THERMAL-BEHAVIOR OF AMMONIUM DIHYDROGEN PHOSPHATE CRYSTALS IN THE TEMPERATURE-RANGE 25-600-DEGREES-C

A slow evaporation technique was used to grow single crystals of ammonium dihydrogen phosphate (ADP) from supersaturated aqueous solutions. An identification of crystal structure for as-grown ADP crystals was achieved. Simultaneous TG, DTG and DTA showed that the ADP crystals decompose at 210-degrees-C to orthophosphoric acid with evolution of ammonia. The orthophosphoric acid converts to pyrophosphoric acid and then to metaphosphoric acid. A pronounced resolution of the overlap between phase transition, melting and decomposition of ADP crystals was obtained at relatively high heating rates. Differential scanning calorimetry (DSC) confirmed the phase transition in ADP crystals at 175-degrees-C. Kissinger and Ozawa methods were applied to calculate the activation energies for the thermal processes occurring at 210-degrees-C, which are 90.37 kJ mol-1 and 99.12 kJ mol-1 respectively. The phase transitions and thermal decompositions of ADP crystals have been confirmed by X-ray diffraction and infrared absorption spectra.