A study of the mechanism of the solvent extraction of metal ions by carboxylic acids

A study of the species involved in the extraction of metal ions from aqueous solutions into organic solvents with long-chain carboxylic acids has been made. The theoretical analysis of the extraction data in terms of a single equilibrium approach and a multiple equilibria method has been made. The two approaches have been shown to be equivalent in the case of one extracted complex, and an extraction expression has been proposed to describe the extractions. The analysis of the data has been considered in terms of this expression by graphical and regression procedures. Under the equilibrium extraction conditions studied, the extracted species in the case of copper (II), cobalt (II) and nickel (II) systems appear to be predominantly dimeric in nature and are usually solvated with carboxylic acid molecules in the organic phase. Results obtained from the extraction of copper from an aqueous solution into free-falling drops of n-octoic acid dissolved in carbon tetrachloride indicate that the rate of extraction does not vary appreciably over the pH range studied. The measurement of the interfacial tensions in the same system suggests that no abrupt change in the nature of the interfacial region occurs as the pH of extraction increases.