Analysis of Ion-association Equilibrium of Precipitable Dipicrylaminate Ion in Aqueous Solution by Capillary Zone Electrophoresis
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The ion-association equilibrium of the dipicrylaminate ion (DPA<sup>−</sup>) 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<sup>−</sup> is precipitable with the pairing cations of K<sup>+</sup> and Cs<sup>+</sup> in a homogenous aqueous solution, a low concentration of DPA<sup>−</sup> below its solubility suppressed the formation of the precipitates, and DPA<sup>−</sup> was detected as a usual CZE signal. Changes in the electrophoretic mobility of DPA<sup>−</sup> 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<sup>+</sup>. 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<sup>−</sup> with K<sup>+</sup> and Cs<sup>+</sup> 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.
- Analytical Sciences
Analytical Sciences 30(9), 919-924, 2014
The Japan Society for Analytical Chemistry