Ammonium bifluoride

Ammonium bifluoride

Sample of Ammonium bifluoride
Names
IUPAC name Ammonium hydrogen fluoride
Other names Ammonium acid fluoride Ammonium hydrofluoride Ammonium difluoride Ammonium hydrogendifluoride Ammonium hydrogen difluoride
Identifiers
CAS Number	1341-49-7 Y
ChemSpider	21241205 N
Jmol 3D model	Interactive image Interactive image
InChI InChI=1S/F2H.H3N/c1-3-2;/h;1H3/q-1;/p+1 N Key: XIONBZYFLSJNBW-UHFFFAOYSA-O N InChI=1/2FH.H3N/h2*1H;1H3 Key: KVBCYCWRDBDGBG-UHFFFAOYAG
SMILES [NH4+].F[H-]F [F-].F.[NH4+]
Properties
Chemical formula	F2H5N
Molar mass	57.04 g·mol−1
Appearance	White crystals
Density	1.50 g cm−3
Melting point	126 °C (259 °F; 399 K)(decomposes)
Boiling point	240 °C (464 °F; 513 K)
Solubility in water	63g/100ml 20 °C
Solubility in alcohol	slightly soluble
Refractive index (nD)	1.390
Structure
Crystal structure	Cubic, related to the CsCl structure
Coordination geometry	[NH4]+ cation: tetrahedral [HF2]− anion: linear
Vapor pressure	{{{value}}}
Related compounds
Other cations	potassium bifluoride
Related compounds	ammonium fluoride
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

Ammonium hydrogen fluoride is the inorganic compound with the formula NH₄HF₂ or NH₄F·HF. It is produced from ammonia and hydrogen fluoride. This colourless salt is a glass-etchant and an intermediate in a once-contemplated route to hydrofluoric acid.

Structure

Ammonium bifluoride, as its name indicates, contains a bifluoride, or hydrogen(difluoride) anion: HF₂⁻. This centrosymmetric triatomic anion features the strongest known hydrogen bond, with a F−H length of 114 pm. and a bond energy greater than 155 kJ mol⁻¹.

In solid [NH₄][HF₂], each ammonium cation is surrounded by four fluoride centers in a tetrahedron, with hydrogen-fluorine hydrogen bonds present between the hydrogen atoms of the ammonium ion and the fluorine atoms. Solutions contain tetrahedral [NH₄]⁺ cations and linear [HF₂]⁻ anions.

Production and applications

Ammonium bifluoride is a component of some etchants. It attacks silica component of glass:

SiO₂ + 4 [NH₄][HF₂] → SiF₄ + 4 [NH₄]F + 2 H₂O

Potassium bifluoride is a related more commonly used etchant.

Ammonium bifluoride has been considered as an intermediate in the production of hydrofluoric acid from hexafluorosilicic acid. Thus, hexafluorosilicic acid is hydrolyzed to give ammonium fluoride, which thermally decomposes to give the bifluoride:

H₂SiF₆ + 6 NH₃ + 2 H₂O → SiO₂ + 6 NH₄F

2 NH₄F → NH₃ + [NH₄]HF₂

The resulting ammonium bifluoride is converted to the sodium bifluoride, which thermally decomposes to release HF.

See also

• Bifluoride anion
• Potassium bifluoride

References

1. A. F. Wells (1984) Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK.

• Emsley, J. (1980) Very strong hydrogen bonds, Chemical Society Reviews, 9, 91–124. doi:10.1039/CS9800900091
• Jean Aigueperse, Paul Mollard, Didier Devilliers, Marius Chemla, Robert Faron, Renée Romano, Jean Pierre Cuer (2005), "Fluorine Compounds, Inorganic" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim. doi:10.1002/14356007.a11 307