Tetrafluoroboric acid
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| Names | |
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Preferred IUPAC name
Tetrafluoroboric acid
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| Other names
tetrafluoroboric acid, oxonium tetrafluoroboranuide, oxonium tetrafluoridoborate(1-)
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| Identifiers | |
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3D model (JSmol)
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| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.037.165 |
| EC Number | 240-898-3 |
| 21702 | |
| MeSH | Fluoroboric+acid |
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PubChem CID
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| RTECS number | ED2685000 |
| UNII | |
| UN number | 1775 |
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| Properties | |
| BF4H | |
| Molar mass | 87.81 g·mol−1 |
| Appearance | Colourless liquid |
| Melting point | −90 °C (−130 °F; 183 K) |
| Boiling point | 130 °C (266 °F; 403 K) |
| Acidity (pKa) | ~1.8 (MeCN solution) |
| Hazards | |
| Safety data sheet | External MSDS |
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EU classification (DSD) (outdated)
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 C |
| R-phrases (outdated) | R34 |
| S-phrases (outdated) | (S1/2), S26, S27, S45 |
| NFPA 704 | |
| Related compounds | |
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Related compounds
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Hydrogen fluoride |
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Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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 (what is   ?) |
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| Infobox references | |
Fluoroboric acid or tetrafluoroboric acid is an inorganic compound with the chemical formula H
3OBF
4.
It is mainly produced as a precursor to other fluoroborate salts. 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. It is structurally similar to perchloric acid, but lacks the hazards associated with oxidants.
Although the solvent-free HBF4 has not been isolated, its solvates are well characterized. These salts consist of protonated solvent as a cation, e.g., H3O+ and H
5O+
2, and the tetrahedral BF−
4 anion. The anion and cations are strongly hydrogen-bonded.
Aqueous solutions of HBF4 are produced by dissolving boric acid in aqueous hydrofluoric acid. Three equivalents of HF react to give the intermediate boron trifluoride and the fourth gives fluoroboric acid:
Anhydrous solutions can be prepared by treatment of aqueous fluoroboric acid with acetic anhydride.
The acidity of fluoroboric acid is complicated by the fact that the name refers to several different species H(OEt2)+BF−
4, H3O+BF−
4, and HF.BF3 – each with a different acidity. The aqueous pKa is quoted as −0.44. Titration of NBu+
4BF−
4 in acetonitrile solution indicates that HBF4, i.e., HF.BF3, has a pKa of 1.6 in that solvent. Its acidity is thus comparable to that of fluorosulfonic acid.
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