A new approach to determination of hydration equilibria constants for the case of [Er(EDTA)(H2O)n]− complexes

Abstract

Two anionic complexes [Er(EDTA)(H₂O)₂]⁻ and [Er(EDTA)(H₂O)₃]⁻ were obtained in the form of the following compounds: [C(NH₂)₃]₂[Er(EDTA)(H₂O)₂]ClO₄·6H₂O (I) and Na[Er(EDTA)(H₂O)₃]·5H₂O (II), respectively. The UV-vis-NIR absorption spectra of both monocrystals were measured at room temperature and at 4.2 K. The influence of the coordination number changes on intensities of the f–f transitions and the crystal field splitting of ^2S+1L_J multiplets are discussed. The weighted sum of molar absorption coefficients of f–f transitions in the spectra of I and II was used to reproduce the absorption bands of the Er³⁺–EDTA complex in aqueous solution. This approach allowed us to estimate that the complex in solution exists in 95% as the 8-coordinate [Er(EDTA)(H₂O)₂]⁻ species and in 5% as the 9-coordinate [Er(EDTA)(H₂O)₃]⁻ ones as well as to calculate the conditional hydration equilibrium constant (K_aqua) of the reaction: [Er(EDTA)(H₂O)₃]⁻ ↔ [Er(EDTA)(H₂O)₂]⁻ + H₂O which is rather difficult to determine by using other methods. The K_aqua value was found to be 19 ± 1.