Extractive separation of cadmium(II) from zinc(II) with capriquat-diethyldithiocarbamate

Abstract

Based on the Hard and Soft Acids and Bases (HSAB) principle, the possibility of extractive separation of cadmium(II) from zinc(II) with some chelating agents was discussed. The separation factor (S.F. = logD^Cd-logD^Zn) for extraction of cadmium(II) from zinc(II) is derived from the HSAB principle as follows: S.F.=(S_A^Cd-S_A^Zn)S_B +(σ_A^Cd-σ_A^Zn)σ_B where S_A, σ_A, S_B and σ_Brepresent strength factor of a metal ion, soft factor of a metal ion, strength factor of chelating agent and soft factor of chelating agent, respectively. If it is assumed that S.F.≥4 is required for the quantitative separation of cadmium(II) from zinc(II), the above equation leads to the following relationship (where S_A^Zn =32.2, S_A^Cd=28.1, σ_A^Zn=-26.6, σ_A^Cd =-12.6): σ_B≥0.286+0.293S_B. This equation suggests that the possibility of the quantitative separation of cadmium(II) from zinc (II) with a suitable chelating agent can be estimated from its S_B and σ_B values. In this paper, some chelating agents such as thenoyltrifluoroacetone (TTA, S_B = 0.312, σ_B=0.101), monothiothenoyltrifluoroacetone (STTA, S_B = 0.26, σ_B-=0.31), diethyldithiocarbamic acid (DDTC, S_B=0.569, σ_B=0.433), dithizone (S_B=0.42, σ_B=0.20) and 8-quinolinol (S_B=0.623, σ_B=0.133) were applied. As a result, it was concluded that DDTC is the most effective chelating agent for the separation of cadmium(II) from zinc(II) although quantitative separation cannot be achieved. However, the use of an ionpair extraction system using a mixture of DDTC and capriquat enables the quantitative separation of cadmium(II) from zinc(II).