1999 Volume 48 Issue 7 Pages 693-697
The chelate extraction constants for the extraction of copper(II), zinc(II) and cadmium(II) with diphenylthiocarbazone (dithizone) and monothiothenoyltrifluoroacetone (STTA) were measured at various temperatures, and the thermodynamic quantities {enthalpy change (ΔH) and entropy change (ΔS)} were estimated by van't Hoff plot. In the case of dithizone extraction, ΔH for zinc (II) was positive, whereas that for cadmium (II) was negative, and ΔS for zinc (II) was much larger than that for cadmium(II). The extraction for copper(II) gave a negative ΔH and a large positive ΔS. From a comparison of these values, it was indicated that the larger extraction constant for copper(II) than that for zinc(II) can be attributable to a difference in ΔH, and than that for cadmium(II) is due to a difference in ΔS. On the other hand, STTA extractions for zinc(II) and cadmium(II) gave large positive ΔH and small positive ΔS, whereas that for copper(II) gave large negative ΔH and positive ΔS. For this reason, the extraction constant for copper(II) became much larger than that for zinc(II) and cadmium(II).
