Abstract
The kinetics of back extraction have been studied on 2-(2-thiazolylazo)-4-methylphenol (TAC) chelates of zinc(II) and cadmium(II) from chloroform solution into aqueous CyDTA solution. The spectrophotometric determination of Zn(II) and Cd(II) in their mixtures was made simultaneously by the aid of the difference in the rate of hack extraction. TAC chelates of Zn(II) and Cd(II) were extracted at pH 8.210.0 and 8.710.2, respectively. These chelates showed their absorption maximum at 600 nm, and moler absorbances at this wavelength were 3.36 × 104 for Zn(II) chelate and 3.67 × 104 for Cd(II) chelate. The rate of the back extraction of Zn(II) chelate was much lower than that of Cd(II) chelate. The conditional rate constants for Zn(II) and Cd(II) chelates were 1.8 × 10-3 and 1.6 × 10-2 s-1, respectively at pH 9.18, free TAC concentration of 4.8 × 10-4 M, μ=0.15 and 20°C. The back extraction reactions were of first-order with respect to Zn(II) or Cd(II) chelate and inversely-first order with respect to free TAC and hydrogen ion. The concentration of CyDTA had no effect on the reactions so far as CyDTA was in excess of TAC chelates. Linear extrapolation method was applied to the analysis of the mixtures of Zn(II) and Cd(II) based on the following equation;
(Eo-Et)=εZn·l·[ZnL2]k'Zn·t+εCd·l·[CdL2]
whereE0, Et are the absorbance of the organic phase at reaction time 0 and t, εZn, εCd are the molar absorbances of Zn(II) and Cd(II) chelates of TAC, l is the length of light pass and k'Zn is the conditional rate constant for Zn(II) chelate. The interferences of Ni(II), Co(II), Cu(II), and Hg(II) were eliminated by preliniinaly extraction with TAC at pH 7.1. Fe (II, III), Mn(II), Cr(III) and Al(III) had no interference.
