Ammonium bifluoride

Ammonium hydrogen fluoride, NH4HF2, is a chemical compound which can be produced from ammonia and hydrogen fluoride. It is the major constituent in glass-etching fluid (though sometimes replaced by potassium hydrogen fluoride), etching occurring due to the fluorination of silicon:

SiO2(s) + 4[NH4][HF2](aq) → SiF4(l) + 4[NH4]F(aq) + 2H2O(l)

The structure of [NH4][HF2] contains tetrahedral [NH4]+ cations and linear [HF2]− anions. Each cation is surrounded by four fluorine atoms in a tetrahedron, with hydrogen - fluorine hydrogen bonds present between the hydrogen atoms of the ammonium ion and the fluorine atoms.