Ion-pair formation of cadmium-18-crown-6 ethers complex ions with picrate and chloride ions in water and distribution of its picrate ion-pair complex into benzene

Overall ion-pair formation constants (β2/mol− 2 dm6) of CdL2+ 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 ) → zero were 1.8 × 107 for Cd(18C6)(Pic)2; 2.3 × 107 for Cd(B18C6)(Pic)2; 1.4 × 106 for Cd(18C6)Cl2; 2.7 × 106 for Cd(B18C6)Cl2. Using the β2 value for CdL(Pic)2 at L = B18C6 and other equilibrium constants, extraction constants {Kexk, k (stoichiometric number of L) = 1, 2} and an association one (KML2A2) of CdL(Pic)2 with L in benzene were re-calculated from the extraction data reported before: Kex1 = [CdL(Pic)2]o/[Cd2+][L]o[Pic−]2 = 102.37, Kex2 = [CdL2(Pic)2]o/[Cd2+][L]o2[Pic−]2 = 104.21, and KML2A2 = [CdL2(Pic)2]o/[CdL(Pic)2]o[L]o = 70 at the acidic w-phase of I = 0.095 mol dm− 3 (HNO3). 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.