NEW 8-HYDROXYQUINOLINE DERIVATIVE EXTRACTANTS FOR PLATINUM-GROUP METALS SEPARATION .4. KINETICS OF PD(II) EXTRACTION AND STRIPPING

The extraction and stripping kinetics of Pd(II) by chloroform solution s of 7-substituted-8-hydroxyquinoline derivatives, 8-HQ, (labelled TN 1911, an unsaturated and sterically-hindered derivative and TN 2336, a saturated and sterically-free derivative) from aqueous chloride media are reported. The extraction process is shown to go through the fast formation of the ion-pair PdCl42-.(H(2)Q(+))(2) (where H(2)Q(+) repres ents the protonated extractant) and the slow intramolecular transforma tion of the latter to an inner sphere (chelate) complex. As the slow i ntramolecular conversion reaction (the postulated rate-limiting step) apparently occurs away from the interface, the extraction reaction was found to be independent of the interfacial area and the agitation spe ed. The order of the extraction reaction, in the HCl concentration ran ge 0.20-2.00M was found to be 1 with respect to [PdCl42-], 0 with resp ect to [H+], 2 with respect to [HQ](org) and 2 with respect to [Cl-] f or both derivatives. The respective orders for the stripping reaction were 1 for [PdQ(2)](org), 2 for [H+] and 2 for [Cl-] for both extracta nts again. Pd(II) extraction and stripping rate constants, k(ext) and k(strip), were found to be 0.36 +/- 0.05 s(-1) and 6.8 +/- 0.3 (x10(-6 )) s(-1) for TN 1911 and 0.6 +/- 0.1 s(-1) and 8.9 +/- 0.3(x10(-6)) s( -1) for TN 2336. It was also observed that the presence of Pt(IV) in t he organic phase slows down (although to a modest degree) the extracti on rate of Pd(II). It is demonstrated finally that an exchange reactio n takes place between Pd(II) and Pt(IV) when the two metals are co-ext racted from a mixed metals solution. Thus Pt(IV) appears to be initial ly extracted very rapidly as an ion-pair and then is crowded out by th e slow loading of Pd(II) as chelate in the organic phase.