Ammonium fluoride

Crystal structure

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 N−H···F hydrogen bonds between the anions and cations. This structure is very similar to ice, and ammonium fluoride is the only substance which can form mixed crystals with water.

Reactions

On passing hydrogen fluoride gas (in excess) through the salt, ammonium fluoride absorbs the gas to form the addition compound ammonium bifluoride. The reaction occurring is: :NH4F + HF → NH4HF2

Ammonium fluoride sublimes when heated—a property common among ammonium salts. In the sublimation, the salt decomposes to ammonia and hydrogen fluoride; the two gases can still recombine, i.e. the reaction is reversible: :[NH4]F ⇌ NH3 + HF

Uses

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 opposed to the "acid salt" (NH4HF2). The acid salt is usually used in preference to the neutral salt in the etching of glass and related silicates. This property is shared among all soluble fluorides. For this reason it cannot be handled in glass test tubes or apparatus during laboratory work.

Ammonium fluoride is a critical component of buffered oxide etch (BOE), a wet etchant used in microfabrication. It acts as the buffering agent in a solution of concentrated HF, creating an etchant with a more controllable rate of etching (than that of simple concentrated HF solutions).

It is also used for preserving wood, as a mothproofing agent, in printing and dyeing textiles, and as an antiseptic in breweries.

References

References

1. "Ammonium Fluoride".
2. "Ammonium Fluoride".
3. (January 2012). "Fluoride--the danger that we must avoid.". Romanian journal of internal medicine = Revue roumaine de medecine interne.
4. A. F. Wells, ''Structural Inorganic Chemistry'', 5th ed., Oxford University Press, Oxford, UK, 1984.
5. Brill, R.. (1954). "Mixed Crystals of Ice and Ammonium Fluoride". Nature.
6. (1986). "Silicon Processing for the VLSI Era: Volume 1 - Process Technology".
7. Aigueperse, Jean. (2005). "Encyclopedia of Industrial Chemistry". Wiley-VCH.