AMMONIUM BIFLUORIDE
Thursday, October 2nd, 2008Formula: NH4HF2; MW 57.04
Synonym: ammonium hydrogen fluoride
Uses
NH4HF2 is used to solubilize silica and silicates in siliceous rocks of oil wells, thus to regenerate oil flow; as a neutralizer for alkalies in textile plants and commercial laundries; for removing stains from fabrics; for treating, polishing and rapid frosting of glass plates, window panes, picture frames, ampoules and optical lenses; to produce pure salts of metal fluorides; in treatment processes to prevent corrosion on magnesium and its alloys; in the preservation of wood; and in aluminum anodizing formulations.
Physical Properties
Orthorhombic or tetragonal crystals; etches glass; deliquescent; density 1.50 g/cm3; refractive index 1.390; melts at 125.6°C; very soluble in water; slightly soluble in alcohol.
Preparation
Commercial grade salt containing 1% NH4F is made by gas-phase reaction of one mole of anhydrous ammonia with two moles of hydrogen fluoride:
NH3 + 2 HF → NH4HF2
It may also be prepared in the anhydrous form by dehydration of ammonium fluoride solution, followed by thermal decomposition of dry crystals.
Reactions
Thermal dissociation produces ammonium fluoride and ammonia; at elevated temperatures products contain ammonia and hydrogen fluoride. It forms a colorless double salt, ammonium iron fluoride 3NH4F•FeF3, with iron, a reaction of commercial application for removing stains from fabric. It reacts with many metal oxides at elevated temperatures forming double fluorides:
2NH4HF2BeO→(NH4)2BeF4+H2O
6NH4HF2+Al2O3→2(NH4)2AlF6+3H2O
On further heating, the double fluorides decompose to metal fluorides, liberating ammonia and hydrogen fluoride:
(NH4)2BeF4→BeF2+2NH3+2HF
(NH4)3A1F6→A1F3+3NH3+3HF
The above reactions are employed commercially for obtaining metal fluorides in high purity. Similar reactions occur with many metal carbonates at elevated temperatures, producing double fluorides. The latter decompose to metal fluorides on further heating:
2NH4HF2+MgCO2→(NH4)2MgF4+CO2
(NH4)2MgF4→MgF2+2NH3+2HF
Analysis
Elemental composition: F 66.61%; H 8.83%; N 24.55% A measured amount of salt is thermally decomposed to ammonia and hydrogen fluoride. These gases liberated in stoichiometric amounts are
(NH4)2BeF4→BeF2+2NH3+2HF
(NH4)3A1F6→A1F3+3NH3+3HF
absorbed in excess standard sulfuric acid solution. Ammonia is measured by back titration of excess acid against a standard solution of caustic soda, using methyl orange indicator. Fluoride ion is measured with an ion-specific electrode. Ammonia may be collected and measured by alternative techniques (see Ammonia).
