Extraction equilibrium of palladium(II) with 2-methyl-8-quinolinol into supercritical carbon dioxide fluid

The present study is concerned with the extraction behavior and equilibrium of Pd(II) with 2-methyl-8-quinolinol (HMQ) into supercritical fluid CO2 (SF-CO2). Pd(II)–HMQ complex extracted from a weakly acidic solution (pH 2–3) into SF-CO2 was determined to be Pd(MQ)2 on the basis of a slope analysis. The extraction constant Kex,SF (=[Pd(MQ)2]SF[H+]2[Cl−]4[PdCl42−]−1[HMQ]−2) was determined to be 104.3±0.2 at 8.5 MPa, 45 °C and I=0.4 M (H, Na)Cl (1 M=1 mol dm−3). The distribution behavior of HMQ between an aqueous and a SF-CO2 phase was examined so as to discuss quantitatively the extraction equilibrium. The extraction constant (Kex,Cy) of Pd(II) with HMQ into cyclohexane with a similar polarity to SF-CO2 was determined and the Kex,SF was compared with the Kex,Cy. Pd(II) at the concentration range of 10−5–10−4 M in the aqueous solution (pH<3) containing relatively high concentration of chloride ion was found to be extracted efficiently by the SF-CO2 extraction.