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. = logDCd-logDZn) for extraction of cadmium(II) from zinc(II) is derived from the HSAB principle as follows: S.F.=(SACd-SAZn)SB +(σACd-σAZn)σB where SA, σA, SB and σBrepresent 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 SAZn =32.2, SACd=28.1, σAZn=-26.6, σACd =-12.6): σB≥0.286+0.293SB. 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 SB and σB values. In this paper, some chelating agents such as thenoyltrifluoroacetone (TTA, SB = 0.312, σB=0.101), monothiothenoyltrifluoroacetone (STTA, SB = 0.26, σB-=0.31), diethyldithiocarbamic acid (DDTC, SB=0.569, σB=0.433), dithizone (SB=0.42, σB=0.20) and 8-quinolinol (SB=0.623, σB=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).