{{Chembox | Verifiedfields = changed | Watchedfields = changed | verifiedrevid = 476998855 | ImageFile = Oxonium tetrafluoroborate.png | ImageFileL1 = Hydronium-3D-vdW.png | ImageFileR1 = Tetrafluoroborate anion 3D spacefill.png | ImageName = Canonical, skeletal formula of oxonium tetrafluoroborate | ImageCaption = Hydronium tetrafluoroborate | ImageCaptionL1 = Hydronium cation | ImageCaptionR1 = Tetrafluoroborate anion | PIN = Tetrafluoroboric acid<ref>{{cite web|url=https://moam.info/nomenclature-of-inorganic-chemistry-iupac_59c16afb1723ddd2fb171db1.html|title=Nomenclature of Inorganic Chemistry|last=IUPAC|access-date=2021-04-08}}</ref> | OtherNames = {{ubl |Hydrogen tetrafluoroborate |Oxonium tetrafluoroboranuide |Oxonium tetrafluoridoborate(1-) |Oxonium tetrafluoroborate }} | Section1 = {{Chembox Identifiers | CASNo1_Ref = {{cascite|changed|??}} | CASNo1 = 14219-41-1 | CASNo1_Comment = ({{chem2|[H3O]+[BF4]−}}) | CASNo2_Ref = {{cascite|correct|CAS}} | CASNo2 = 16872-11-0 | CASNo2_Comment = (solvent free) | CASNo3_Ref = {{cascite|changed|??}} | CASNo3 = 80628-99-5 | CASNo3_Comment = ({{chem2|[H5O2]+[BF4]−}}) | CASNo_Ref = {{cascite|changed|CAS}} | UNII_Ref = {{fdacite|correct|FDA}} | UNII = H429WZ9FBQ | PubChem = 28118 | ChemSpiderID = 26156 | ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}} | EINECS = 240-898-3 | UNNumber = 1775 | MeSHName = Fluoroboric+acid | ChEBI = 38902 | ChEBI_Ref = {{ebicite|correct|EBI}} | RTECS = ED2685000 | Gmelin = 21702 | SMILES = F[B-](F)(F)[FH+] | SMILES2 = [H+].F[B-](F)(F)F | StdInChI = 1S/BF4H/c2-1(3,4)5/h2H | StdInChI_Ref = {{stdinchicite|changed|chemspider}} | StdInChIKey = YKRRMQXMWYXWJW-UHFFFAOYSA-N | StdInChIKey_Ref = {{stdinchicite|changed|chemspider}} }} | Section2 = {{Chembox Properties | Formula = {{chem2|H[BF4]}} | H=1 | B=1 | F=4 | Appearance = Colourless liquid | Density ={{val|1.4|u=g/mL}} (48% solution in water)<ref name="Sigma">{{Sigma-Aldrich|sigald|id=207934|name=Tetrafluoroboric acid, 48%}}</ref> | MeltingPtC = -90 | MeltingPt_ref = <ref name="FisherSDS"/> | BoilingPtC = 130 | BoilingPt_ref = <ref name="FisherSDS"/> | pKa = ~1.8 (MeCN solution)<ref name="Esti">{{cite journal |last1=Kütt |first1=A. |last2=Rodima |first2=T. |last3=Saame |first3=J. |last4=Raamat |first4=E. |last5=Mäemets |first5=V. |last6=Kaljurand |first6=I. |last7=Koppel |first7=I.A. |last8=Garlyauskayte |first8=R.Yu. |last9=Yagupolskii |first9=Y.L. |last10=Yagupolskii |first10=L.M. |last11=Bernhardt |first11=E. |last12=Willner |first12=H. |last13=Leito |first13=I. |title=Equilibrium Acidities of Superacids |journal=The Journal of Organic Chemistry |date=21 January 2011 |volume=76 |issue=2 |pages=391–395 |doi=10.1021/jo101409p}}</ref> | pKb = | VaporPressure = {{cvt|5|mmHg}} ({{cvt|20|C|F K}})<ref name="Sigma"/> }} | Section7 = {{Chembox Hazards | GHS_ref=<ref name="Sigma"/> | GHSPictograms = {{GHS05}} | GHSSignalWord = Danger | HPhrases = {{H-phrases|314}} | PPhrases = {{P-phrases|264|280|301+330+331|303+361+353|304+340+310|305+351+338+310|405|501}} | NFPA-F = 1 | NFPA-H = 3 | NFPA-R = 0 | NFPA-S = - | NFPA_ref = <ref name="FisherSDS">{{cite web |title=SDS - Tetrafluoroboric acid, 48% |url=https://documents.thermofisher.com/DirectWebViewer/private/document.aspx?prd=ALFAA11484~~PDF~~MTR~~AGHS~~EN~~2025-12-27%2011:32:31~~Tetrafluoroboric%20acid%20%2048%%20min%20w/w%20aqueous%20solution~~ |website=www.thermofisher.com |publisher=ThermoFisher Scientific |access-date=2 February 2026 |date=27 December 2025}}</ref> | TLV = | TLV-TWA = {{val|2.5|u=mg/m3}} | TLV-STEL = {{val|6|u=mg/m3}}<ref name="FisherSDS"/> | TLV-C = | LD50 = | LDLo = | LC50 = | LCLo = | PEL = {{val|2.5|u=mg/m3}} (TWA) | REL = | IDLH = {{val|250|u=mg/m3}} }} | Section9 = {{Chembox Related | OtherCompounds = {{ubl |Hydrogen fluoride |Triflic acid |Hexafluorophosphoric acid }} }} }}
'''Fluoroboric acid''' or '''tetrafluoroboric acid''' (archaically, '''fluoboric acid''') is an inorganic compound with the simplified chemical formula {{chem2|H+[BF4]−|auto=1}}. Solvent-free tetrafluoroboric acid ({{chem2|H[BF4]}}) has not been reported. The term "fluoroboric acid" usually refers to a range of compounds including hydronium tetrafluoroborate ({{chem2|[H3O]+[BF4]−}}), which are available as solutions. The ethyl ether solvate is also commercially available, where the fluoroboric acid can be represented by the formula {{chem2|[H((CH3CH2)2O)_{''n''}]+[BF4]−}}, where {{mvar|n}} is 2.
It is mainly produced as a precursor to other fluoroborate salts.<ref name="eEROS">{{cite journal |last1=Friestad |first1=Gregory K. |last2=Branchaud |first2=Bruce P. |title=Tetrafluoroboric Acid |journal=Encyclopedia of Reagents for Organic Synthesis |date=15 April 2001 |doi=10.1002/047084289X.rt035}}</ref> It is a strong acid. Fluoroboric acid is corrosive and attacks the skin. It is available commercially as a solution in water and other solvents such as diethyl ether. It is a strong acid with a weakly coordinating, non-oxidizing conjugate base.<ref name=Esti/> It is structurally similar to perchloric acid, but lacks the hazards associated with oxidants.
== Structure and production == Pure {{chem2|H[BF4]}} has not been described. The same holds true for the superacids that are known by the simplified formulas [[Hexafluorophosphoric acid|{{chem2|H[PF6]}}]] and [[Hexafluoroantimonic acid|{{chem2|H[SbF6]}}]].<ref>{{Cite journal|last1=Juhasz|first1=Mark|last2=Hoffmann|first2=Stephan|last3=Stoyanov|first3=Evgenii|last4=Kim|first4=Kee-Chan|last5=Reed|first5=Christopher A.|date=2004-10-11|title=The Strongest Isolable Acid|journal=Angewandte Chemie International Edition|language=en|volume=43|issue=40|pages=5352–5355|doi=10.1002/anie.200460005|pmid=15468064|issn=1433-7851|doi-access=free}}</ref><ref>{{Cite journal|last=Reed|first=Christopher A.|date=2005|title=Carborane Acids. New "strong yet gentle" acids for organic and inorganic chemistry|journal=Chem. Commun.|language=en|issue=13|pages=1669–1677|doi=10.1039/B415425H|pmid=15791295|issn=1359-7345|url=https://escholarship.org/content/qt90p8v05h/qt90p8v05h.pdf?t=o9pbfh |format=pdf}}</ref> However, a solution of {{chem2|BF3}} in HF is highly acidic, having an approximate speciation of {{chem2|[H2F]+[BF4]–}} (fluoronium tetrafluoroborate) and a Hammett acidity function of −16.6 at {{val|7|u=mol %}} {{chem2|BF3}}, easily qualifying as a superacid.<ref>{{Cite book|title=Superacid chemistry |isbn=9780471596684 |edition=2nd |publisher=Wiley |location=Hoboken, N.J. |oclc=191809598 |last1=Olah |first1 = George A. |last2=Surya Prakash |first2=G. K. |last3=Sommer |first3=Jean |last4=Molnar |first4=Arpad |date=2009-02-03}}</ref>{{page needed|date=February 2026}} Although the solvent-free {{chem2|H[BF4]}} has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g., {{chem2|H3O+}} and {{chem2|H5O2+}}, and the tetrahedral {{chem2|BF4−}} anion. The anion and cations are strongly hydrogen-bonded.<ref>{{cite journal |last1=Mootz |first1=Dietrich |last2=Steffen |first2=Michael |title=Kristallstrukturen von Säurehydraten und Oxoniumsalzen. XX. Die Oxoniumtetrafluoroborate {{chem|H|3|O|B|F|4}}, {{chem|[|H|5|O|2|]|B|F|4}} und {{chem|[|H|(|C|H|3|O|H|)|2|]|B|F|4}} |language=de |trans-title=Crystal structures of acid hydrates and oxonium salts. XX. Oxonium tetrafluoroborates H<sub>3</sub>OBF<sub>4</sub>, [H<sub>5</sub>O<sub>2</sub>]BF<sub>4</sub>, and [H(CH<sub>3</sub>OH)<sub>2</sub>]BF<sub>4</sub> |journal=Zeitschrift für anorganische und allgemeine Chemie |date=November 1981 |volume=482 |issue=11 |pages=193–200 |doi=10.1002/zaac.19814821124}}</ref>{{better citation needed |reason=Something English on the English wiki maybe? (Or at least a translated quote)|date=February 2026}} [[File:H3OBF4subunit.png|thumb|left|Subunit of crystal structure of {{chem2|[H3O]+[BF4]−}} highlighting the hydrogen bonding between the cation and the anion]]
Aqueous solutions of {{chem2|H[BF4]}} are produced by dissolving boric acid in aqueous hydrofluoric acid.<ref>{{Ullmann|title = Boron Compounds | doi = 10.1002/14356007.a04_309 | author1 = Brotherton, R. J. |author2=Weber, C. J.|author3=Guibert, C. R.|author4= Little, J. L. }}</ref><ref>{{ OrgSynth | author = Flood, D. T. | title = Fluorobenzene | year = 1933 | volume = 13 | page = 46 | collvol = 2 | collvolpages = 295 | prep = CV2P0295 |doi=10.15227/orgsyn.013.0046}}</ref> Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid: :{{chem2|B(OH)3 + 4 HF -> H3O+ + BF4− + 2 H2O}} An anhydrous fluoroboric acid solution can be prepared by adding aqueous fluoroboric acid to an excess of acetic anhydride at {{cvt|0|C}}, which produces a solution of fluoroboric acid, acetic acid, and residual acetic anhydride.<ref>{{cite journal |last1=Wudl |first1=F. |last2=Kaplan |first2=M. L. |last3=Engler |first3=E. M. |last4=Patel |first4=V. V. |title=2,2′‐Bi‐L,3‐Dithiolylidene (Tetrathiafulvalene, Ttf) and its Radical Cation Salts |journal=Inorganic Syntheses |date=January 1979 |volume=19 |pages=27–34 |doi=10.1002/9780470132500.ch7}}</ref>
===Acidity=== Determination of the acidity of fluoroboric acid is complicated by the fact that its name refers to a range of different compounds, e.g. {{chem2|[H(CH3CH2)2O]+[BF4]−}} (dimethyloxonium tetrafluoroborate), {{chem2|[H3O]+[BF4]−}} (oxonium tetrafluoroborate), and {{chem2|HF*BF3}} (hydrogen fluoride-boron trifluoride 1:1 adduct) – each with a different acidity. The aqueous {{math|p''K''{{sub|a}}}} is quoted as −0.44.<ref name=eEROS/> Titration of {{chem2|[N((CH2)3CH3)4]+[BF4]−}} (tetrabutylammonium tetrafluoroborate) in acetonitrile solution indicates that {{chem2|H[BF4]}}, i.e., {{chem2|HF*BF3}}, has a {{math|p''K''{{sub|a}}}} of 1.6 in that solvent. Its acidity is thus comparable to that of fluorosulfonic acid.<ref name=Esti/>
== Applications == Fluoroboric acid is the principal precursor to fluoroborate salts, which are typically prepared by treating the metal oxides with fluoroboric acid. The inorganic salts are intermediates in the manufacture of flame-retardant materials and glazing frits, and in electrolytic generation of boron. {{chem2|H[BF4]}} is also used in aluminum etching and acid pickling.{{cn|date=February 2026}}
=== Organic chemistry === {{unreferenced section|date=February 2026}} {{chem2|H[BF4]}} is used as a catalyst for alkylations and polymerizations. In carbohydrate protection reactions, ethereal fluoroboric acid is an efficient and cost-effective catalyst for transacetalation and isopropylidenation reactions. Acetonitrile solutions cleave acetals and some ethers. Many reactive cations have been obtained using fluoroboric acid, e.g. tropylium tetrafluoroborate ({{chem2|C7H7+[BF4]−}}), triphenylcarbenium tetrafluoroborate ({{chem2|Ph3C]+[BF4]−}}), triethyloxonium tetrafluoroborate ({{chem2|Et3O]+[BF4]−}}), and benzenediazonium tetrafluoroborate ({{chem2|[PhN2]+[BF4]−}}).
===Electroplating=== Solutions of {{chem2|H[BF4]}} are used in the electroplating of tin and tin alloys. In this application, methanesulfonic acid is displacing the use of {{chem2|H[BF4]}}.<ref name=electrol>{{cite journal|doi = 10.1080/00202967.2003.11871526|title = Methanesulphonic acid in electroplating related metal finishing industries|journal = Transactions of the Imf|volume = 81|issue = 5|pages = 154–158|year = 2003|last1 = Balaji|first1 = R.|last2 = Pushpavanam|first2 = Malathy|s2cid = 91584456}}</ref> Fluoroboric acid is also used for high-speed electroplating of copper in fluoroborate baths.<ref>{{cite journal |last1=Barauskas |first1=Romualdas "Ron" |title=Copper plating |journal=Metal Finishing |date=January 1, 2000 |volume=98 |issue=1 |pages=234–247 |doi=10.1016/S0026-0576(00)80330-X |language=en |issn=0026-0576}}</ref>
==Safety== Fluoroboric acid is toxic and attacks skin and eyes. It attacks glass.<ref name=eEROS/> It hydrolyzes, releasing corrosive, volatile hydrogen fluoride.<ref name=electrol/>
==Other fluoroboric acids== A series of fluoroboric acids is known in aqueous solutions. The series can be presented as follows:<ref>{{Greenwood&Earnshaw}}</ref>{{page needed|date=February 2026}} *{{chem2|H+[B(OH)4]−}} (hydrogen tetrahydroxyborate) (not a fluoroboric acid) *{{chem2|H+[BF(OH)3]−}} (hydrogen fluoro(trihydroxy)borate) *{{chem2|H+[BF2(OH)2]−}} (hydrogen difluoro(dihydroxy)borate) *{{chem2|H+[BF3(OH)]−}} (hydrogen trifluoro(hydroxy)borate) *{{chem2|H+[BF4]−}} (hydrogen tetrafluoroborate)
==See also== * Fluorosulfuric acid * Fluoroantimonic acid
==References== {{reflist}}
{{Hydrogen compounds}} {{fluorine compounds}}
Category:Tetrafluoroborates Category:Mineral acids