Influence of Methanol as a Cosolvent on Complexation Constants

Abstract

Complexation constants are typically determined in a fully aqueous solvent system. In literature, there has been a small amount of reported work where complexation constants have been determined in mixed solvents or fully non-aqueous solvent systems but this is not common. The use of non-aqueous solvent systems complicates the measurement of complexation constants, as there are numerous physical property changes between non-aqueous solvents and water. Some organic solvents are also hazardous to work with or add other difficulties that need to be worked around for their use as a solvent. Methanol is the simplest organic solvent and the most similar to water, and as a result most of the non-aqueous work in the literature is completed using this as a cosolvent or full solvent medium. In this work, the pKa values of acetic acid, 2-hydroxyisobutyric acid, 2-hydroxy-2-methylbutyric acid have been determined in solvent systems containing various concentrations of methanol. With the knowledge of the acid dissociation constants, the complexation constants between these three carboxylic acid ligands and curium(III), plutonium(IV), neptunium(V), and uranium(VI) have been determined, also in the presence of varying methanol concentrations. These complexation constant values were measured using capillary electrophoresis inductively coupled plasma mass spectrometry. In the presence of methanol, both the measured pKa and complexation constants increased. This knowledge was applied in the separation of lanthanides from one another, where there was a corresponding increase in the separation of lanthanides along with the methanol concentration.