Ammonium monofluophosphate

Ammonium monofluophosphate, (NH4)2POsF, is formed in about equal amount with the difluophosphate by the reaction of phosphorus(V) oxide with ammonium fluoride. Monofluophosphates can also be prepared by carefully hydrolyzing difluophosphates/ which may be obtained by hydrolysis of phosphorus(V) oxyfluoride. The following sequence of reactions takes place  

Under certain conditions, the last reaction is reversible, as considerable amounts of monofluophosphoric acid are formed from hydrofluoric acid and concentrated orthophos-phoric acid according to the equation 

In the preparation of ammonium difluophosphate (synthesis 47) 4.15 g. of impure ammpnium monofluophosphate is obtained. If this is dissolved in 5.8 ml. of hot water and filtered into a platinum crucible cooled in ice, 0.58 g. of pure, well-crystaUized salt separates. The crystals are dried on filter paper in the open air. However, it is preferable to dissolve the finely powdered crude product in water at room temperature in a platinum dish and to evaporate the solution in a vacuum over sulfuric acid. Then the 

Ammonium monofiuophosphate 1-hydrate crystallizes from water in compact rectangular prisms, which effloresce in the air. At 105°, the crystals become anhydrous. At higher temperatures, ammonia is slowly lost, and, at 220°, the acid ammonium salt, NH4HPO3F, is formed. This melts at 225° without decomposition. 

The salt is very soluble in water and gives a neutral solution. After some time, however, salts of orthophos-phoric acid are formed by hydrolysis. If possible, solutions should be stored in platinum containers. 

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The monofluophosphates closely resemble the sulfates in their solubilities. Therefore, monofluophosphates can be obtained by allowing ammonium monofluophosphates in water solution to react with easily soluble salts of those cations which form more difficultly soluble sulfates. The silver compound crystallizes especially well in colorless rhombs. It is used for preparing the alkali salts from solutions of the corresponding alkali chlorides. The solutions so obtained are evaporated in vacuo at room temperature. 

Ammonium difluophosphate crystallizes in colorless rhombic prisms and melts at 213° without decomposition. It is very soluble in water. The solution at first has a neutral reaction but hydrolyzes after a time to acid aiiomo-nium monofluophosphate, this process being greatly accelerated by raising the temperature. Therefore, solutions of the salt should be stored only for a short time and at the lowest possible temperature, especially if handled in glass equipment. 

Silver monofluophosphate is needed in the purification of crude sodium monofluophosphate. It has been prepared by Lange from the ammonium fluoride-phosphorus (V) oxide fusion residues but, using the same general technique, it is more conveniently made from a sodium metaphosphate-sodium fluoride fusion residue. The impurities in such a mixture are the various phosphates whose silver salts are quite insoluble and may be removed from solution as such silver monofluophosphate is prepared by adding solid silver nitrate to the filtrate. 

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