Thermodynamic studies of thorium phosphate diphosphate and phase investigations of Th-P-O and Th-P-H_2O systems

摘要

The standard molar enthalpy of formation of thorium phosphate diphosphate, β-Th_4(PO_4)_4P_2O_7(cr)has been determined using an isoperibol solution calorimeter, a differential scanning calorimeter and phase transition data of high temperature calorimeter. The enthalpy of precipitation of thorium phosphate-hydrogenphosphate hydrate, Th_2(PO_4)_2(HPO_4)·H_2O(cr), was measured at 298.15 K using an isoperibol solution calorimeter. Heat capacities of α-Th_4(PO_4)_4P_2O_7(cr)and β-Th_4(PO_4)_4P_2O_7(cr)were measured using a Differential Scanning Calorimeter. Combining these experimental data, and other auxiliary data from the literature, thermo-chemical reaction scheme was devised to calculate the standard molar enthalpy of formation of β-Th_4(PO_4)_4P_2O_7(cr)to be {-10565.5±13.6} kJmol~(-1). The thermodynamic functions for β-Th_4(PO_4)_4P_2O_7(cr)have been computed using Δ_fH_m~o(298.15K), C_(p,m)~o(T)and literature data. The Gibbs phase diagram and predominant area diagrams for Th-P-O system have been computed using the thermodynamic information of the various phase present in Th-P, P-O and ThO_2-P_2O_5 systems. E_H-pH diagram of Th-P-H_2O system has been computed to determine stability of Th_4P_6O_(23)(cr)in ground water.