural water samples Al and Fe are more important than Si in complexing fluoride.