Binding Ability of Crown Ethers Towards Pb(II) Ions in Binary Water/Organic Solvents Using Solvent Extraction Method

Abstract

The presence of crown ethers 12C4, 15C5 and 18C6 in binary water/X solvents (X is methanol, ethanol, acetonitrile, dimethylformamide and dimethylsulfoxide) influences the extraction of lead ions by the acidic extractant bis(2-ethylhexyl)phosphoric acid in cyclohexane. In fact, complexation of Pb(II) ions in the aqueous phase by the studied crown ethers causes a shift toward the higher pH region of the extraction curves. Analysis of the extraction data allows evaluation of the stability formation constant of [Pb·CE]2+ complexes in the studied solvents. Regardless of the composition of solvents, the ability of the crown ethers to complex lead ions varies as 18C6 > 15C5 > 12C4. The stability formation constant of [Pb·CE]2+ complexes depends on the type of solvent X. A decrease in binding ability of the studied crown ethers towards Pb(II) ions was observed by enhancing the water/X ratio. The influence of temperature on the extraction of lead ions from the studied solvents in the absence and in the presence of the crown ethers was studied in the temperature range 290–310 K. This study has allowed estimation of the thermodynamic parameters, i.e. standard Gibbs energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) changes associated with the extraction process, as well as those for complexation of lead ions by 12C4, 15C5 and 18C6 in the investigated water/X solvents.