Reaction of Np(VI) with Cyclohexanediaminetetraacetic Acid in HClO4 Solutions

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H(4)chdta) in 0.05 M HClO4 solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115-0.98 M HClO4 solutions (the ionic strength of 1.0 was supported with LiClO4) containing 3-29 mM CHDTA at 20-45 degrees C, Np(VI) at a concentration of 0.2-3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of -1.2 with respect to [H+]. The activated complex is formed with the loss of one and two H+ ions. The activation energy is 82.3 +/- 3.8 kJ mol(-1).