Thermal Decomposition of Sodium Phosphates

Sodium tripolyphosphate (STPP, Na5P3O10) was prepared by the thermal intermolecular dehydration of a spray-dried mixture of sodium phosphates, so that Na/P was 1.67. Thermal analysis, X-ray diffraction, Fourier transform infrared (FT-IR) analysis, and microscopic analysis with the application of a scanning electron microscope and an optical microscope with a heating system were applied in investigations. Temperatures of the formation of Na2H2P2O7 (similar to 463 K) and Na4P2O7 (similar to 488 K), as well as Na3P3O9 (>513 K) and NaPO3 (similar to 593 K), were established for the process in which sodium tripolyphosphate is obtained. At over 616 K NaPO3 melts and is the source of the liquid phase in the reaction system. The formation of anhydrous tripolyphosphate attains the highest speed around 563 K.