Uncertainty Principle in the Elucidation of the Extraction Mechanism of Ions from Aqueous towards Ionic Liquid Phases. PtCl₆²⁻ and [C₁C₈IM][NTf₂] as a Textbook Case

Abstract

The extraction mechanism of Pt(IV) from aqueous HCl solutions towards the ionic liquid 1-octyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C 1 C 8 IM][NTf 2 ]) is deciphered. To that end, aqueous speciation of Pt(IV) as a function of HCl was studied by 195 Pt NMR. Influence of the concentration of HCl (ranging from 1 to 12 M) and of the initial concentration of Pt (IV) (ranging from 10 À5 to 10 À2 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 PtCl 6 2À through an ion-pair formation and that of Cl À towards the ionic liquid phase. D values were then predicted over a wider range of concentrations for HCl and PtCl 6 2À. Alternatively, the extraction of PtCl 6 2À 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 PtCl 6 2À 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 [a] Prof. N. Papaiconomou

Rh(III) Aqueous Speciation with Chloride as a Driver for Its Extraction by Phosphonium Based Ionic Liquids

“…Based on the large D values for large [RhCl 4 ] − amounts, the proposed mechanism cannot cope without involving [RhCl 4 ] − . Rh extraction can be either described by an ion exchange model or by an ion pair formation process [14]. The ion exchange model can be written as:RhCl4−+trueCl−¯⇔trueRhCl4−¯+Cl− where species capped with a bar correspond to species in the IL phase.…”

Application of Ionic Liquids in Solvent Extraction of Platinum Group Metals

“…ILs have low viscosity. An in-depth study of the Pt extraction mechanism was reported by Papaiconomou et al in 2016 [56]. They investigated the extraction mechanism of Pt using hydrophobic imidazolium-based ILs (e.g., C 8 mimTf 2 N).…”

“…They found that the mechanism of metal extraction is typically mainly based on either ion exchange or ionpair formation, with the latter being more favorable because it avoids undesirable loss of the ILs in the aqueous phase [55]. They also found that both the ion exchange and ion-pair formation mechanisms are equally applicable to metal extraction [56]. This uncertainty principle can also be applied to extraction of charged metal complexes with any hydrophobic IL.…”

In situ electrochemical dissolution of platinum and gold in organic-based solvent

“…Furthermore, understanding the corrosion of noble metals has started to emerge as an important issue from the perspective of environmentally friendly recycling strategies, 16 where organic solvents show much promise. [17][18][19][20] ICP-MS analysis of organic matrices is not as common as in case of aqueous matrices and presents a much greater challenge in tackling instrumental settings and several unusual spectral interferences. 21 The presence of high levels of organic solvents increases the vapor pressure of the sample solution, which is circumvented by cooling the ICP-MS spray chamber to −5°C.…”