Ammonium difluophosphate

Ammonium difluophosphate has been prepared in large quantities by the reaction of phosphorus (V) oxide with ammonium fluoride. In this reaction di- and monofluo-phosphates are formed in equimolecular amounts. After the molten mixture has cooled, ammonium difluophosphate, NH4PO2F2, is extracted with boiling alcohol. 

To prepare the pure salt, the crude material is dissolved in 3 ml. of hot water, and the solution is quickly filtered into a platinum crucible. The filtrate is cooled in ice and the crystals are filtered and dried on filter paper 1.6 g. of analytically pure substance is obtained (20 per cent of the product present in the fusion). The product is dried in a desiccator over sulfuric acid and solid potassium hydroxide and stored in the absence of moisture. In a platinum container it is stable indefinitely, but glass is attacked in the course of time. 

When larger amounts of the pure salt are required, several reactions are carried out with the indicated quantities. The collected raw products should be dissolved in hot water in one portion and recrystallized very quickly. 

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. 

The salts of difluophosphoric acid have solubilities resembling those of the perchlorates. However, the shghtly soluble perchlorates are somewhat less soluble than the corresponding difluophosphates. These solubility relationships allow the conversion of ammonium difluophosphate to less soluble salts by metathesis. 

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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 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 ... 

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