Ammonium bifluoride

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
CAS Number	1341-49-7 ;
3D model (JSmol)	Interactive image; Interactive image;
ChemSpider	21241205 ;
ECHA InfoCard	100.014.252
EC Number	215-676-4;
PubChem CID	14935;
UNII	C2M215358O;
UN number	1727
CompTox Dashboard (EPA)	DTXSID9029645 ;
	InChI InChI=1S/F2H.H3N/c1-3-2;/h;1H3/q-1;/p+1 Key: XIONBZYFLSJNBW-UHFFFAOYSA-O ; InChI=1/2FH.H3N/h2*1H;1H3 Key: KVBCYCWRDBDGBG-UHFFFAOYAG;
	SMILES [NH4+].F[H-]F; [F-].F.[NH4+];
Properties
Chemical formula	F2H5N
Molar mass	57.044 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 labelling:
Pictograms	class="wikitable collapsible" style="min-width: 50em;"

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.^[3] and a bond energy greater than 155 kJ mol⁻¹.^[4]

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.^{[citation needed]} 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 the sodium bifluoride, which thermally decomposes to release HF.^[5]

References

^ ^a ^b "Globally Harmonized System of Classification and Labelling of Chemicals" (pdf). 2021. Annex 3: Codification of Statements and Pictograms (pp 268–385).
^ ^a ^b ^c Sigma-Aldrich Co., Ammonium bifluoride. Retrieved on 2013-07-20.
^ Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
^ 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

[GHS2021_annex-3-1] "Globally Harmonized System of Classification and Labelling of Chemicals" (pdf). 2021. Annex 3: Codification of Statements and Pictograms (pp 268–385).

[sigma-2] Sigma-Aldrich Co., Ammonium bifluoride. Retrieved on 2013-07-20.

[3] Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

[4] Emsley, J. (1980) Very strong hydrogen bonds, Chemical Society Reviews, 9, 91–124. doi:10.1039/CS9800900091

[Aigueperse-5] 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

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[2]

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[5]

Code	Symbol description	Image link
GHS01	{{GHS exploding bomb}}	Image:GHS-pictogram-explos.svg	Explosive
GHS02	{{GHS flame}}	Image:GHS-pictogram-flamme.svg
GHS03	{{GHS flame over circle}}	Image:GHS-pictogram-rondflam.svg
GHS04	{{GHS gas cylinder}}	Image:GHS-pictogram-bottle.svg
GHS05	{{GHS corrosion}}	Image:GHS-pictogram-acid.svg	Corrosive
GHS06	{{GHS skull and crossbones}}	Image:GHS-pictogram-skull.svg	Accute Toxic
GHS07	{{GHS exclamation mark}}	Image:GHS-pictogram-exclam.svg	Irritant
GHS08	{{GHS health hazard}}	Image:GHS-pictogram-silhouette.svg	Health Hazard
GHS09	{{GHS environment}}	Image:GHS-pictogram-pollu.svg	Environment