Some interesting aspects of the extraction of zinc
Realizing that, for a forty five minute talk, a title of "Solvent Extraction of Metal Chelates" is entirely too ambitious, I have chosen instead to speak about the experiences my students, associates and myself have encountered in the extraction of zinc, hoping that you will find this an appropriate illustration of the interesting variety of behaviour and information obtainable in the study of metal chelate extractions. One interesting aspect of the solution chemistry of Zn(II) is the variation of coordination number observed in zinc complexes; e.g., six, as in Zn(H20)62+, Zn(NH3)62+, Zn(CN)64 and four, as in ZnC142, and Zn (C204)22—. The zinc complexes of such chelating extractants as acetylacetone and 8-quinolinol occur as the hexacoordinated dihydrates, which upon heating, transform to the quadridentate anhydrous chelates. As a divalent metal ion of coordination number six, zinc chelates with bidentate monoanionic reagents would not be as readily extractable with coordinated water as with other, more hydrophobic, "auxiliary" ligands. Several years ago our attention was attracted by conflicting reportsl'2 of the efficacy of 8-quinolinol as a zinc extractant. A study of the extraction equilibria of zinc with 8-quinolinol and its 2-methyland 4-methyl-analogues helped to resolve the conflict by revealing that the extracted species in the 8-quinolinol (HOx) extraction is ZnOx2HOx [slope of log D versus pH was found to be 2; slope of log D versus (HOx)o was 3]3. The extraction of such a complex, termed a seif-adduct, calls for significantly higher 8-quinolinol concentrations than would be needed for the simple chelate; thus, the results directed attention to and explained the significance of the higher (HOx)o used in the publication' reporting successful Zn extraction, resolving the apparent conflict. The course of the extraction of zinc with 4-methyl-8-quinolinol (HL) was found to parallel that with 8-quinolinol, but that with 2-methyl-8-quinolinol (HQ) differed, as the extracted species was found to be a simple chelate ZnQ2, rather than a self-adduct.3 This difference probably arises from the steric hindrance to adduct formation presented by the substituent in the 2-position, which makes it likely that the adducting ligand coordinates through the quinoline nitrogen. Strengthening this hypothesis is the observation that the adduct formed by 4-methyl-8-quinolinol is more stable than that of the parent ligand, reflecting the increasing basicity of the nitrogen in the former ligand (Table 1).