Uncertainty Principle in the Elucidation of the Extraction Mechanism of Ions from Aqueous towards Ionic Liquid Phases. PtCl 6 2− and [C 1 C 8 IM][NTf 2 ] as a Textbook Case
Résumé
The extraction mechanism of Pt(IV) from aqueous HCl solutions towards the ionic liquid 1‐octyl‐3‐methylimidazolium bis(trifluoromethanesulfonyl)imide ([C1C8IM][NTf2]) is deciphered. To that end, aqueous speciation of Pt(IV) as a function of HCl was studied by 195Pt NMR. Influence of the concentration of HCl (ranging from 1 to 12 M) and of the initial concentration of Pt(IV) (ranging from 10−5 to 10−2 M) in the aqueous phase on the distribution coefficients of Pt(IV) was systematically investigated. The decrease in D with the concentrations of HCl and Pt(IV) observed is quantitatively very well described with a model using three chemical equilibria, namely the solubility of the ionic liquid in water, the extraction of PtCl62− through an ion‐pair formation and that of Cl− towards the ionic liquid phase. D values were then predicted over a wider range of concentrations for HCl and PtCl62−. Alternatively, the extraction of PtCl62− was described using an anion exchange hypothesis and it is demonstrated that these two apparently different mechanisms are actually equivalent within hydrophobic ionic liquids. Because the two versions of the extraction model presented here are not specific to PtCl62− or the ionic liquid, they can be generalized to the extraction of any charged (metal) complex and any hydrophobic ionic liquid, allowing us to outline a general uncertainty principle in the determination of extraction mechanisms in systems involving ionic liquids