The Vaporization of NH4NO3

Abstract

The total vapor pressure and vapor molecular weight of ammonium nitrate (NH₄NO₃) were determined. The vapor pressure was determined by the torsion + effusion method, and vapor composition was determined by effusion-beam mass spectrometry. Total vapor pressures of NH₄NO₃ were measured by using two effusion cells with different orifice diameters over the pressure range of (10⁻⁶ to 10⁻³) kPa, between (313 and 360) K. The equilibrium vapor pressure equation was zero-extrapolated from measurements with different orifice Knudsen cells, P1 and P2 cells, and is given as: log P_T (kPa) = (10.400 ± 0.0002) − (4783.16 ± 0.07)/T. The measured molecular weight of NH₄NO₃ is 48.7 g/mol for P1 cell and 50.7 g/mol for P2 cell, both of which are much less than the theoretical molecular weight of NH₄NO₃ (approximately 80.04 g/mol). This significant difference in molecular weight suggests that there is disproportionation of NH₄NO₃ sample. The mass spectroscopic results revealed that NH₄NO₃ decomposes to NH₃ and HNO₃; it was interesting to note that the expected N₂, O₂, and H₂O gases were not evolved during vaporization. The partial pressures of the three gas phase species (NH₄NO₃, NH₃, and HNO₃) that were evolved during vaporization of NH₄NO₃ sample were determined as: P1 cell: PNH4NO3/P_T = 0.1490, cell: PNH3/P_T = 0.2911, PHNO3/P_T = 0.5599, and P2 cell: PNH4NO3/P_T = 0.2101, PNH3/P_T = 0.2702, and PHNO3/P_T = 0.5197. The standard Gibbs energy change (ΔG°) for NH₄NO₃ decomposition and sublimation reactions are obtained from the partial pressure results. Details of total and partial pressures of vaporization of NH₄NO₃ and disproportionation aspects of the evolved gases are presented.