Environmental context
Knowledge of metal-chelating agent speciation is integral to our ability to predict and interpret the behaviour of synthetic chelating agents in the environment. Capillary electrophoresis can be used to separate metal–ligand complexes with similar spectroscopic characteristics but different stoichiometries, thereby providing insight into metal–ligand speciation that is not possible by any other technique. Here, we demonstrate the utility of capillary electrophoresis for the determination of metal–ligand stoichiometries and evaluate its limitations.
Abstract
Job’s method of continuous variation is a traditional method used to determine the stoichiometry of metal–ligand complexes. The method is often applied to whole-sample absorbance measurements but its utility is limited when two or more complexes are present at significant concentrations and have similar absorption spectra. Here we employ capillary electrophoresis (CE), which separates complexes on the basis of charge and hydrodynamic radii, to extend the capabilities of Job’s method. Solutions containing nickel(II) and diethylenetriaminepentaacetic acid (DTPA) yield three CE peaks. Job’s method plot maxima, based on areas for each of the three CE peaks, coincide with nickel(II)-to-DTPA ratios of 1 : 1 and 1 : 2, which correspond to two complexes previously identified using whole-sample measurements, plus a ratio of 3 : 2, which corresponds to a previously unreported complex. We demonstrate how CE peak areas and electromigration times can be used to determine complex stoichiometries and formation constants. We discuss the strengths and weaknesses of Job’s Method coupled with CE and implications for speciation determination in environmentally relevant systems.
Exchange reactions between multidentate ligands (also known as chelating agents) contribute to kinetic control of metal ion speciation in aquatic environments. However, the complexity of the stepwise reaction mechanism complicates predictions of kinetic behavior (rates, rate laws, and mechanisms). Clarity is achieved with the adjunctive-semijunctive-disjunctive paradigm, which categorizes multidentate ligand exchange pathways along a continuum according to the decreasing ease of forming intermediate mixed-ligand ternary complexes. In order to better understand how steric interaction between entering and leaving ligands affects reaction pathways and kinetic behavior, we use a capillary electrophoresis method to monitor exchange between trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate (CDTA) and nickel(II) complexes with each of the following N-substituted iminodiacetate ligands (XIDA), iminodiacetate (IDA), methyliminodiacetate (MIDA), and benzyliminodiacetate (BIDA). Kinetic modeling indicates that reactions between CDTA and 1:1 nickel−XIDA complexes occur via parallel adjunctive and disjunctive pathways. With greater steric bulk of N-substituents on iminodiacetate, product formation via a disjunctive pathway increases while formation via the adjunctive pathway decreases. Kinetic analysis demonstrates how the shift in reaction pathways has a nonlinear effect on both the magnitude of the overall rate and the rate dependence on ligand concentrations. Furthermore, we discuss the implications of this work for understanding dynamic metal ion speciation in the environment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.