Formation of super-concentrated hydrochloric acid in the third phase in tertiary amine N235-PtCl62−-HCl system and its influences on the Pt microemulsion extraction

In this paper, we have investigated the formation of the third-phase in tertiary amine (N235)-PtCl62−-HCl system and the microscopic phase structural evolution of platinum-loaded organic phases before and after the occurrence of the third-phase. The third-phase is characterized by various spectroscopic techniques, and the small angle X-ray scattering (SAXS) experiments demonstrate the appearance of nano-aggregates, i.e., water-in-oil reversed micelles, in the third phase. The experimental results indicate that (1) formation of the third phase is related to the aggregation behaviors of nano-reversed micelles in which a super-concentrated hydrochloric acid formed with the H+ to H2O molar ratio being much higher than that of the conventional 37 wt% saturated hydrochloric acid. (2) The occurrence of the super-concentrated HCl results in a great amount of H+ and Cl− ions enriched and confined within the nano-water pools of W/O reversed micelles in third phase. Therefore, the coordination behaviors of platinum complex ions in that super-concentrated hydrochloric acid are very different from their corresponding behaviors in bulk aqueous solutions. It is possible that H+ ions participate in the formation of such complexes as HmPtCl6z+ in the super-concentrated hydrochloric acid. (3) The relative contents of various HmPtCl6z+ complexes are different corresponding to the H+ ion concentrations in confined nano-water pools. The association ability of the acidified tertiary amine N235 molecules (R3NH+) with various HmPtCl6z+ complexes plays an important role in affecting the platinum extraction behaviors.