Silver monofluophosphate

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. 

The potassium salt may be prepared in a like fashion by fusing potassium metaphosphate and potassium fluoride. 

Marquina Rev. acad. cienc. exact, fisqulm. y nat. Madrid, 30, 382 (1933). 

Methods of Analysis of the Association of Official Agricultural Chemists, 6th ed., p. 22, George Banta Publishing Company (The Collegiate Press), Monasha, Wis., 1945, 

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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. 

Silver monofluophosphate forms the anhydrous salt. It is sparingly soluble in water according to Lange, the solubility is 5.93 X 10 mol per liter at 20°. Like all mono-fluophosphates, it undergoes slow hydrolysis to orthophos-... 

The reagent is added to the supernatant of a diluted portion of a centrifuged or filtered sample. Addition of the silver nitrate solution to the concentrated solution is apt to be misleading since silver monofluophosphate has a limited solubility. 

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. 

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