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

648 A. Discussion of selected references0.7 M, only the first complex, PuI 2+ , was found to be formed with a constant ofK 1 (PuI 2+ , 1M, 303.15 K) = (1.09 ± 0.07). This experiment was not describedin more detail by the authors. For the thiocyanate complexes, the followingconstants were reported: β 1 (PuSCN 2+ , 1 M, 303.15 K) = (2.18 ± 0.17) andβ 2 (Pu(SCN) + 2, 1M, 303.15 K) = (4.10 ± 0.21).In order to obtain some information on the thermodynamic properties of the complexes,the authors performed additional experiments at 15, 37 and 45 ◦ C. The resultsare given in Figure 3 of [74KHO/MAT]. The formation constants obtained forPuSCN 2+ are: log 10 β 1 = 0.31 (15 ◦ C), 0.34 (30 ◦ C), 0.33 (37 ◦ C)and0.31(45 ◦ C).The temperature variation of this constant is thus zero within the experimental error.The authors reported, for the formation of PuSCN 2+ : r H m (1M, 303.15 K) =(0.25 ± 0.75) kJ·mol −1 .Khopkar and Mathur [74KHO/MAT] drew attention to the “reversal” behaviour ofPu(III) and Am(III) in the thiocyanate complex stability order. They expected the 1:1complex of Pu(III) to be less stable than that of Am(III) due to a decrease in the atomicradii. However, Choppin and Ketels [65CHO/KET] have found a higher stability forthe Am(III) complex. In any case, the formation constants of the complexes PuSCN 2+and AmSCN 2+ are very similar, cf. [95SIL/BID], and the differences are smaller thanthe estimated uncertainties.[74MEF/KRO]The hydrolysis of Np(III) was studied by potentiometric titration of ca. 0.044 millimolesamples of Np(III) in 0.1 M NaClO 4 , using 0.1 M NaOH and a LPU-1 pH meter andglass(?) electrode, in a deaerated system protected from air. The experiments werecarried out at 20 ◦ C (Table 1). The titrations in chloride medium at 23 ◦ C (Figure 1) werepresumably not used in the final calculations of the hydrolysis constant. The Np(III)was produced by electrolysis of 0.022 M Cs 2 NpCl 6 (aq) in 0.1 M HCl using a mercurycathode in an inert atmosphere. Solutions of Pr 3+ and Nd 3+ were titrated in a similarmanner, and the three sets of titration curves were compared to obtain the ratios of thehydrolysis constants. A ratio ∗ β 1 (Nd)/ ∗ β 1 (Pr) = 1.35 was obtained, in agreement withan earlier value of 1.32 [66FRO/KUM], obtained at 0.3 M NaClO 4 (25 ◦ C). The valuesreported in the paper by Frolova, Kumok, and Serebrennikov, [66FRO/KUM] forthefirst hydrolysis constants, log ∗ 10 β 1,are−8.43 for Nd 3+ ,and−8.55 for Pr 3+ .Thevaluefor the first Np(III) hydrolysis constant at (25 ◦ C), ∗ β 1 = (3.7 ± 0.9) × 10 −8 , was thencalculated for a medium of 0.3 M NaClO 4 (log10 ∗β1 = 7.43 ± 0.11) using the ratios ofthe hydrolysis constants of Np(III) with those of Pr(III) amd Nd(III).Considering the comparison of values obtained by [66FRO/KUM] for 0.3 and 3 MNaClO 4 , from titrations using both NaOH and Ba(OH) 2 , and literature values cited bythe same authors for the lanthanide ion hydrolysis constants in 3 M NaClO 4 , it seemslikely that the error in the Np(III) hydrolysis constant is considerably greater than ±0.1.Systematic errors may also be present, and as the paper does not show any measuredpoints, the statistical uncertainty cannot be evaluated. Usinglog ∗10 β 1(I = 0) = log ∗10 β 1(I) + 4D + εI − log 10 a H2 O