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

A. Discussion of selected references 575[49HIN2]In this paper there are some data on hydrolysis of Pu(IV) from spectrophotometricmeasurements for 0.1 to 1.1 M HCl solutions at constant total Cl − concentration(1.1 M). A value of log 10 K = 1.5 to 1.6 was found. The values are comparable toothers for this system in similar media.[49HIN/MAG2]The authors studied the NpO 2+2 /NpO+ 2couple by emf measurements. Potentials weremeasured after addition of aliquots of tin(II) chloride to a NpO + 2solution in 1 M HCl.The investigators believed that the reaction is reversible, but note that the precision oftheir measurements was affected a) by the slow reduction of Np(VI) by chloride in thepresence of the platinum electrode and b) by the kinetically slow reduction of Np(VI)by Sn(II).They reported for the reaction in 1 M HCl at 25 ◦ C,NpO 2+2+ 1 2 H 2(g) Å NpO + 2 + H+ ,the formal potential E ◦′ = (1.14 ± 0.02) V.Because no correction was made for the possible complexation of the metal ionsby chloride, the value is not used in this review.[49KAS]Values of 7.9 × 10 −56 and 6.3 × 10 −56 were reported for log 10 K s,0 of plutonium(IV)hydroxide for solutions 0.0615 M in HCl and (0.0520 M HClO 4 +0.95MNaClO 4 )respectively. Equilibrium with colloidal plutonium(IV) hydroxide (detected spectrophotometrically)was assumed, but only 6.2% and 3.0% of the plutonium solution specieswere Pu(IV). It was assumed (incorrectly) that no hydrolysis of the Pu(IV) hadoccurred.A value of 3 × 10 −56 from calculations based on the work of Kraus (as describedin: Metallurgical Project Report, CN-2289, Nov. 1, 1944, p.17) was also reported.[49KIN]This is a careful solubility study performed at (25.00 ± 0.07) ◦ C, at relatively high andconstant concentrations of HNO 3 (0.35 M, 0.832 M and 2.08 M), while varying thephosphoric acid concentration over a wide range (0-1.2 M). The composition of thesolid phase was determined chemically. A crystalline Pu(IV) phosphate was shownby X-ray analysis to be isomorphous with the corresponding Th(IV) and Ce(IV) phosphates,having the formula Pu 2 H(PO 4 ) 3·yH 2 O(s). Solubility measurements on thiscrystalline phase have shown that in solution it transforms to a more compact form ofthe gelatinous Pu(HPO 4 ) 2·xH 2 O(s). Hence the solubility measurements have been performedwith gelatinous Pu(HPO 4 ) 2·xH 2 O(s) as analysed chemically. Care was takento assure a constant ionic strength, especially at the lowest concentration of HNO 3 .