Something about Ammonium hydrogen！

　　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 hydrogen

　　1.Structure
　　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 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.

　　2.production and application
　　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.