Ammonium bifluoride

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.

Ammonium bifluoride
Space fill model of the ammonium cation
Space fill model of the bifluoride anion
Names
IUPAC name
Ammonium hydrogen fluoride
Other names
Ammonium acid fluoride

Ammonium hydrofluoride
Ammonium difluoride
Ammonium hydrogendifluoride

Ammonium hydrogen difluoride
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.014.252
EC Number
  • 215-676-4
UNII
UN number 1727
Properties
F2H5N
Molar mass 57.044 g·mol−1
Appearance White crystals
Density 1.50 g cm−3
Melting point 126 °C (259 °F; 399 K)
Boiling point 240 °C (464 °F; 513 K)(decomposes)
63g/100ml 20 °C
Solubility in alcohol slightly soluble
1.390
Structure
Cubic, related to the CsCl structure
[NH4]+ cation: tetrahedral
[HF2]− anion: linear
Hazards
GHS pictograms [1]
H301, H314[1]
P280, P301+310, P305+351+338, P310[1]
NFPA 704 (fire diamond)
Flammability code 0: Will not burn. E.g. waterHealth code 3: Short exposure could cause serious temporary or residual injury. E.g. chlorine gasReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
3
0
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).
N verify (what is YN ?)
Infobox references

Structure

Ammonium bifluoride, as its name indicates, contains an ammonium cation (NH4+) and a bifluoride, or hydrogen(difluoride), anion (HF2−). The centrosymmetric triatomic bifluoride anion features the strongest known hydrogen bond, with a F−H 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

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

SiO2 + 4 [NH4][HF2] → SiF4 + 4 NH4F + 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 sodium bifluoride, which thermally decomposes to release HF.[4]

Ammonium bifluoride is also used as an additive in tin-nickel plating processes as the fluoride ion acts as a complexing agent with the tin, allowing for greater control over the resulting composition and finish.

Toxicity

Ammonium bifluoride is toxic to consume and a skin corrosion agent. Upon exposure to skin, rinsing with water followed by a treatment of calcium gluconate is required.[1]

References
  1. 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 978-0-08-037941-8.
  3. Emsley, J. (1980) Very Strong Hydrogen Bonds, Chemical Society Reviews, 9, 91–124. doi:10.1039/CS9800900091
  4. Aigueperse, Jean; Mollard, Paul; Devilliers, Didier; Chemla, Marius; Faron, Robert; Romano, René; Cuer, Jean Pierre (2000). "Fluorine Compounds, Inorganic". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a11_307.
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