The vaporization of NH 4 NO 3 Wen-Ming Chien a , Dhanesh Chandra a, * , K.H. Lau b , D.L. Hildenbrand b , A.M. Helmy c a Chemical and Materials Sciences, University of Nevada, Reno, NV 89557, United States b SRI International, Menlo Park, CA 94025, United States c Science and Technology Applications, 13762 Stagecoach Trail, Moorpark, CA 93021, United States article info Article history: Received 23 July 2009 Received in revised form 1 January 2010 Accepted 29 January 2010 Available online 4 February 2010 Keywords: NH 4 NO 3 Vapor pressure Torsion + effusion method Mass spectrometry Air bags abstract The total vapor pressure and vapor molecular weight of ammonium nitrate (NH 4 NO 3 ) were determined. The vapor pressure was determined by the torsion + effusion method, and vapor composition was deter- mined by effusion-beam mass spectrometry. Total vapor pressures of NH 4 NO 3 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 NH 4 NO 3 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 NH 4 NO 3 (approximately 80.04 g/mol). This significant difference in molecular weight suggests that there is dis- proportionation of NH 4 NO 3 sample. The mass spectroscopic results revealed that NH 4 NO 3 decomposes to NH 3 and HNO 3 ; it was interesting to note that the expected N 2 ,O 2 , and H 2 O gases were not evolved during vaporization. The partial pressures of the three gas phase species (NH 4 NO 3 , NH 3 , and HNO 3 ) that were evolved during vaporization of NH 4 NO 3 sample were determined as: P1 cell: P NH 4 NO 3 /P T = 0.1490, P NH 3 /P T = 0.2911, and P HNO 3 /P T = 0.5599, and P2 cell: P NH 4 NO 3 /P T = 0.2101, P NH 3 /P T = 0.2702, and P HNO 3 / P T = 0.5197. The standard Gibbs energy change (DG°) for NH 4 NO 3 decomposition and sublimation reac- tions are obtained from the partial pressure results. Details of total and partial pressures of vaporization of NH 4 NO 3 and disproportionation aspects of the evolved gases are presented. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Ammonium nitrate (AN) is of great interest for use in sealed gas generators for automobile air bag systems. NH 4 NO 3 is used as an oxidizer along with propellants and other ingredients in gas gener- ators for automobile air bags. In addition, there are new prospects for use in rocket motors which are also sealed. In processing these materials, the knowledge of vapor pressures and species that are evolving during processing are very important for safety in unit operations of air bag processing, and for personnel safety. The vaporization of NH 4 NO 3 in small or large sealed containers requires accurate knowledge of the pressure experienced by the contain- ment vessel. In the important temperature range of the gas gener- ators for automobile air bag (40 to +100) °C, phase transitions cause abrupt changes in specific volume for NH 4 NO 3 , leading to irreversible growth of cast charges. Ammonium nitrate undergoes four different solid-state phase transitions when heated [1–4]. Pure ammonium nitrate has five different solid-state phases: AN I, AN II, AN III, AN IV, and AN V phases. The high-temperature AN I phase (above 125 °C and before melting) was reported as cubic structure (Pm  3m) and lattice parameter a = 0.437 nm [5,6]. Shinnaka [7] and Lucas et al. [8] reported the AN II phase was stable between (84 and 125) °C with the tetragonal structure with space group P  42 1 m (a = 0.57193 nm and c = 0.49326 nm). The orthorhombic structure (Pnma) of the AN III phase with a = 0.77184 nm, b = 0.58447 nm and c = 0.0.71624 nm, was reported to be stable between (32 and 84) °C [9,10]. The low-temperature AN IV and AN V phases were reported as orthorhombic structure with space group Pmmn [stable (16 to 32) °C] [8,11] and orthorhombic struc- ture with space group Pccn (stable below 16°C) [12,13], respec- tively. Feick [14] determined the vapor pressure of liquid ammonium nitrate from (190 to 240) °C using a modified boiling point apparatus. Later, Brandner et al. [15] reported the vapor pres- sure of solid ammonium nitrate from (76 to 165) °C using a tran- spiration method. The purpose of this study was to determine the total equilib- rium vapor pressures of formulated gas generator materials; mainly second law thermodynamic measurements. In order to understand the vaporization behavior of NH 4 NO 3 sample, the total vapor pressure and vapor molecular weight were studied by the torsion + effusion method, and vapor composition was also studied 0021-9614/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2010.01.012 * Corresponding author. Tel.: +1 775 784 4960; fax: +1 775 784 4316. E-mail address: dchandra@unr.edu (D. Chandra). J. Chem. Thermodynamics 42 (2010) 846–851 Contents lists available at ScienceDirect J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct