Thermodynamics of complexed aqueous uranyl species. 1. Volume and heat capacity changes associated with the formation of uranyl sulfate from 10 to 55.degree.C and calculation of the ion-pair equilibrium constant to 175.degree.C

Apparent molar heat capacities and volumes of aqueous solutions containing UO{sub 2}SO{sub 4} in dilute H{sub 2}SO{sub 4} have been obtained from 10 to 55{degree}C. These results have been analyzed considering the ion-pair-formation reaction of uranyl with sulfate ion, and considering the contributions to the measured heat capacities from chemical relaxation. Young's rule was used to extract apparent molar volumes and heat capacities for the solute species UO{sub 2}SO{sub 4} from 10 to 55{degree}C. The temperature dependence of the standard-state heat capacity and volume functions for UO{sub 2}SO{sub 4}(aq) are represented by the following equations: {bar V}{degree}/(cm{sup 3} mol{sup {minus}1}) = {minus}93.87 + 0.7784T - 0.001 093 T{sup 2} and {bar C}{sub p}{degree}/(J K{sup {minus}1} mol{sup {minus}1}) = {minus} 2,773.1 + 13.636T - 0.015 322T{sup 2}, which are accurate from 10 to 55{degree}C. The volumetric and heat capacity changes associated with the complexation of uranyl ion with sulfate are compared with other results for divalent and trivalent monatomic cations. The results for uranyl sulfate and other metal sulfates have also been examined in reference to the Eigen-Tamm three-step ion-pair-formation model. The highly negative standard heat capacities of this neutral solute along with the structural data for uranyl sulfate provide informationmore » regarding the enhanced solute-water dipole interactions not present in systems containing other neutral solutes.« less