Stability of Metal-Organic Complexes in Acetone- and Methanol-Water Mixtures

摘要

Contamination of soils and groundwaters by trace metals, organic ligands, and organic solvents has been identified at some DOE sites and hazardous landfills. Complexation of metal ions with organic ligands impacts the transport and fate of these contaminants. In this study, aqueous potentiometric titration was extended to cosolvent-water solutions to examine the effects of cosolvents on complexation stability. In H_2O-acetone and H_2O-methanol solutions, conditional stability constants, K_c, increased with increasing organic cosolvent content for complexes between Co-(Ⅱ) and oxalate, citrate, nitrilotriacetate (NTA), and ethylenediaminetetraacetate (EDTA). In most cases, the value of log K_c varied linearly with 1/ε (where ε is the dielectric constant of the solution). The stability of complexes between Cu(Ⅱ) and EDTA declined as the content of the organic cosolvent increased, and the stability constants for Fe(Ⅲ)-EDTA complexes exhibited little dependence on the concentration of organic cosolvent. Cupric electron paramagnetic spin measurements showed that the bonding between Cu and EDTA was more ionic than covalent, consistent with the linear dependence of log K_c on the value of 1/ε.