Analytical Sciences

Ion-pair Formation between Cd(II), Na(I), and Ag(I) Complex Ions with 18-Crown-6 Ether Derivatives and Various Pairing Anions in Water: an Understanding of the Ion-pair Formation Based on the HSAB Principle

Yoshihiro KUDO,* Tsukasa KOIDE,** Yujie ZHAO,* Shoichi KATSUTA,* and Yasuyuki TAKEDA*

*Graduate School of Science, Chiba University, Chiba 263-8522, Japan
**Department of Chemistry, Faculty of Science, Chiba University, Chiba 263-8522, Japan

The ion-pair formation constants (K_MLX⁰/mol⁻¹ dm³) of CdL²⁺ with Br⁻ or NaL⁺ with N,N-diethyldithiocarbamate ion (DDTC⁻) in water were determined potentiometrically at 25°C; ionic strength (I)→0: L denotes 18-crown-6 ether (18C6) and its mono-benzo derivative for the CdBr₂-L system and 15-crown-5 ether and 18C6 for the NaDDTC-L one. The formation constant corresponding to the simple salt, NaDDTC, in water was also determined at I→0. Using the log K_CdLX⁰ values of CdLCl⁺, CdLBr⁺, CdLPic⁺, and CdLSO₄, then CdL²⁺ and picrate ion (Pic⁻) in water have been classified with the hard and soft acids and bases principle, where the values were available in the literature, except for CdLBr⁺. The same classification was examined in NaX-L systems with X⁻ = DDTC⁻, trifluoroacetate ion, MnO₄⁻, ReO₄⁻, Pic⁻, and BPh₄⁻ and the AgPic-L one. Consequently, CdL²⁺, NaL⁺, and AgL⁺ were classified as the hard acids, while Pic⁻ and BPh₄⁻ as the hard bases. These results reflected the reactivities of the complex ions in ion-pair formation with X⁻ and SO₄²⁻ in water.

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