Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

The work herein presents the first high temperature Knudsen effusion mass spectrometric measurements carried out on the sodium neptunate a -Na 2 NpO 4 . The material s vaporization behaviour was investigated under vacuum conditions, in a tungsten metal cell up to T = 2700 K, and in an alumina cell up to T = 1900 K. Gaseous and condensed phases are in equilibrium under Knudsen conditions. The equilibrium decomposition reaction of a -Na 2 NpO 4 to neptunium oxide, sodium oxide, and oxygen was established in the temperature range 1030 K to 1206 K. The enthalpy of formation of a -Na 2 NpO 4 at 298.15 K was fur- thermore estimated at ( 1761.0 +- 7.7) kJ mol 1 using a second law analysis in the interval 1030 K to 1115 K, in good agreement with the value reported by Goudiakas et al. [25] ( 1763.9 +- 7) kJ mol 1 , who used solution calorimetry. In the interval 1115 K to 1206 K, where the vaporization of sodium oxide formed by decomposition of a -Na 2 NpO 4 is considered, the analysis yielded at 298.15 K ( 1761.9 +- 20.4) kJ mol 1 and ( 1758.8 +- 22.3) kJ mol 1 taking into account two different sodium oxide sublimation mechanisms suggested by Hildenbrand and Lau [56] and Steinberg and Schofield [37] respectively. Finally, the detection of the masses 253 and 292 in the temperature range 1350 to 1620 K suggests a probable sublimation of the high temperature tetragonal phase of the Na 2 NpO 4 com- pound in parallel with the decomposition. The method developed in the present paper holds promise for the thermodynamic investigation of other sodium neptunates and plutonates phases