Mechanism of transformation of Np(V) into Np(IV) in sodium 1,2-cyclohexanediaminetetraacetate solutions.

The kinetics of the transformation of Np(V) into Np(IV) in 0.1 M potassium biphthalate solutions containing 5-74 mM sodium 1,2-cyclohexanediaminetetraacetate (NaCHDTA) or in a 96-97 mM NaCHDTA solution at 25-45°С was studied. The reaction rate at NaCHDTA concentrations in the range 5-60 mM and pH 3.5-5.9 is described by the equation V = k[Np(V)][CHDTA], and at NaCHDTA concentrations in the range 70-100 mM and pH 4.1-5.2, by the equation V = k [Np(V)]. Neptunium(V) forms with the CHDTA ion an activated complex in which Np(V) is reduced to Np(IV). The dimer {Np(V)} forming another activated complex with the CHDTA ion is formed concurrently. The latter complex decomposes along the disproportionation pathway to give Np(IV) and Np(VI). Np(VI) is reduced with the CHDTA ion to Np(V). [ABSTRACT FROM AUTHOR]