Thermodynamic studies of thorium phosphate diphosphate and phase investigations of Th-P-O and Th-P-H2O systems

Abstract

Abstract The standard molar enthalpy of formation of thorium phosphate diphosphate, β-Th4(PO4)4P2O7(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, Th2(PO4)2(HPO4)·H2O(cr), was measured at 298.15 K using an isoperibol solution calorimeter. Heat capacities of α-Th4(PO4)4P2O7(cr) and β-Th4(PO4)4P2O7(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-Th4(PO4)4P2O7(cr) to be {−10565.5 ± 13.6} kJ mol−1. The thermodynamic functions for β-Th4(PO4)4P2O7(cr) have been computed using Δ f H m o ( 298.15 K ) , 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 ThO2-P2O5 systems. EH–pH diagram of Th-P-H2O system has been computed to determine stability of Th4P6O23(cr) in ground water.