Study of Ca_nUO₂(CO₃)₃^(4-2n)- complexes using CE-ICP-MS with polyetheretherketone (PEEK) capillaries

The formation constants of Ca_nUO₂(CO₃)₃^(4-2n)- complexes were determined directly using capillary electrophoresis coupled with inductively coupled plasma mass spectrometry (CE-ICP-MS) in 0.1 M NaCl and at room temperature. Instead of conventional fused silica, polyetheretherketone (PEEK), a neutral organic material, was used as the capillary material to avoid interactions between uranyl and silica. Since PEEK is not optically transparent, a macrocyclic neutral complex, Ga-NOTA, was used to measure the electroosmotic flux directly by ICP-MS. The impact of the Joule effect was evaluated to control the temperature during electrophoresis. To alter the reversible interaction that arose from continuous use, a two-mode electrophoresis strategy was adopted. The observed mobilities of the samples showed a satisfactory correlation with the theoretical charge calculated from previous data (C. Shang and P. E. Reiller, Dalton Trans., 2020, 49, 466-481). The successive formation constants obtained in this work, log₁₀ K(CaUO₂(CO₃)₃^2-) = 5.28 ± 0.39 and log₁₀ K(Ca₂UO₂(CO₃)₃(aq)) = 8.46 ± 0.67 in 0.1 M NaCl, were extrapolated to infinite dilution using the Davies equation, yielding log₁₀ β°(CaUO₂(CO₃)₃^2-) = 27.12 ± 0.39 and log₁₀ β°(Ca₂UO₂(CO₃)₃(aq)) = 30.30 ± 0.67. These values are in excellent agreement with Shang and Reiller (C. Shang and P. E. Reiller, Dalton Trans., 2020, 49, 466-481). This further verifies the thermodynamic data available for these species and validates PEEK-based capillary electrophoresis for the measurement of thermodynamic constants of inorganic complexes in alkaline media.