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

578 A. Discussion of selected references[49MAG/LAC]Redox titrations and spectrophotometric experiments were done for Np(III)-Np(IV) in1 M HCl. No hydrolysis values are given.[49ZEB/NEU]The reference (which reported results of a study of complexation of Pu(IV) with nitratein aqueous solution) was not available to the reviewers. Hindman [54HIN] cited resultsfrom this extraction study for “I = 6.0” (units, probably mol·dm −3 , were not statedand the medium was not described) as “K 1 = 8.3” at 45 ◦ C, and “K 1 = 4.7”, “K 2 =0.96” and “K 3 = 0.33” at an unstated temperature (presumably a room temperaturenear 25 ◦ C). The reported value of “5.3 kJ·mol −1 ” for the enthalpy of reaction for thefirst complexation step is consistent with the K 1 value of 4.7 dm·mol −1 having beendetermined at 25-26 ◦ C, and presumably K 2 and K 3 were also determined at the sametemperature. The information from this study was not used in determining selectedvalues in the present review.[50BUS/COW]OH − m 0.5Cl − m PutotPotentiometric titrations of Pu(III) solutions provide information on the hydrolysis ofPu(III) and the solubility of Pu(III) hydroxide at I = 0.04. The titration curve indicatesextensive hydrolysis of Pu(III) occurred before precipitation. No account was taken ofhydrolysis in the calculation of the solubility product, and the final solid had a stoichiometryclose to Pu(OH) 2.5 Cl 0.5 . The “solubility product” for this chlorohydroxide,seemingly described as a 2.5OH − mCl 0.5 − (0.5m Putot ) was then reported as 9.1 × 10 21 . Recalculationsuggests 8.4 × 10 −21 , whereas in later sources referring to this paper, e.g.[56KRA], a value of 2 × 10 −20 (a 2.5(?)) is noted. None of these valuesis actually∗ K s,0 , nor is there adequate information to calculate the value. Thequantitative results from this paper are not used in the present review. This work indicatesthe value of the first hydrolysis constant for Pu(III) near room temperature islog10 ∗K1(I = 0.04 M(chloride)) =−(7.2 5 ± 0.4).[50KRA/NEL]Spectrophotometric measurements (470 nm) were done in combination with pH measurementsto measure the hydrolysis of Pu 4+ in 0.5 M chloride and perchlorate solutions.The uncertainties in the logarithm (base 10) of the equilibrium constants areestimated in the present review to be about 0.2 (0.3 for the solution of lowest acidity).The molar absorbance of the hydrolysed species was assumed to be zero at 470 nm,however, Rabideau and Kline [60RAB/KLI] reanalysed the data for the perchloratesolutions and deduced a value of (10 ± 3) for the molar absorbance of PuOH 3+ (andhence, log10 ∗β1 =−(1.72 ± 0.05)). Considerable difficulty was reported with polymerizationand disproportionation of the Pu(IV) solutions (7.2 × 10 −4 M in aqueousplutonium species). Somewhat larger values for ∗ β 1 were found from the measurementsin the chloride solutions, and this was attributed to chloride complexation.