Hexafluorosilicic acid

{{short description|Octahedric silicon compound}} {{redirect|Fluorosilicate|Fluorosilicate glass and glass-ceramics|Fluorosilicate glass}} {{Chembox | Watchedfields = changed | verifiedrevid = 477003251 | ImageFile = Hexafluorosilicic acid molecular structure.png | ImageClass = skin-invert-image | ImageFile2 = | PIN = Hexafluorosilicic acid | SystematicName = Dihydrogen hexafluorosilicate | OtherNames = Fluorosilicic acid, fluosilic acid, hydrofluorosilicic acid, silicofluoride, silicofluoric acid, oxonium hexafluorosilanediuide, oxonium hexafluoridosilicate(2−) | Section1 = {{Chembox Identifiers | CASNo = 16961-83-4 | CASNo_Ref = {{cascite|correct|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 53V4OQG6U1 | PubChem = 21863527 | ChemSpiderID = 17215660 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | EINECS = 241-034-8 | UNNumber = 1778 | RTECS = VV8225000 | SMILES = [H+].[H+].F[Si-2](F)(F)(F)(F)F | SMILES1 = [H+].[H+].F[Si--](F)(F)(F)(F)F | StdInChI_Ref = {{stdinchicite|correct|chemspider}} | StdInChI = 1S/F6Si/c1-7(2,3,4,5)6/q-2/p+2 | InChI = 1/F6Si/c1-7(2,3,4,5)6/q-2/p+2 | StdInChIKey_Ref = {{stdinchicite|correct|chemspider}} | StdInChIKey = OHORFAFFMDIQRR-UHFFFAOYSA-P | InChIKey = OHORFAFFMDIQRR-SKRXCDHZAM}} | Section2 = {{Chembox Properties | H=2 | Si=1 | F=6 | Appearance = transparent, colorless, fuming liquid | Odor = sour, pungent | Density = 1.22 g/cm³ (25% soln.)<br/>1.38 g/cm³ (35% soln.)<br/>1.46 g/cm³ (61% soln.) | MeltingPt = {{circa}} {{convert|19|C|F K}} {{nowrap|(60–70% solution)}}<br/><&nbsp;{{convert|-30|C|F K}} {{nowrap|(35% solution)}} | BoilingPtC = 108.5 | BoilingPt_notes = (decomposes) | Solubility = miscible | RefractIndex = 1.3465 | pKa = p''K''_a1 < 0<br/>p''K''_a2 = 1.83 or 0.65 }} | Section3 = {{Chembox Structure | MolShape = Octahedral SiF₆^{2<nowiki>&minus;</nowiki>} }} | Section7 = {{Chembox Hazards | NFPA-H = 3 | NFPA-F = 0 | NFPA-R = 0 | NFPA-S = | ExternalSDS = [https://www.sigmaaldrich.com/US/en/sds/sial/01302 External MSDS] | GHSPictograms = {{GHS05}} | GHSSignalWord = Danger | HPhrases = {{H-phrases|314}} | PPhrases = {{P-phrases|260|264|280|301+330+331|303+361+353|304+340|305+351+338|310|321|363|405|501}} | FlashPt = Non-flammable | LD50 = 430 mg/kg (oral, rat) }} | Section8 = {{Chembox Related | OtherCations = Ammonium hexafluorosilicate<br /> Sodium fluorosilicate | OtherAnions = Hexafluorotitanic acid<br />Hexafluorozirconic acid | OtherCompounds = Hexafluorophosphoric acid<br/>Fluoroboric acid}} }}

'''Hexafluorosilicic acid''' is an inorganic compound with the chemical formula {{Chem|H|2|SiF|6}}. Aqueous solutions of hexafluorosilicic acid consist of salts of the cation and hexafluorosilicate anion. These salts and their aqueous solutions are colorless.<ref name=Urbansky>{{cite journal |last1=Urbansky |first1=Edward Todd |title=Fate of Fluorosilicate Drinking Water Additives |journal=Chemical Reviews |date=2002 |volume=102 |issue=8 |pages=2837–2854 |doi=10.1021/cr020403c |pmid=12175269 }}</ref>

Hexafluorosilicic acid is produced naturally on a large scale in volcanoes.<ref name="Palache">Palache, C., Berman, H., and Frondel, C. (1951) Dana’s System of Mineralogy, Volume II: Halides, Nitrates, Borates, Carbonates, Sulfates, Phosphates, Arsenates, Tungstates, Molybdates, etc. John Wiley and Sons, Inc., New York, 7th edition.</ref><ref name="Anthony">Anthony, J.W., Bideaux, R.A., Bladh, K.W., and Nichols, M.C. (1997) Handbook of Mineralogy, Volume III: Halides, Hydroxides, Oxides. Mineral Data Publishing, Tucson. *[https://www.handbookofmineralogy.org/pdfs/bararite.pdf Bararite] *[https://www.handbookofmineralogy.org/pdfs/cryptohalite.pdf Cryptohalite]</ref> It is manufactured as a coproduct in the production of phosphate fertilizers. The resulting hexafluorosilicic acid is almost exclusively consumed as a precursor to aluminum trifluoride and synthetic cryolite, which are used in aluminium processing. Salts derived from hexafluorosilicic acid are called '''hexafluorosilicates'''.

== Structure == thumb|left|Structure of (H₅O₂)₂SiF₆. The hydrogen bonding between the fluoride and protons are indicated by dashed lines. Color code: green = F, orange = Si, red = O, gray = H.<ref name=ZAAC>{{cite journal |doi=10.1002/zaac.19885590103|title=The Crystalline Hydrates of Hexafluorosilicic Acid: A Combined Phase-Analytical and Structural Study |year=1988 |last1=Mootz |first1=D. |last2=Oellers |first2=E.-J. |journal=Zeitschrift für anorganische und allgemeine Chemie |volume=559 |issue=1 |pages=27–39 |bibcode=1988ZAACh.559...27M }}</ref> Hexafluorosilicic acid has been crystallized as various hydrates. These include (H₅O₂)₂SiF₆, the more complicated (H₅O₂)₂SiF₆·2H₂O, and (H₅O₂)(H₇O₃)SiF₆·4.5H₂O. In all of these salts, the octahedral hexafluorosilicate anion is hydrogen bonded to the cations.<ref name=ZAAC/>

Aqueous solutions of hexafluorosilicic acid are often described as {{chem2|H2SiF6}}, but the main fluoride-containing species in such solutions are {{chem2|SiF6](2-)}} and various products from hydrolysis such as F^-.<ref name=Urbansky/>

==Production and principal reactions== Hexafluorosilicic acid is produced commercially from fluoride-containing minerals that also contain silicates. Specifically, apatite and fluorapatite are treated with sulfuric acid to give phosphoric acid, a precursor to several water-soluble fertilizers. This is called the wet phosphoric acid process.<ref name=USGS>USGS. [https://minerals.usgs.gov/minerals/pubs/commodity/fluorspar/fluormcs07.pdf Fluorspar].</ref> As a by-product, approximately 50&nbsp;kg of hexafluorosilicic acid is produced per tonne of HF owing to reactions involving silica-containing mineral impurities.<ref name= NTP >{{Cite web| url=https://ntp.niehs.nih.gov/ntp/htdocs/chem_background/exsumpdf/fluorosilicates_508.pdf| title=Sodium Hexafluorosilicate [CASRN 16893-85-9] and Fluorosilicic Acid [CASRN 16961-83-4] Review of Toxicological Literature| website=National Toxicology Program (U.S.)| access-date=13 July 2017| url-status=dead| archive-url=https://web.archive.org/web/20121022152457/http://ntp.niehs.nih.gov/ntp/htdocs/Chem_Background/ExSumPDF/Fluorosilicates.pdf| archive-date=22 October 2012}}</ref>{{rp|p=3}}

Some of the hydrogen fluoride (HF) produced during this process in turn reacts with silicon dioxide (SiO₂) impurities, which are unavoidable constituents of the mineral feedstock, to give silicon tetrafluoride. Thus formed, the silicon tetrafluoride reacts further with HF.{{Citation needed|date=May 2022}} The net process can be described as:<ref name="Ullmann">{{Ullmann|first1=J. |last1=Aigueperse |first2=P. |last2=Mollard |first3=D. |last3=Devilliers |first4=M. |last4=Chemla |first5=R. |last5=Faron |first6=R. |last6=Romano |first7=J. P. |last7=Cuer |title=Fluorine Compounds, Inorganic |date=2005 |doi=10.1002/14356007.a11_307}}</ref>{{Page needed|date=May 2022}} :{{chem2|6 HF + SiO2 → SiF6(2-) + 2 H3O+}} Hexafluorosilicic acid can also be produced by treating silicon tetrafluoride with hydrofluoric acid.<ref name=Ullmann/>

== Reactions == Hexafluorosilicic acid is a strong acid, similar to sulfuric acid. However, the second deprotonation can be quantified, but here differing values were reported (pK₂ = 0.65 or pK₂ = 1.83).<ref name=":0" />

Hexafluorosilic acid is only stable in hydrogen fluoride or acidic aqueous solutions. In any other circumstance, it acts as a source of hydrofluoric acid. Thus, for example, hexafluorosilicic acid pure or in oleum solution evolves silicon tetrafluoride until the residual hydrogen fluoride re-establishes equilibrium:<ref name="Ullmann" /> :H₂SiF₆&nbsp;{{eqm}} 2&nbsp;HF(''l'') + SiF₄(''g'')

In alkaline-to-neutral aqueous solutions, hexafluorosilicic acid readily hydrolyzes to fluoride anions and amorphous, hydrated silica ("SiO₂"). Strong bases give fluorosilicate salts at first, but any stoichiometric excess begins hydrolysis.<ref name=Ullmann/> At the concentrations usually used for water fluoridation, 99% hydrolysis occurs:{{r|NTP}}<ref name=":0">{{cite journal|last2=Wilson|first2=Erin|last3=Callender|first3=Andrew|last4=Morris|first4=Michael D.|last5=Beck|first5=Larry W.|year=2006|title=Reexamination of Hexafluorosilicate Hydrolysis by ¹⁹F NMR and pH Measurement|journal=Environ. Sci. Technol.|volume=40|issue=8|pages=2572–2577|doi=10.1021/es052295s|last1=Finney|first1=William F.|pmid=16683594|bibcode=2006EnST...40.2572F}}</ref> :{{Chem|SiF|6|2-}} + 2&nbsp;H₂O → 6&nbsp;F⁻ + SiO₂ + 4&nbsp;H⁺

=== Alkali and alkaline earth salts === Neutralization of solutions of hexafluorosilicic acid with alkali metal bases produces the corresponding alkali metal fluorosilicate salts: :H₂SiF₆ + 2&nbsp;NaOH → Na₂SiF₆ + 2 H₂O The resulting salt Na₂SiF₆ is mainly used in water fluoridation. Related ammonium and barium salts are produced similarly for other applications. At room temperature 15–30% concentrated hexafluorosilicic acid undergoes similar reactions with chlorides, hydroxides, and carbonates of alkali and alkaline earth metals.<ref name=":2">{{cite book |title=Inorganic Syntheses |year=1953 |volume=4 |pages=147–8 |doi=10.1002/9780470132357.ch47 |vauthors=Hoffman CJ, Gutowsky HS, Schumb WC, Breck DW |chapter=Silicon Tetrafluoride }}</ref>

Sodium hexafluorosilicate for instance may be produced by treating sodium chloride ({{chem2|NaCl}}) by hexafluorosilicic acid:{{r|NTP|p=3}}<ref name=":1">{{Cite patent|country=Us|number=A345458|title=Patent Silicon tetrafluoride generation|status=Granted|pubdate=January 3, 1982|gdate=1982|invent1=Keith|invent2=L. Yaws|inventor1-first=C. Hansen|inventor2-first=Carl}}</ref>{{rp|7}}

: {{chem2|2NaCl + H2SiF6}} {{overset|27 °C|→}} {{chem2|Na2SiF6↓ + 2 HCl}} : {{chem2|BaCl2 + H2SiF6}} {{overset|27 °C|→}} {{chem2|BaSiF6↓ + 2 HCl}}

Heating sodium hexafluorosilicate gives silicon tetrafluoride:{{R|name=:1|page=8}} :{{chem2|Na2SiF6}} {{overset|>400 °C|→}} {{chem2|SiF4 + 2 NaF}}

== Uses == The majority of the hexafluorosilicic acid is converted to aluminium fluoride and synthetic cryolite. These materials are central to the conversion of aluminium ore into aluminium metal. The conversion to aluminium trifluoride is described as:<ref name=Ullmann/> :H₂SiF₆ + Al₂O₃ → 2&nbsp;AlF₃ + SiO₂ + H₂O

Hexafluorosilicic acid is also converted to a variety of useful hexafluorosilicate salts. The potassium salt, Potassium fluorosilicate, is used in the production of porcelains, the magnesium salt for hardened concretes and as an insecticide, and the barium salts for phosphors.

Hexafluorosilicic acid and the salts are used as wood preservation agents.<ref>{{cite journal |author=Carsten Mai, Holger Militz |year=2004 |title=Modification of wood with silicon compounds. inorganic silicon compounds and sol-gel systems: a review |journal=Wood Science and Technology |volume=37 |issue=5 |page=339 |doi=10.1007/s00226-003-0205-5 |s2cid=9672269}}</ref>

=== Lead refining === Hexafluorosilicic acid is also used as an electrolyte in the Betts electrolytic process for refining lead.

=== Rust removers === Hexafluorosilicic acid (identified as hydrofluorosilicic acid on the label) along with oxalic acid are the active ingredients used in ''Iron Out'' rust-removing cleaning products, which are essentially varieties of laundry sour.

===Niche applications=== H₂SiF₆ is a specialized 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.<ref>{{cite book|last1=Pilcher|first1= A. S. |last2=DeShong |first2=P. |chapter=Fluorosilicic Acid |title=Encyclopedia of Reagents for Organic Synthesis|date=2001 |publisher=John Wiley & Sons |doi=10.1002/047084289X.rf013|isbn= 0471936235 }}</ref>

=== Treating concrete === The application of hexafluorosilicic acid to a calcium-rich surface such as concrete will give that surface some resistance to acid attack.<ref>Properties of Concrete by A M Neville</ref> :CaCO₃ + H₂O → &nbsp;Ca²⁺ + 2&nbsp;OH⁻ + CO₂ :H₂SiF₆ → 2&nbsp;H⁺ + {{Chem|SiF|6|2-}} :{{Chem|SiF|6|2-}} + 2&nbsp;H₂O → 6&nbsp;F⁻ + SiO₂ + 4&nbsp;H⁺ : &nbsp;Ca²⁺ + 2&nbsp;F⁻ → CaF₂

Calcium fluoride (CaF₂) is an insoluble solid that is acid resistant.

==Natural salts== Some rare minerals, encountered either within volcanic or coal-fire fumaroles, are salts of the hexafluorosilicic acid. Examples include ammonium hexafluorosilicate that naturally occurs as two polymorphs: cryptohalite and bararite.<ref name="Cryptohalite, Mindat">{{Cite web|url=https://www.mindat.org/min-1163.html|title=Cryptohalite}}</ref><ref name="Bararite, Mindat">{{Cite web|url=https://www.mindat.org/min-511.html|title = Bararite}}</ref><ref name="Kuszewski et al. 2020">{{cite journal | doi=10.1016/j.scitotenv.2019.134274 | title=Carbon‑nitrogen compounds, alcohols, mercaptans, monoterpenes, acetates, aldehydes, ketones, SF6, PH3, and other fire gases in coal-mining waste heaps of Upper Silesian Coal Basin (Poland) – a re-investigation by means of in situ FTIR external database approach | year=2020 | last1=Kruszewski | first1=Łukasz | last2=Fabiańska | first2=Monika J. | last3=Segit | first3=Tomasz | last4=Kusy | first4=Danuta | last5=Motyliński | first5=Rafał | last6=Ciesielczuk | first6=Justyna | last7=Deput | first7=Ewa | journal=Science of the Total Environment | volume=698 | article-number=134274 | pmid=31509784 | bibcode=2020ScTEn.69834274K | s2cid=202563638 }}</ref>

==Safety== Hexafluorosilicic acid can release hydrogen fluoride (HF) when evaporated, so it has similar risks. Inhalation of the vapors may cause lung edema. Like hydrogen fluoride, it attacks glass and stoneware.<ref>{{cite web |url=http://niosh.dnacih.com/nioshdbs/ipcsneng/neng1233.htm |title=Fluorosilicic Acid – International Chemical Safety Cards |website=NIOSH |access-date=2015-03-10}}</ref> The LD₅₀ value of hexafluorosilicic acid is 430&nbsp;mg/kg.{{r|NTP}}

==See also== * Ammonium fluorosilicate * Sodium fluorosilicate * Potassium fluorosilicate

==References== {{Reflist|30em}}

{{Hydrogen compounds}}

Category:Mineral acids Category:Hydrogen compounds Category:Nonmetal halides Category:Hexafluorosilicates Category:Fluoro complexes