Calorimetric study of calcium nitrate tetrahydrate and magnesium nitrate hexahydrate

Abstract

The heat capacity and enthalpy of fusion of calcium nitrate tetrahydrate and magnesium nitrate hexahydrate were determined from 234.15 K to melting temperature by DSC. The modified stepwise method was used for heat capacity measurement, and enthalpy of fusion was determined from continuous heating by the rate of 10 K min−1 (Pilař et al. in J Therm Anal Calorim 118:485–491, 2014). Determined values were used for the calculation of entropy and Gibbs energy in the experimental temperature range. Melting point and enthalpy of fusion of calcium nitrate tetrahydrate are 317.1 ± 0.3 K and 36.6 ± 0.2 kJ mol−1, and for the magnesium nitrate hexahydrate, the values are 362.9 ± 0.4 K and 40.8 ± 0.5 kJ mol−1. One solid–solid phase transformation was observed for the magnesium salt at 345.7 ± 0.9 K with enthalpy of transition 3.1 ± 0.2 kJ mol−1. The available accumulated energy composed of sensible (heating) and latent heat (phase transformations) is 43.4 and 63.8 kJ mol−1 for the hydrated calcium and magnesium nitrate, respectively. The kinetics of solid–solid phase transformation for magnesium salt was studied under non-isothermal conditions by DSC, and the process was described using autocatalytical model with parameters in the range of 0.50–0.85 for m and range of 2.58–1.48 for n, respectively.