Effect of Changes in Structure of Phosphoric Acid Esters on Solvent Extraction of Cobalt (II) and Nickel (II)

Abstract

Complexation behaviors of phosphoric acid esters with cobalt (II) and nickel (II) in solvent extraction have been assessed to elucidate the relationship between the structure of phosphoric acid esters and the extraction phenomena. By using phosphoric acid esters with aryl and alkyl groups, it is concluded that the structure of the organic groups in the phosphates plays an important role in the formation of the complexes and the extraction behaviors. In the case of di (nonylphenyl) phosphoric acid, the octahedral complexes of cobalt (II) and nickel (II) form in the extraction, and the compound extracts the metal ions at the lowest pH among the phosphates used. By changing the phenyl group with an alkyl group, the extraction curve shifts to higher pH, and the formation of the tetrahedral complex of cobalt increases in the extraction. Furthermore, the increase of the steric hindrance of the alkyl groups in the phosphates causes the shift of the extraction curves to higher pH. In addition, the selectivity of cobalt over nickel increases with increasing the steric hindrance of the phosphates.