Extraction of Palladium(II) and Platinum(IV) from Hydrochloric Acid Solutions with Trioctylammonium Nitrate Ionic Liquid without Dilution

The fundamental properties and extraction capability of an ionic liquid (IL), trioctylammonium nitrate ([HTOA][NO₃]), for Pd^II and Pt^IV, are investigated. At room temperature, [HTOA][NO₃] is a solid (melting point: 30.7 °C), but it becomes a liquid (melting point: 16.7 °C) when saturated with water. Water-saturated [HTOA][NO₃] exhibits a viscosity of 267.1 mPa·s and an aqueous solubility of 2.821?×?10^?4 mol·dm^?3 at 25 °C, and can be used as an extraction solvent without dilution. [HTOA][NO₃] exhibits an extremely high extraction capability for Pd^II and Pt^IV in dilute hydrochloric acid (0.1–2 mol·dm^?3 HCl); the distribution ratio reaches 3 × 10⁴ for both the metals. From electrospray ionization mass spectrometry analysis, the species extracted in the IL phase are [PdCl₃]^? and [PdCl₂(NO₃)]^? for Pd^II and [PtCl₆]^2? and [PtCl₅]^? for Pt^IV. A majority of the other transition metals are considerably less or marginally extracted into [HTOA][NO₃] from a 0.1 mol·dm^?3 hydrochloric acid solution. The extraction capacity of [HTOA][NO₃] is greater than that of other hydrophobic ILs such as [HTOA]Cl and bis(trifluoromethanesulfonyl)imide-based ILs. The metals extracted into the IL phase are quantitatively back-extracted using an aqueous solution containing thiourea and nitric acid. By controlling the thiourea concentration and shaking time, Pd^II and Pt^IV are mutually separated to some extent in the back extraction process. The IL phase used for the back extraction can be reused for the forward extraction of these metals after scrubbing it with an aqueous nitric acid solution.