Sodium monofluophosphate

An intimate mixtiu e of 40 g. of anhydrous sodiiun trimetaphosphate and 16.48 g. of anhydrous sodium fluoride is placed in a 100-ml. platinum dish. The dish and its contents are then placed in a muffle furnace at 800 for about 6 minutes, no more than long enough to effect complete fusion of the mixture. The fused mass is removed from the furnace, allowed to cool to below red heat in the open air, and then cooled to room temperature in a desiccator. This product consists of about 90 per cent sodium monofluophosphate. A small weight loss (0.2 to 0.3 per cent), due to the decomposition of sodium monofluophosphate, accompanies the fusion reaction. 

To analyze the product for phosphorus and fluorine, the latter is first distilled as hexafluosilicic acid in the presence of perchloric acid. The fluorine content of the distillate is then determined by titrating with standard thorium nitrate solution at a pH of about 3, using sodium alizarinsulfonate as the indicator, The phosphorus may be determined in the residue remaining after distillation by precipitating ammonium 12-molybdophosphate, dissolving the precipitate in an excess of standard sodium hydroxide, and titrating the excess base with standard acid. AtuU. Calcd. for NajPOjF F, 13.19 P, 21.53. Found F, 13.17 P, 21.53. 

For a general statement of the properties of monofluo-phosphates, see Lange, Inokganic Syntheses, 2,156 (1946), 

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Sodium monofluophosphate is very soluble in water. The heat of solution is large enough to cause a temperature rise when a concentrated solution is prepared. The solution is neutral and fairly stable at room temperature, but does undergo slow hydrolysis to orthophosphate and fluoride. Hydrolysis takes place rapidly in acid solution. 

Sodium monofluophosphate undergoes thermal decomposition at higher temperatures, yielding pyrophosphate. In a wet atmosphere the decomposition takes place through the evolution of hydrogen fluoride. 

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. 

NasPtOi is the mctapbosphate chosen for this fusion reaction since it may be prepared readily in the anhydrous state. Other sodium metaphosphates, such as Graham s salt, may also be employed, but even small amounts of moisture will bring about some hydrolysis of the monofluophosphate with a resulting decrease in the yield. 

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