Mechanism of Solvent Extraction of Cobalt with 5-Chloro-2thiophenecarbaldehyde 2-Benzothiazolylhydrazone

Abstract

The equilibrium and rate of solvent extraction of cobalt ion in aqueous solution (μ= 0.1)with 5-chloro-2-thiophenecarbaldehyde 2-benzothiazolylhydrazone (CTBH) in benzene have been studied at 25°C. The binding ratio of the complex was assumed to be 1: 3 (Co: CTBH) by the slope of distribution ratio against CTBH concentration, and the valence state of cobalt ion in the complex, which was measured by Evans' method, was +3. It was, therefore, concluded that the extracted complex was identified as the neutral chelate, CoR₃, where R represents dissociated CTBH. The distribution constant (log K'DC) of the complex between benzene and water was 2.23. The rate of the complexation reaction has been found to be first-order with respect to cobalt 00 ion and CTBH concentrations, and inverse first-order with respect to hydrogen ion concentration. From these results, the controlling reaction was assumed to be formation of 1: 1 chelate of cobalt (II) on with dissociated CTBH (R-) in aqueous phase. The rate constant for the reaction between cobalt (II) on and R. - was found to be 2.9×10⁶ mol^-1 ⋅dm³⋅ s^-1 at 25°C. The thermodynamic data for the reaction system were calculated. These values are Ea: 9.7kcal/mol, ΔH: 9.1kcal/mol, ΔG: 8.6 kcal/mol and ΔS:1.6 e. u. It was also assumed that the structure of the extracted complex was the octahe dral configuration.