Analysis of Ion-association Equilibrium of Precipitable Dipicrylaminate Ion in Aqueous Solution by Capillary Zone Electrophoresis

Abstract

The ion-association equilibrium of the dipicrylaminate ion (DPA -) was investigated in an aqueous solution with alkali metal ions and with its 18-crown-6 ether complexes as pairing cations by capillary zone electrophoresis (CZE). Although DPA -is precipitable with the pairing cations of K + and Cs + in a homogenous aqueous solution, a low concentration of DPA -below its solubility suppressed the formation of the precipitates, and DPA -was detected as a usual CZE signal. Changes in the electrophoretic mobility of DPA -was analyzed for its ion-association equilibrium. The ion association constants determined were almost comparable among alkali metal ions, while the ion association constants were meaningfully large with a hydrophobic 18-crown-6 ether complex of K + . The CZE separation was also proved to be useful for the equilibrium analysis under precipitating conditions. It was suggested that the precipitable property of DPA -with K + and Cs + could not be attributed to the ion-association process in an aqueous solution, but to the condensation process concerning the intermolecular CH···O bond in the precipitate crystals, as reported.Keywords Dipicrylaminate ion, alkali metal ions, 18-crown-6 ether complexes, ion-association equilibrium, aqueous solution, capillary zone electrophoresis

Development of Novel Analysis and Characterization Methods Utilizing Reaction Dynamics in a Separation Capillary

“…Aromatic-aromatic interaction was also revealed with divalent viologen cations [39]. It is also mentioned that such CE analysis of the ion-association equilibrium is applicable to precipitable ion-associates because of the superb hydrophobicity [40]. Factors contributing to the ion-associability in an aqueous solution are summarized in Table 1 [1].…”