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

A. Discussion of selected references 573to be higher than those shown on the curve in [49KRA/DAM], especially for OH − :Puratios near 0.5. The reported values for log 10 K s,0 vary from −8.3 to−9.5, with thevalues being most negative for solutions having lower initial concentrations of Pu(V),and at low ratios of OH − :Pu(V). It is not clear how the disproportionation during theprecipitation [49KRA/DAM] was addressed.The hydrolysis constant was based on the data from a titration of an aqueous perchloratesolution of Pu(V) with 0.58 M NaOH. The following (logarithmic) values wererecommended for the solubility product and the hydrolysis constantPuO + 2 + OH− Å PuO 2 OH(am) log 10 K s,0 =−8.6PuO + 2 + H 2O(l) Å PuO 2 OH(aq) + H + log10 ∗β1 < −9.7i.e., log ∗10 K s,0 = 5.4, for solutions having ionic strengths between 2.60 × 10 −3 and4.43 × 10 −2 , and assuming log 10 K w =−14.0. Ionic strength effects on ∗ β 1 are expectedto be small.The results on plutonium(VI) hydrolysis are based on data also discussed in[49KRA/DAM2]and[56KRA].[48KRA/NEL2]This report is mainly a description of a spectrophotometric investigation of actinide(IV) species in 0.5 M NaCl or NaClO 4 . Little difference was found between the spectrain perchlorate medium and those for < 0.5 MCl − . Polymerization of Pu(IV) wasfound to be rapid compared to depolymerization for < 0.1 MH + . A more completeanalysis of these results was reported in a later paper [50KRA/NEL]. Results of titrationof Pu(III) with NaOH (from Kraus and Dam [46KRA/DAM2]) are reported. Inperchlorate medium (I = 0.069 M) log10 ∗β1 =−7.1. The titration study was laterreported in detail in [49KRA/DAM3], and is discussed further in the Appendix A entryfor that reference.[49HIN]This is a careful spectrophotometric study performed at 25 ◦ C and 2 M H(NO 3 ,ClO 4 ).Measurements in solutions 0.5-14.6 M HNO 3 with variable ionic strength have alsobeen carried out to study the higher complexes. Here the data at constant ionic strengthare discussed first, while for the variable and high ionic strength data only some qualitativecomments can be made. The author defines the constant:K =[PuNO3+ 3 ][Pu 4+ ]·[NO − 3 ] = K◦·fPu 4+·f NO − 3f PuNO3+3(A.1)where the symbol f i represents activity coefficients in molar scale. The experimentaldata are then explained in terms of the formation of one complex with constantlog K(A.1, I = 2M) = (0.46 ± 0.10).