AQUEOUS PARTIAL MOLAR HEAT-CAPACITIES AND VOLUMES FOR NATCO4 AND NAREO4

A flow microcalorimeter/densimeter system has been commissioned to measure heat capacities and densities of solutions containing radioactive species as a function of temperature. Measurements were made for NaTcO4(aq) at six temperatures (289.15 K to 373.15 K for the heat capacities, 287.43 K to 396.67 K for the densities) over the molality range 0.01 to 0.29 mol-kg-1. Measurements for NaReO4(aq) (NaReO4 is a common nonradioactive analogue for NaTcO4) were made under similar conditions, bat for eight temperatures and a more extensive range of molalities, 0.05 to 0.65 mol-kg-1. Heat capacities of NaCl(aq) reference solutions were also measured from 293.15 K to 398.15 K. The heat capacity and density data are analysed using Pitzer's ion-interaction model. Equations for the apparent molar heat capacities and volumes are reported. Values of the NaReO4(aq) partial molar heat capacities are compared to literature values based on integral heats of solution. The agreement between the two sets of NaReO4 results is good below 330 K, but only fair at the higher temperatures. Values of the partial molar volumes have also been derived. Using literature values and the results of our experiments, it is calculated that the disproportionation of hydrated TcO2(s) to form TcO4-(aq) and Tc(cr) occurs more readily at high temperatures. The uncertainties introduced by using thermodynamic values for ReO4-(aq), in the absence of values for TcO4-(aq), are discussed.