4182. Fluosilicic Acid

Nomenclature

CAS number: 16961-83-4
Hydrogen hexafluorosilicate; hexafluosilicic acid; hydrosilicofluoric acid; hydrofluosilicic acid; silicofluoric acid; fluorosilicic acid.
F6H2Si; mol wt 144.09.
F 79.11%, H 1.40%, Si 19.49%.
H2SiF6.

Description and references

Prepd from HF + SiO2; also prepd by the action of water on SiF4; by the action of H2SO4 on BaSiF6: Hempel, Ber. 18, 1438 (1885); Baur, Glaessner, Ber. 36, 4215 (1903); Sll, FIAT-Review 23, 257 (1946); Kwasnik in Handbook of Preparative Inorganic Chemistry vol. 1, G. Brauer, Ed. (Academic Press, New York, 2nd ed., 1963) p 214-215; Lange in Fluorine Chemistry vol. I, J. H. Simon, Ed. (New York, 1950) p 129. Commercial grades of fluosilicic acid soln are obtained as a by-product in the superphosphate industry. Review: Colton, J. Chem. Educ. 35, 562-563 (1958).

Properties

Liquid, when anhydr dissociates almost instantly into SiF4 and HF. Marketed as aq soln only. A 60-70% soln solidifies around 19°, forming a cryst dihydrate. May be distilled without decompn only as a 13.3% aq soln. Fairly strong acid. Sour, pungent odor. d17.517.5 5% soln: 1.0407; 10%: 1.0834; 15%: 1.1281; 20%: 1.1748; 25%: 1.2235; 30%: 1.2742; 34%: 1.3162. The more concd solns (but not the anhydr liq) can be stored in glass, although some etching will take place around the surface. Usually stored in iron containers.

Caution

Severe corrosive effect on skin, mucous membranes.

Use

A 1-2% soln is used widely for sterilizing equipment in brewing and bottling establishments. Other concns are used in the electrolytic refining of lead, in electroplating, for hardening cement, crumbling lime or brick work, for the removal of lime from hides during the tanning process, to remove molds, as preservative for timber.