Thermodynamic properties of aqueous gadolinium perrhenate and gadolinium chloride from high dilution calorimetry at extreme temperatures and pressures.

The heat of solution of solid cubic gadolinium oxide has been measured in noncomplexing perrhenic acid solutions at very high dilutions (10(-4) m) up to 596.30 K, from which the standard state thermodynamic properties of aqueous gadolinium perrhenate were determined up to 623.15 K. From the measured differences between similar properties of aqueous sodium chloride and perrhenate, thermodynamic properties for aqueous gadolinium chloride were obtained by ionic additivity. Data for the hydrolysis of Gd3+(aq) were obtained by separate determinations. The enthalpy of solution of gadolinium chloride at 623.15 K obtained from this research (-2.7 MJ mol(-1)) is apparently larger than any other recorded for a chemical reaction involving aqueous systems. Standard state partial molal heat capacities for ThCl4(aq) were predicted up to 623.15 K.