Ammonium bifluoride

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Ammonium bifluoride
Space fill model of the ammonium cation Space fill model of the bifluoride anion
Sample of Ammonium bifluoride
Identifiers
CAS number 1341-49-7 YesY
ChemSpider 21241205 N
Jmol-3D images Image 1
Image 2
Properties
Molecular formula H5F2N
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
Hazards
GHS pictograms GHS-pictogram-acid.svgGHS-pictogram-skull.svg[1]
GHS hazard statements H301, H314[1]
GHS precautionary statements P280, P301+310, P305+351+338, P310[1]
NFPA 704
Flammability (red): no hazard code Health code 3: Short exposure could cause serious temporary or residual injury. E.g., chlorine gas Reactivity (yellow): no hazard code Special hazards (white): no codeNFPA 704 four-colored diamond
Related compounds
Other cations potassium bifluoride
Related compounds ammonium fluoride
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
 N (verify) (what is: YesY/N?)
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[edit]

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 FH length of 114 pm.[2] and a bond energy greater than 155 kJ mol−1.[3]

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[edit]

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.[4]

See also[edit]

References[edit]

  1. ^ a b c Sigma-Aldrich Co., Ammonium bifluoride. Retrieved on 2013-07-20.
  2. ^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419. 
  3. ^ Emsley, J. (1980) Very strong hydrogen bonds, Chemical Society Reviews, 9, 91–124. doi:10.1039/CS9800900091
  4. ^ 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
  1. A. F. Wells (1984) Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK.