Complexation Behavior of Iron(III) with Aloxime 800, D2EHPA, CYANEX 272, CYANEX 302, and CYANEX 301

The extraction of iron(III) has been studied from chloride solutions with di(2‐ethylhexyl)phosphoric acid (D2EHPA), bis(2,4,4‐trimethylpentyl)phosphinic acid (CYANEX 272) and its sulfur‐substituted analogs, called CYANEX 302, and CYANEX 301, and 5‐dodecylsalicylaldoxime (Aloxime 800). Job's method was applied for the characterization of the iron(III) complexes dissolved in hexane. In the case of D2EHPA and CYANEX 272, a 1:1 ligand‐to‐metal ratio was observed, thus inferring the coordination of additional compounds. No chloride transport occurred during extraction, therefore suggesting the formation of [Fe(OH)2L] complexes. With CYANEX 302, a ratio of 2:1 was obtained, whereas for CYANEX 301, the results of Job's method indicated the presence of four extractant molecules around the metal ion. Less hydrolysis or the possible oxidation of the sulfur‐substituted organophosphinic acids and the corresponding reduction of Fe(III) towards Fe(II) may explain this behavior. In the case of Aloxime 800, the formation of the [FeL3] species is suggested. A comparative study was carried out to identify the ligand‐to‐metal ratio of the iron(III) complexes in anhydrous circumstances. These studies showed that 1:1 ligand‐metal complexes are easily formed in the case of the organophosphoric‐ and organophosphinic‐acid extractants. A higher ligand‐metal ratio may be possible, but is not always a necessary condition for iron(III) extraction. Even the coexistence of [FeCl2L], [FeClL2] and to a smaller extent [FeL3] is quite presumable in anhydrous media. Finally, FT‐IR spectra as well as UV‐VIS spectra of the hexane phases make it possible to gain a better insight into the complexation characteristics of iron(III).