Studies of metal-carbonate complexes. 14. Composition and equilibria of trinuclear neptunium(VI)- and plutonium(VI)-carbonate complexes

The chemical composition of the trinuclear complexes (MO2)3(CO3)66- and the equilibrium constants for the reaction 3MO2(CO3)34- in equilibrium (MO2)3(CO3)66- + 3CO32-, where M = Np or Pu, have been determined by spectrophotometric and emf methods. The values of the equilibrium constants at I = 3 M (NaClO4) and T = 22 +/- 1 0C are log K/sub 3,6/(Np) = -10.1 +/- 0.1 and log K/sub 3,6/(Pu) = -7.4 +/- 0.2; the constant for U determined previously is -11.3 +/- 0.1. The range of stability of the trinuclear plutonium complex is much larger than those of uranium and neptunium, a fact that might be due to a lower stability of the limiting PuO2(CO3)34- complex. The formation of mixed complexes of the type (MO2)/sub x/(M'O2)/sub 3-x/(CO3)66- in the U(VI)-Np-(VI)-Pu(VI)-carbonate system, formed by the isomorphic substitution of U(VI) b another actinide is demonstrated. Spectral characteristics and estimated stabilities are given for (UO2)2(MO2)(CO3)66- (M = Np, Pu). The equilibrium constants for the reaction 2UO2(CO3)34- + MO2(CO3)34- in equilibrium (UO2)2(MO2)(CO3)66- + 3CO32-, where M = Np or Pu, are equal to log K(Np) = -10.0 +/- 0.1 and log K(Pu) = -8.8. 14 references, 6 figures, 1 table