In order to determine the ion-pair formation constant of a crown ether-metal salt 1:1:1 complex in water, an equation is derived from regular solution theory and its predictions are verified experimentally by the solvent extraction method using benzo-18-crown-6 (B18C6), potassium picrate (KA), and various diluents of low dielectric constant. The distribution constants of B18C6 itself and the overall extraction constants of KA with B18C6 were determined at 25±0.2°C. The distribution constants of the neutral K(B18C6)A complex were calculated from these data. The literature value for the complex-formation constant of K(B18C6)+ in water and the ion-pair formation constant (K K(B18C6)A ) for K(B18C6)A in water determined in this study were log K K(B18C6)A =3.12±0.23 at 25°C). The distribution behavior of B18C6 and K(B18C6)A is explained in terms of regular solution theory. The molar volumes V (cm3·mol−1) and solubility parameters δ (cal1/2-cm−3/2) are as follows: V B18C6 =249±36; V K(B18C6)A =407±56; δB18C6 = 11.5 ± 0.5; and δK(B18C6)A = 11.5 ± 0.5.
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Takeda, Y., Sato, H. & Sato, S. Solvent effects on extraction of potassium picrate with benzo-18-crown-6. A new equation for the determination of ion-pair formation of crown ether-metal salt complexes in water. J Solution Chem 21, 1069–1080 (1992). https://doi.org/10.1007/BF00651734
- Solvent extraction
- ion-pair formation constant in water
- solvent effects
- regular solution theory
- molar volume
- solubility parameter
- potassium picrate