Experimental evaluation of barium bromide-ammonia equilibrium lines

Abstract

This work has experimentally evaluated the position of thermochemical equilibrium lines of barium bromide (BaBr₂) reacting with ammonia (NH₃). BaBr₂ samples, contained in discs of Expanded Natural Graphite, were placed in an experimental Large Temperature Jump rig and a new methodology, termed the Fast Temperature Ramp, was used to reveal adsorption and desorption reactions. Reaction onset points were plotted to generate equilibrium lines and calculate reaction enthalpies and entropies. Significant hysteresis between adsorption and desorption reactions was found, with the degree of hysteresis increasing as the salt becomes more deammoniated. The hysteresis effect was greatest for BaBr₂(2–1) and BaBr₂(1–0) reactions and also increased at higher pressures, exceeding 20 °C at 900 kPa for BaBr₂(1–0). Adsorption reactions were found to occur over a very small temperature range, giving rise to a single transition ‘zone’ less than 20 °C wide. TGA experiments confirmed the position of equilibrium lines, although they were not successful in differentiating between two of the four ammoniated states. The close position of all equilibrium lines gives rise to a temperature ‘zone’ encompassing all four reactions, and in future work it may be advantageous to consider BaBr₂ as a pseudo single-transition salt instead.