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.