SYNTHESIS OF CRYSTALLINE AMERICIUM HYDROXIDE, AM(OH)(3), AND DETERMINATION OF ITS ENTHALPY OF FORMATION - ESTIMATION OF THE SOLUBILITY-PRODUCT CONSTANTS OF ACTINIDE(III) HYDROXIDES

This paper reports a new synthesis of pure, crystalline Am(OH)(3), its characterization by X-ray powder diffraction and infrared spectroscopy, and the calorimetric determination of its enthalpy of solution in dilute hydrochloric acid. From the enthalpy of solution the enthalpy of formation of Am(OH)(3) has been calculated to be -1371.2+/-7.9 kJ.mol(-1) at 298.15 K, which represents the first experimental determination of an enthalpy of formation of any actinide hydroxide. Because of the need for reliable values of foe energies of heterogeneous equilibria of actinide materials, the solubility-product constant of Am(OH)(3) at 298.15 K (K-sp = 3.4X10(-30), log K-sp = -29.5 +/- 1.6) was calculated from our enthalpy of formation and entropy estimates and is compared with literature measurements under near-equilibrium conditions. Since many properties of the tripositive lanthanide and actinide ions (e.g., hydrolysis, complex-ion formation, and thermochemistry) change in a regular manner, these properties can be interpreted systematically in terms of ionic size. This paper compares the thermochemistry and solubility equilibrium of Am(OH)(3) with those of lanthanide hydroxides. A combined structural and acid-base model is used to explain the systematic differences in enthalpies of solution between the oxides and hydroxides of the 4f(n) and 5f(n) subgroups and to predict and to recommend solubility-product constants for the actinide(III) hydroxides of Pu through Cf. For Pu(OH)(3) and Am(OH)(3), recommended logK(sp) values are -(26.5+/-2) and -(27.5+/-2) respectively.