CdI₂ Extraction with 18-Crown-6 Ether into Various Diluents: Classification of Extracted Cd(II) Complex Ions Based on the HSAB Principle

Abstract

CdI 2 in water was extracted with 18-crown-6 ether (L) into 10 diluents at 298 K. The following equilibrium constants were determined or evaluated: some extraction constants ( K ex /mol -3 ·dm 9 & K ex,ip /mol -2 ·dm 6 for CdLI 2 , K ex± /mol -2 ·dm 6 for CdLI + with I - , & K ex2± /mol -1 ·dm 3 for CdL2+ with 2I - ), conditional distribution constants ( K D,I for I - , K D,CdLI for CdLI + , & K D,CdL for CdL2+) between the two phases, and an ion-pair formation constant ( K 1,org /mol -1 ·dm 3 ) for CdLI + and that ( K 2,org /mol -1 ·dm 3 ) for CdLI 2 in the organic (org) phases. Using the K 1,org and K 2,org values, acidities of the complex ions, CdL2+ and CdLA+ (A - = I - , Br - , & Cl - ), in the 11 diluents were classified by applying the HSAB rule. Especially, the CdLA+ ions were classified as the soft acids in 9 diluents. Also, molar volumes ( V j /cm 3 ·mol -1 ) of j = CdLI 2 and CdL2+ were determined with the regular-solution-theory plot of log K ex,ip vs . log K D,L and its pseudo-plot of log K D,CdL , respectively. Here, K D,L denotes the distribution constant of L between the two phases. So, sizes among CdLA2 and CdL2+ were compared by using the V j values. Additionally, some distribution equilibrium potentials (dep/V) between the water and org bulk phases were topically calculated from an equation of K D,I with K S D,I , where the symbol K S D,I shows a standard distribution constant of I - at dep = 0 V for a given diluent.