ABSTRACT

A calorimetric and thermodynamic investigation of three alkali-metal uranyl tungstates of general formula A2[(UO2)2(WO5)O], with A = K, Rb and Cs, was undertaken. All three phases were synthesized by high-temperature solid-state reaction of a mixture of the respective alkali-metal nitrates and tungsten (VI) and gamma uranium (VI) oxide. The synthetic products were characterized by X-ray powder diffraction and X-ray fluorescence methods. The enthalpy of formation of the three phases was determined using HF-solution calorimetry giving DfHm (T = 298 K, K2[(UO2)2(WO5)O], cr) = 4185 +- 8 kJ mol 1, DfHm (T = 298 K, Rb2[(UO2)2(WO5)O], cr) = 4185 +- 10 kJ mol 1, and DfHm (T = 298 K, Cs2[(UO2)2(WO5) O], cr) = 4198 +- 9 kJ mol 1. Their low-temperature molar heat capacity, Cp,m , was measured using adiabatic calorimetry from T = 9 to 329 K for K2[(UO2)2(WO5)O], from T = 9 to 321 K for Rb2[(UO2)2(WO5)O] and from T = 6 to 320 K for Cs2[(UO2)2(WO5)O]. The respective molar third law entropy at T = 298.15 K, Sm , was calculated giving 380 +- 1 J K 1 mol 1 for K2[(UO2)2(WO5)O], 406 +- 1 J K 1 mol 1 for Rb2[(UO2)2(WO5)O] and 419 +- 1 J K 1 mol 1 for Cs2[(UO2)2(WO5)O]. These new calorimetric results, combined with literature data, have been used to calculate the Gibbs free energy of formation, DfGm , for all the studied phases giving: DfGm (T = 298 K, K2[(UO2)2(WO5)O], cr) = 3914 +- 8 kJ mol 1, DfGm (T = 298 K, Rb2[(UO2)2(WO5)O], cr) = 3915 +- 10 kJ mol 1 and DfGm (T = 298 K, Cs2[(UO2)2(WO5)O], cr) = 3926 +- 9 kJ mol 1. Smoothed Cp,m (T) values between 0 K and 320 K are presented along with the values for Sm and the functions [Hm (T) Hm (0)] and [Gm (T) Hm (0)] for all three phases. The phase relations of various solid phases and solution complexes in the K2WO4-UO3-H2O and K2MoO4-K2WO4- UO3-H2O systems with and without CO2 at T = 298 K were calculated using a Gibbs energy minimization approach. The results show that there is no uranium-containing solid phase stable at pH less than 2.2 in either CO2-free or CO2-bearing systems. Here, uranium (VI) is bound in solution complexes. Various solid phases are stable from pH 2.2 to pH 10 in the CO2-free system, but at pH greater than 10 and in the presence of CO2 no solid is present.

Compounds

#	Formula	Name
1	K	potassium
2	W	tungsten
3	U	uranium
4	O2	oxygen
5	Rb	rubidium
6	Cs	cesium
7	K2O10U2W	potassium tungsten uranium oxide (K2WU2O10)
8	O10Rb2U2W	rubidium tungsten uranium oxide (Rb2WU2O10)
9	Cs2O10U2W	cesium tungsten uranium oxide (Cs2WU2O10)

Datasets

The table above is generated from the ThermoML associated json file (link above). POMD and RXND refer to PureOrMixture and Reaction Datasets. The compound numbers are included in properties, variables, and phases, if specificied; the numbers refer to the table of compounds on the left.
Type	Compound-#	Property	Variable	Constraint	Phase	Method	#Points
POMD	7	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	7	Molar enthalpy, kJ/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	7	Molar entropy, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	7	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	145
POMD	8	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	8	Molar enthalpy, kJ/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	8	Molar entropy, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	8	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	143
POMD	9	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	9	Molar enthalpy, kJ/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	9	Molar entropy, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	19
POMD	9	Molar heat capacity at constant pressure, J/K/mol ; Crystal	Temperature, K; Crystal	Pressure, kPa; Crystal	Crystal	Small (less than 1 g) adiabatic calorimetry	153
RXND	7 1 2 3 4	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1
RXND	8 5 2 3 4	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1
RXND	9 2 6 3 4	Molar enthalpy of reaction, kJ/mol				Solution calorimetry	1

Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

A calorimetric investigation of A2[(UO2)2(WO5)O] compounds with A = K, Rb and Cs and calculated phase relations in the K2WO4-UO3-H2O and K2MoO4-K2WO4-UO3-H2O systems

Lelet, M. I.[Maxim I.], Borodulina, M. L.[Maria L.], Suleimanov, E. V.[Evgeny V.], Geiger, C. A.[Charles A.], Bosbach, D.[Dirk], Alekseev, E. V.[Evgeny V.]

ABSTRACT