Thermodynamic Characteristics of the Thermal Decomposition of Th(NO₃)₄⋅5H₂O

Derivative thermogravimetry (DTG) and tensimetry are used to establish that the thermal decomposition of Th(NO₃)₄·5H₂O consists of five steps in the 300–550 K range of temperatures. The first four steps correspond to dehydration and proceed at temperatures of 300–325, 330–350, 350–390, and 400–425 K under conditions of equilibrium. One water molecule is split off in each of the first two stages of sample dehydration, and two are split off in the third. The fifth stage (427–440 K) corresponds to the thermal decomposition of Th(NO₃)₄ with the formation of thorium(IV) oxide and gaseous nitrogen(IV) oxide and oxygen. Tensimetry is used to establish that the temperatures at the beginning of the stages under equilibrium conditions shift to lower temperatures (by almost 50–70 K), compared to DTG data. Thermodynamic characteristics (enthalpy, entropy, and the Gibbs energy of the stages of dehydration and thermal decomposition of Th(NO₃)₄⋅5H₂O) are calculated using tensimetry data. DTG and tensimetry provide mutually consistent enthalpies of different processes.