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.
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