Ion-pair formation in water Distribution of cadmium ion-pair complex into benzene Cadmium picrate Crown ethers Potentiometry with ISE Overall ion-pair formation constants (Ξ² 2 /mol -2 dm 6 ) of CdL 2+ with picrate ion (Pic -) and Cl -in water (w) were determined at 298 K potentiometrically, where L denotes 18-crown-6 ether (18C6) or benzo-18C6 (B18C6). These values at ionic strength (I ) β zero were 1.8Γ10 7 for Cd(18C6)(Pic) 2 ; 2.3Γ10 7 for Cd(B18C6)(Pic) 2 ; 1.4Γ 10 6 for Cd(18C6)Cl 2 ; 2.7Γ10 6 for Cd(B18C6)Cl 2 . Using the Ξ² 2 value for CdL(Pic) 2 at L = B18C6 and other equilibrium constants, extraction constants {Kex k , k (stoichiometric number of L) = 1, 2} and an association one (K ML2A2 ) of CdL(Pic) 2 with L in benzene were re-calculated from the extraction data reported before:
70 at the acidic w-phase of I = 0.095 mol dm -3 (HNO 3 ). Here, the subscript "o" denotes the benzene phase. These values were somewhat larger than those reported previously. From a thermodynamic cycle of an extraction model proposed here, the distribution constant of Cd(B18C6)(Pic) 2 into benzene was calculated to be 10 -3.56 . A difference (4.50) in logarithmic distribution constant between Cd(B18C6)(Pic) 2 and B18C6 suggested the stronger hydration of the former than the latter in the w-phase.