Heptavalent state of neptunium and plutonium

Abstract

In alkaline media Np(VI) is oxidised to heptavalent state by O 3, oxygen compounds of hexa- and octavalent Xe, persulphate, periodate, hypochlorite, and hypobromite ions, bismuthates and, electrolytically, on platinum anode. The redox potential of the pair Np(VII)/Np(VI) in 1 M alkaline solutions is in the range of 0·40-0·76 V. To obtain Pu(VII) in diluted alkaline solutions O 3 and S 2O 8 = ions can be used. In strong alkali Pu(VI) is also oxidized by hypochlorite, hypohromite and perxenate ions. The redox potential of the pair Pu(VII)/Pu(VI) in 1 M KOH is in the range of 0·76-1·24 V. Absorption spectra of alkaline solutions of Np(VII) and Pu(VII) are independent of the way they were taken and are practically unaffected by changing concentration of OH − ions over a broad range. The colour of solutions of Pu(VII) and Np(VII) in the visible part of the spectrum is in a satisfactory agreement with the Lambert-Beer law. Np(VII) and Pu(VII) can be converted in alkaline solutions to hexavalent state by means of numerous reducers. The stoichiometric evidence obtained for a number of reduction reactions confirms the monoelectronic metal transfer from hepta to hexavalent form. Compounds [Co(NH 3) 6].NpO 5.3H 2O, [Pt(NH 3) 5Cl]NpO 5.H 2O, Ba 3(NpO 5) 2. nH 2O, S 3(NpO 5) 2. nH 2O and Ba 3(PuO 5) 2. nH 2O. have been obtained in a solid state. Some of their properties have been studied. It has been found that Np(VII) and Pu(VII) exist in alkaline media as NpO 5 3− and PuO 5 3− anions. The latter are an anionic form of amphoteric hydroxides MeO 2(OH) 3.NpO 2(OH) 3. nH 2O has been obtained in a solid state. Some peculiar properties of heptavalent state of Np and Pu are discussed.