Magnesium Pyrophosphate

Sodium pyrophosphate precipitates amorphous Mg₂P₂O₇.3H₂O from magnesium sulphate solutions, which becomes the crystalline trihydrate when dissolved in sulphurous acid and heated. A gelatinous trihydrate separates from a heated solution of acidified sodium pyrophosphate in which magnesia has been dissolved. It is slightly soluble in water, but is soluble in nitric, hydrochloric, and pyrophosphoric acids. According to Popp, the amorphous precipitate spontaneously changes to a crystalline pentahydrate.

2MgO.P₂O₅.7H₂O, and two modifications of 2MgO.P₂O₅.15H₂O, have been prepared from magnesium sulphate and sodium phosphate under different conditions. Ammonium magnesium phosphate is converted into the first by soaking in glycerol.

These hydrates, MgHPO₄, and the double phosphate of ammonium and magnesium, ignite to the anhydrous pyrophosphate. Popp referred the incandescence which sometimes accompanies the last ignition to a passage from the crystalline to the amorphous condition. When there is no incandescence, the pyrophosphate is white and loose in texture; when there is, it is dark, lava-like, and more resistant to acids. Since the dark colour is due to enclosed and unburnt carbon, incandescence can be prevented by excluding organic matter. Slow precipitation at boiling temperature, evaporation before ignition with an ammonium salt, or very slow ignition also prevents it. Phosphides may occur during ignition if organic matter is present.

At the temperature of the electric furnace the pyrophosphate decomposes with the formation of phosphorus and magnesia. It also loses weight when heated in a blast.

Nitric acid liberates pyrophosphoric acid from magnesium pyrophosphate and forms magnesium nitrate.

Digestion with concentrated sulphuric acid converts it into orthophosphate. It dissolves in acids and does not combine with ammonia. It has been crystallised from the molten condition.