Thermodynamics of lanthanide chelation in mixed aqueous solvents

The present work discusses the aqueous solution chemistry of lanthanide complexes in terms of previously observed trends in stability and enthalpy and entropy changes. The theory of the glass electrode method for determining stability constants is reviewed in detail, and its application to mixed aqueous solvent systems is critically discussed. In this investigation, the method of Bjerrum was used to determine the stability constants of the tripositive lanthanide--o-aminophenol and --4-methylcatechol complexes in 50 percent ethanol, dioxane, and dimethyl sulfoxide. Some a-hydroxyisobutyric acid complexes were studied at various solvent concentrations. The enthalpies and entropies of chelation are reported as obtained by the differential temperature method. The results are interpreted in terms of a generalized solvation model for mixed solvent systems. The model is shown to agree with the established conclusions for water solution.