Evidence of different stoichiometries for the limiting carbonate complexes across the lanthanide(III) series:: A capillary electrophoresis-mass spectrometry study

The electrophoretic mobilities (mu(ep,Ln)) of twelve lanthanides (not Ce, Pr and Yb) were measured by CE-ICP-MS in 0.15 and 0.5 mol L-1 Alk(2)CO(3) aqueous solutions for Alk(+) = Li+, Na+, K+ and Cs+. In 0.5 mol L-1 solutions, two different mu(ep,Ln) values were found for the light (La to Nd) and the heavy (Dy to Tm) lanthanides, which suggests two different stoichiometries for the carbonate limiting complexes. These results are consistent with a solubility study that attests the Ln(CO3)(3)(3-) and Ln(CO3)(4)(5-) stoichiometries for the heavy (small) mu(Alk2CO3)(ep,Ln) and the light (big) lanthanides, respectively. The Alk(+) counterions influence the mu(ep,Ln) (Alk2CO3) values, but not the overall shape of the pep,Ln plots as a function of the lanthanide atomic numbers: the counterions do not modify the stoichiometries of the inner sphere complexes. The influence of the Alk(+) counterions decreases in the Li+ > Na+ >> K+ > Cs+ series. The K-3,K-Ln stepwise formation constants of the Ln(CO3)(3)(3-) complexes slightly increase with the atomic numbers of the lanthanides while K-4,K-Ln, the stepwise formation constants of Ln(CO3)(4)(5-) complexes, slightly decrease from La to Th, and is no longer measurable for heavier lanthanides.