Spectrophotometric Study of the Dissociation of the Pu(IV) Carbonate Limiting Complex

The dissociation of the limiting complex of Pu(IV) was investigated by spectrophotometry, in carbonate and bicarbonate media. The initial Na₂CO₃ solution (from 0.1 to 1.5 M) was transformed into a NaHCO₃ one by bubbling CO₂. The presence of an isobestic point is a strong indication that only two major species are in equilibrium in the solution under study: the quantitative interpretation indicates that only one CO₃^2- is exchanged between these two complexes, that no OH is exchanged over 3 pH units, and that there is no evidence of polymerisation. It is assumed for the interpretation, that the limiting complex is Pu(CO₃)₅^6-. The new species formed is then Pu(CO₃)₄^4-. This point is discussed and compared with published works on Pu(IV) solubility and spectrophotometry. The influence of the ionic strength on the dissociation constant is modelled by the S.I.T., then lg K₅(0) = -1.36±0.09 is deduced. The results are compared with the values published for chemical analogues U, Np, Pu, and Am(IV).