The various factors involved in the extraction of alkali metal picrates by calixarene ester derivatives in the mutually saturated water–dichloromethane solvent system

Abstract

The distribution of alkali metal picrates in the mutually saturated water–dichloromethane solvent system at 298.15 K was investigated. From distribution data, the partition and the distribution constants of these salts in this solvent system and the ion-pair formation constants of alkali metal picrates in water saturated dichloromethane were calculated. For salts containing the smaller alkali metal cations, significant differences were found between the partition and the transfer Gibbs energies in this solvent system. The former data are referred to the mutually saturated solvents whereas the latter involve water and dichloromethane in their pure state. In order to (i) formulate an equation which is representative of the extraction process taking place when an aqueous solution containing the free metal ion salt is equilibrated with a solution of calixarene ester in dichloromethane and (ii) gain information regarding the behaviour of calixarene ester salts in non-aqueous media, conductance measurements were performed in acetonitrile and pure and wet dichloromethane at 298.15 K. Whenever possible, the ion-pair formation constants and the limiting molar conductances were calculated from the analysis of data using a well established treatment. The results obtained are discussed. From distribution experiments in the presence of ethyl p-tert-butylcalix[4]arene tetraethanoate in the mutually saturated solvents, the distribution and extraction constants for alkali metal picrates were calculated. Good agreement was found between the distribution constants derived from data in the absence and presence of the ligand. The individual processes involved in the overall extraction of cations by ethyl p-tert-butylcalix[4]arene tetraethanoate were assessed. The results were compared with corresponding data for the mutually saturated water–benzonitrile solvent system. It is concluded that the higher extraction constants obtained in the latter solvent system are mainly attributed to the more favourable distribution constants in water–benzonitrile relative to water–dichloromethane. Based on the experimental evidence shown in this paper, some of the statements made in the literature regarding these systems are discussed. The effect of the solvent on the selective extraction of alkali metal cations from water is discussed.