Ion association reactions in aqueous solutions between non-UV-absorbing crown ether–alkali metal complexes with picrate ion by capillary zone electrophoresis

Abstract

The ion association potential of non-UV-absorbing crown ether [12-crown-4 (12C4), 15-crown-5 (15C5) and 18-crown-6 (18C6)]–metal complexes with picrate ion was established through the change in mobility of picrate ion by capillary zone electrophoresis. The apparent electrophoretic mobility of picrate ion was found to decrease considerably with increasing concentration of 18C6 in the migrating solution in the presence of alkali metal ion. However, on increasing the alkali metal concentration in the presence of 18C6, the apparent electrophoretic mobility of picrate ion also decreased at concentrations up to 15 mM and then started to increase gradually. The decrease was independent of the kind of counter anion of the alkali metal ion, and was attributed to the interaction between the cationic complex and the picrate ion, whereas the increase was attributed as being due to both concentrated salt effects and competitive ion association towards the cations between picrate ion and the counter anions. No remarkable change in the mobility of picrate ion was observed with cationic12C4 or 15C5, which is attributed to weak complexation capability. The ion association constants (K_ML·X) between the complexes and picrate ion were determined using such mobility changes. 18-Crown-6 was found to provide the best ion associability whereas the ion association constants for 12C4 and 15C5 metal complexes with picrate ion could not be determined. Ion association constants obtained in this study were compared with those of other crown ethers; they decreased in the order 18C6 < benzo-18-crown-6 (B18C6) < dibenzo-18-crown-6 (DB18C6) and hydrophobic interaction was suggested. In all cases, Na⁺ complexes showed higher ion associability than K⁺ complexes.