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ammonium bi fluoride

Ammonium fluoride

Ammonium fluoride, [NH4]F, may be obtained by neutralizing ammonia with hydrofluoric acid. It crystallizes as small prisms, having a sharp saline taste, and is exceedingly soluble in water. It decomposes silicates, and thus glass, on being heated with them, a property shared among all soluble fluorides. For this reason it cannot be handled in glass test tubes or apparatus during laboratory work. It also sublimes when heated - a property common among ammonium salts. This is due to the fact that the salt decomposes to ammonia and hydrogen fluoride when heated, and the two gases react to form ammonium fluoride when cold, i.e. the reaction is reversible:

[NH4]F ↔ NH3 + HF

Ammonium fluoride adopts the wurtzite crystal structure, in which both the ammonium cations and the fluoride anions are stacked in ABABAB... layers, each being tetrahedrally surrounded by four of the other. There are NH...F hydrogen bonds between the anions and cations.

On passing hydrogen fluoride gas (in excess) through the salt, ammonium fluoride absorbs the gas to form the addition compound ammonium hydrogen fluoride. The reaction occurring is:

NH4F + HF → [[Ammonium hydrogen fluoride|[NH4][HF2]]] or [NH4]F·HF

This substance is commonly called "commercial ammonium fluoride". The word "neutral" is sometimes added to "ammonium fluoride" to represent the neutral salt - [NH4]F. As the acid salt contains a higher percentage of fluoride by mass, it is usually used in preference to the neutral salt in the etching of glass.

References

  • A. F. Wells, 'Structural Inorganic Chemistry, 5th ed., Oxford University Press, Oxford, UK, 1984.

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