Effect of sample mass on the apparent Arrhenius parameters observed for the thermal dehydration of sodium carbonate monohydrate

Abstract

The dynamics of the thermal dehydration of sodium carbonate monohydrate was investigated using isothermal and non-isothermal thermogravimetric methods for sample masses between 1 and 10 mg in a 70 μl alumina cell. The apparent activation energy determined from isothermal measurements changed from 70 kJ mol−1 for 10 mg samples to 115 kJ mol−1 for 1 mg samples when the data was fit using the AE-2 mechanism. Fitting non-isothermal data using the Starink method produced apparent activation energies were constant within experimental error for 0.05 < α < 0.6 then decreased rapidly for α > 0.6 indicating there were at least two steps in the dehydration mechanism. The values found for 0.05 < α < 0.6 also depended on the sample mass and were 112, 105, and 100 kJ mol−1 for 1, 5 and 10 mg samples respectively. The experimental uncertainty in each of these measurements is ± 5 kJ mol−1. The Arrhenius parameters found for the dehydration reaction by extrapolating the isothermal reaction rates to zero mass where it was assumed that the rehydration reaction was not contributing to the dynamics were Ed = 116 ± 5 kJ mol−1 and Ad = 1.7 ± 0.9 * 1015 s−1. Since the activation energy determined is less than the value of 130 kJ mol−1 obtained using the thermochemical approach these values could still include a contribution from the rehydration reaction and represent a lower bound for these parameters. Using these values and other measured thermodynamic quantities gives Er = 57 ± 5 kJ mol−1 and Ar = 1.3 ± 0.9 * 107 s−1 for the rehydration reaction. This investigation clearly shows that self-generated atmospheres may affect the measured dynamics for a reversible reaction and should be considered when doing the analysis.