Extraction Equilibria of Palladium(II) and Platinum(IV) with N,N-Di(2-ethylhexyl)aminomethylquinoline from Hydrochloric Acid

Abstract

N,N-Di(2-ethylhexyl)aminomethylquinoline (DEQ) has been newly synthesized to develop selective extractants for precious metals. DEQ is designed so as to exhibit high selectivity for precious metals through the electrostatic force and coordination ability of the tertiary amine and quinoline moieties, respectively. DEQ in toluene exhibits a high selectivity for gold(III), palladium(II) and platinum(IV) over base metals such as copper(II), nickel(II) and cobalt(II) from hydrochloric acid. This is attributed to the strong basicity of the tertiary amine and high coordination ability of quinoline for precious metals. Palladium(II) and platinum(IV) extraction is significantly dependent on the concentration of hydrochloric acid, while gold(III) is quantitatively extracted over the whole concentration region of hydrochloric acid examined. We have quantitatively discussed the extraction equilibria of palladium(II) and platinum(IV) with DEQ in toluene from hydrochloric acid. It is observed that palladium(II) was extracted by the anion exchange mechanism, while platinum(IV) was extracted through the anion exchange and solvation mechanisms. Palladium(II) was extracted as a 1 : 1 complex formed by ion pair formation and coordination bonding, and its extraction equilibrium constant was determined as 1.28 × 10⁷ [-]. Platinum(IV) was extracted as a 1 : 1 complex and a 1 : 2 complex at low and high concentrations of hydrochloric acid, respectively. These extraction equilibrium constants were determined as 4.15 × 10² [mol dm⁻³] and 3.46 × 10⁴ [-], respectively.