Extraction by separation of multi-charge chloride complexes of iridium (IV) and rhodium (III) in segregated water systems

Abstract

Extraction of iridium (IV) and rhodium (III) chloride complexes in a two-phase water system without
the introduction of an additional reagent was studied. A polyethylene glycol (PEG-1500, PEG-115) – NaCl –
(NH₄)₂SO₄ – H₂O two-phase water system is proposed for the selective extraction of iridium (IV) without the introduction of an additional reagent. The purpose of this work was to study the regularities of the interphase distribution of iridium (IV) chloride complexes and to develop an extraction method for its separation from rhodium (III) using a two-phase water system. The quantum-chemical calculations and the extraction method showed the influence of charge value on the central atom of the ion of multi-charge chloride complexes of iridium (IV) and rhodium (III) on their interphase distribution in a two-phase water system. The nature of the water-soluble extractant affected the completeness of the analyte extraction from the concentrated sulphate solutions. The iridium (IV) chloride complex was almost completely extracted with PEG-1500, PEG-115 by 95% and was extracted with ethanol, isopropanol (IPA), and acetone by 23, 11, and 15% respectively. The rhodium (III) chloride complex was extracted with PEG by 4% and was not extracted with ethanol, IPA, or acetone. The composition of the extracted compound was determined by molecular absorption spectrophotometry. Absorption maximums typical for the [IrCl₆]₂- complex were observed at 435 and 495 nm in the PEGbased absorption spectrum of the iridium (IV) extract. After the separation of iridium (IV), absorption bands were observed in the absorption spectra of the aqueous phase at wavelengths of 412 and 518 nm, which are characteristic of the [RhCl₆]₃- complex. As a result of the conducted studies, an extraction method was developed for the separation of iridium (IV) from rhodium (III) in the form of the multi-charge chloride complexes. Optimal conditions for the separation of iridium (IV) from rhodium (III) using PEG were determined. The ratio of the aqueous phase to the organic phase is 3 : 1. Maximum extraction of the iridium (IV) chloride complex and its separation from rhodium (III) was observed with the concentration of C (Cl^-)=1.3∙10^-2-0.5 mol/dm³.
When the iridium (IV) is separated from rhodium (III), the composition of the analytes in the aqueous and
organic phases was determined by a spectrophotometric method based on the colour of [IrCl6]2- and [RhCl⁶]^3-. Under optimal extraction conditions, iridium (IV) has also been separated from ruthenium (III), iron (III), aluminum (III), nickel (II), copper (II), cobalt (II), magnesium (II), zinc (II), and lead (II). The Ir (IV) / Rh (III) separation factor is 456. The separation technique is characterised by its simplicity (single stage extraction), cost effectiveness, rapidity, and the use of a low-risk PEG extractant. The process takes 20 minutes.