The dissociation constant of ammonium nitrate and its dependence on temperature, relative humidity, and particle size

The dissociation constant of NH4NO3(c, IV) is determined from the evaluation of literature data from a variety of sources. Three different methods of calculation using five independent data sets lead to accurate values of the dissociation constant; these are in excellent agreement with each other. The recommended value of the standard free energy of dissociation of NH4NO3 (c, IV) to NH3(g) and HNO3(g) is 93.4±0.3 kJ mol−1. Over the temperature range for which crystalline phase IV is stable (−17 to 32°C), the temperature dependence of the dissociation constant (in units of nb2) is given to within ±12% by

Equations are also given for the computation of the dissociation constant of NH4NO3(aq) to NH3(g) and HNO3(g) as a function of temperature and either solution concentration or equilibrium relative humidity. For saturated solutions these equations are consistent with the results for the solid, and for dilute solutions they are consistent with data on dilute solutions of NH3(aq) and HNO3(aq). For particles with radii less than 0.1 μm, the Kelvin effect has a substantial effect on the dissociation constant. However, particle size has little effect on the deliquescence point.