Fluosilicic Acid

Title: Fluosilicic Acid
CAS Registry Number: 16961-83-4
CAS Name: Hydrogen hexafluorosilicate
Additional Names: hexafluosilicic acid; hydrosilicofluoric acid; hydrofluosilicic acid; silicofluoric acid; fluorosilicic acid
Molecular Formula: F6H2Si
Molecular Weight: 144.09
Percent Composition: F 79.11%, H 1.40%, Si 19.49%
Line Formula: H2SiF6
Literature 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); Söll, 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.
Density: 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
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.
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