FTIR characterization of polymorphic transformation of ammonium nitrate

Hong Bo Wu, Man Nin Chan, Chak K. Chan

Research output: Contribution to journalArticlepeer-review

100 Scopus citations

Abstract

Solid ammonium nitrate (NH4NO3) exists in five stable polymorphic forms (designated as phases I, II, III, IV, and V) below its melting point at around 170°C. Phase IV is stable in a temperature range of -17°C ∼ 32°C and is the only phase that has been considered by the atmospheric research community until recently. In this study, we examine the IV ↔ III phase transition of NH4NO3 and how relative humidity (RH) affects the transition path and the transition temperatures using in-situ microscopic Fourier Transform InfraRed spectroscopy. Two kinds of NH4NO3 samples, powder produced from grinding commercially produced chemicals and single particles obtained by efflorescence of droplets on PTFE filters, were studied. The powder samples exhibit the IV ↔ III phase transition and the transition temperature depends on the RH while the single particle samples exhibit only the IV ↔ II transition at about 52°C (forward) and 48°C (reverse), bypassing phase III, with transition temperatures independent of the RH. However, grinding of the particles produced through efflorescence results in the IV ↔ III transitions. Differences in crystal structure and moisture content may explain the distinct phase transition behaviors of the two types of samples. These results suggest that solid and pure NH4NO3 aerosol particles are stable in phase IV under ambient conditions. Copyright © American Association for Aerosol Research.
Original languageEnglish (US)
Pages (from-to)581-588
Number of pages8
JournalAerosol Science and Technology
Volume41
Issue number6
DOIs
StatePublished - Jun 1 2007
Externally publishedYes

ASJC Scopus subject areas

  • Environmental Chemistry
  • General Materials Science
  • Pollution

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