chemical thermodynamics of neptunium and plutonium - U.S. ...

98 7. Neptunium aqua ionsnon-systematic values for Sm ◦ (MO+ 2(aq), M = U, Np, Pu, Am) (respectively, -25, -46,2and-21J·K −1·mol−1 , cf. Section 16.3 and References [92GRE/FUG, 95SIL/BID])were not indicated in previous reviews [76FUG/OET, 86MOR] that relied moreheavily on estimates. The differences in the entropy values for these key ions appearto be beyond those easily attributable to specific experimental uncertainties, and needto be confirmed or refuted by further experimental work.Using f Hm ◦ (Np4+ , 298.15 K) = −(556.0 ± 4.2) kJ·mol −1 , f G ◦ m (Np4+ ,298.15 K) =−(491.8 ± 5.6) kJ·mol −1 (cf. Section 7.4) and CODATA auxiliary data,the following selected values are calculated: f Hm ◦ (NpO+ 2, aq, 298.15 K) = −(978.2 ± 4.6) kJ·mol−1 f G ◦ m (NpO+ 2, aq, 298.15 K) = −(907.8 ± 5.6) kJ·mol−1Values of ionic apparent molar heat capacities vary strongly with temperature, and to asomewhat lesser degree with ionic strength. The influence of heat capacity on the Gibbsenergy of reaction is usually too small to determine r C p,m or r C p,m (T) from secondand higher order derivatives of functions describing the temperature dependence ofsolubilities or potentials. Lemire and Campbell [93LEM/CAM, 96LEM/CAM] havereported apparent molar heat capacities of NpO 2 ClO 4 (aq) as a function of temperature(18 to 100 ◦ C) for a limited range of ionic strengths (0.05 to 0.22 m). The extrapolatedvalues for the partial molar heat capacities can be expressed as:C ◦ p,m,2 (NpO 2ClO 4 , aq, T) = (3.56770 × 10 3 − 4.95931T−6.32344 × 10 5 T −1 ) J·K−1·mol−1The value of C ◦ p,m,2 (NpO 2ClO 4 , aq) at 298.15 K is −(32 ± 25) J·K −1·mol−1 ,where the uncertainty is assigned in the present review, and comparable uncertaintiesapply to values throughout the temperature range of the measurements. This valueand the equation above are accepted in the present review. For the usual standard state(C ◦ p,m,2 (H+ , aq, 298.15 K) = 0J·K −1·mol−1 ),usingC ◦ p,m,2 (HClO 4, aq, 298.15 K) =−27.8 J·K −1·mol−1 [96LEM/CAM2] the value of the partial molar heat capacity,C ◦ p,m,2 (NpO+ 2 , aq, 298.15 K) =−(4 ± 25) J·K−1·mol−1 , is calculated.Lemire and Campbell [96LEM/CAM] have used their values for C ◦ p,m,2 (NpO 2ClO 4 ,aq, T) with values of the partial molar heat capacities of other electrolytes[89HOV/NGU, 96LEM/CAM2] to estimate the variation of the potential of theNp(VI)/Np(V) couple as a function of temperature. The apparent molar heatcapacity as a function of temperature of NpO 2+2was assumed equal to that ofUO 2+2[89HOV/NGU, 96LEM/CAM]. The uncertainties in values of r C p,m (7.4, T)estimated in that way are estimated here as ±70 J·K −1·mol−1 .7.4 Np 4+The thermodynamic quantities for Np 4+ are derived from the value of the entropy basedon the solubility and the enthalpy of dissolution of NpO 2 (NO 3 ) 2·6H 2 O(s) in water, the