Application of capillary electrophoresis to the uranyl/citrate system in moderately acidic solutions

The capillary electrophoresis method has been applied to the speciation study of uranium(VI) at room temperature, in 0.02M citrate buffer solutions, at pH values between 2.5 and 5.5 and at citrate/U ratios between 20 and 40. No negatively charged species have been pointed out at pH values lower than 3. For a pH value higher than 5, the electropherograms are ill-defined and the signals cannot be analyzed simply (owing to a high and rough baseline). In the pH range 3–5, up to 4 peaks can be attributed to U(VI) species. Two of them are likely due to the expected monomer (UO₂)(Cit)]⁻and dimer (UO₂)₂(Cit)₂]²⁻ complex species and these species are shown to be in quasi-equilibrium with two other species possessing slightly lower migration velocities, (UO₂)H(Cit)(OH)]⁻ and (UO₂)₂H₂(Cit)₂(OH)₂]²⁻, respectively. Speciation diagrams calculated by an exact analytical approach are proposed in order to explain the experimental results. A complete agreement between theoretical and experimental results needs to take into account kinetic and hydrolysis effects.