The standard molar enthalpy of formation of LnPO4(s) (Ln = La, Nd, Sm) by solution calorimetry

The standard molar enthalpy of formation of LaPO4(s), NdPO4(s), and SmPO4(s) has been determined using an isoperibol solution calorimeter. The solution calorimeter vessel was held at 298.15 K. The precipitation reaction between aqueous solution of rare-earth chloride (LnCl3(aq.)) and ammoniacal solution of ammonium dihydrogen phosphate (NH4H2PO4(aq.)) was studied. The temperature of the calorimeter vessel was measured before, during, and after the reaction. The enthalpy change due to precipitation of LaPO4(s), NdPO4(s), or SmPO4(s) from required solutions was measured at 298.15 K. Using these values and other auxiliary data from the literature, thermochemical reaction scheme were devised to calculate the standard enthalpy of formation of each phosphate compound i.e., LaPO4(s), NdPO4(s), and SmPO4(s). The calculated values for LaPO4(s), NdPO4(s), and SmPO4(s) at 298.15 K were found to be −1947.5 ± 3.2, −1938.3 ± 3.6, and −1942.9 ± 3.4 kJ mol−1, respectively.