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Hydrogendifluoride ion.svg
Other names
Bifluoride anion
Hydrogen difluoride anion
3D model (Jmol)
Molar mass 39.00 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Bifluoride is an inorganic anion with the chemical formula HF
(also written [HF2]). It contributes no color to bifluoride salts. Salts of bifluoride are used to etch glass.

Acid-base properties and production[edit]

Bifluoride undergoes the typical[clarification needed] chemical reactions of a weak acid. Upon treatment with a standard acid, it converts to hydrofluoric acid and a metal salt. Oxidation of bifluoride gives fluorine.[citation needed] When heated, bifluoride salts decompose to produce fluoride salts and hydrogen fluoride:

[HF2] HF + F

Bifluoride protonates to give hydrogen fluoride:

+ H+ 2 HF

Because of this capture of a proton (H+), bifluoride has basic character. Its conjugate acid is the reactive intermediate, μ-fluoro-fluorodihydrogen (H2F2), which subsequently dissociates to become hydrogen fluoride. In solution, most bifluoride ions are dissociated.

+ H
2 HF + HO

Bifluoride salts arise by treatment of hydrogen fluoride with base[clarification needed]:

HF + F → [HF2]


This centrosymmetric triatomic anion features a symmetric hydrogen bond, the strongest known hydrogen bond, with an FH length of 114 pm[1] and a bond strength of >155 kJ mol−1.[2] A molecular orbital diagram reveals the atoms to be held together by a 3-center 4-electron bond.[3] It is isoelectronic with the fluoroheliate anion, FHeO, whose existence is suspected[4] but not confirmed. "Hydrogen(difluoride)" is written as one word because it is a unified (covalent) anion; "hydrogen difluoride" would instead imply the electrically neutral compound HF2 (CAS number 12528-21-1).


Some HF2 salts are common, examples include potassium bifluoride (KHF2, also called potassium hydrogen fluoride) and ammonium bifluoride ([NH4][HF2]). Many salts claimed to be anhydrous sources of simple fluoride (F) ions, for example, tetra-n-butylammonium fluoride, can decompose to yield bifluoride instead.[citation needed]

Autodissociation of pure HF[edit]

The bifluoride ion also contributes to the unusually high auto-protolysis constant of liquid anhydrous hydrogen fluoride, which autodissociates in a manner similar to the self-ionization of water. This equilibrium can be denoted as

HF H+ + F

However, both the H+ and F ions are solvated by HF, so a better descriptive equation is

3HF H2F+(HF) + HF2(HF)


  1. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 
  2. ^ Emsley, J., "Very Strong Hydrogen Bonds", Chemical Society Reviews, 1980, 9, 91-124.
  3. ^ Pimentel, G. C. The Bonding of Trihalide and Bifluoride Ions by the Molecular Orbital Method. J. Chem. Phys. 1951, 19, 446-448. doi:10.1063/1.1748245
  4. ^ "Collapse of helium's chemical nobility predicted by Polish chemist" (PDF). Retrieved 2009-05-15.