Transition metal ions, extraction studies

Bond length differences between HS and LS isomers have been determined for a number of iron(II), iron(III) and cobalt(II) complexes on the basis of multiple temperature X-ray diffraction structure studies [6]. The available results have been collected in Table 17. Average values for the bond length changes characteristic for a particular transition-metal ion have been extracted from these data and are obtained as AR 0.17 A for iron(II) complexes, AR 0.13 A for iron(III) complexes, and AR = 0.06 A for cobalt(II) complexes. These values may be compared with the differences of ionic radii between the HS and LS forms of iron(II), iron(III) and cobalt(II) which were estimated some time ago [184] as 0.16, 0.095, and 0.085 A, respectively. 

Bautista and Hard (B8) made a comparative study of the extractability of. several of the first-transition metals from thiocyanate solutions using methyl isobutyl ketone as the organic solvent. The transition metals readily extracted were scandium (III), iron (III), and cobalt (II) while chromium (II) and manganese (II) were not. The principal extractable species were found to be the neutral scandium and iron trithiocyanate complexes, while the extractable cobalt complex was the negatively charged tetrathiocyanate radial Co(SCN)4 . The distribution ratio for scandium, iron, and cobalt decreased with increase in metal ion concentrar tion but increased with increasing ionic strength of the solutions. 

One of the results of contamination of bleach with transition metals is the accumulation of oxygen in pipelines and storage tanks. This has become a safety issue for suppliers and utilities. Many utilities have revised their specifications for transition metal ions in bleach into the ppb range. This requires more sensitivity and precision in analytical methods. Pham [81 ] has studied a number of methods and reported results obtained by chloroform extraction colorimetry and ion chromatography. The latter is the preferred analytical method. 

It would be too long and too tedious to list all the facilitated ion-transfer reactions that have been reported over the years. From aUcali-metal ions to transition metal ions, most cations have been studied with different classes of ionophores ranging from the crown family with N, O, or S electron-donating atoms to caUx-arenes, not to mention all the commercial ionophores developed for ion-selective electrode applications or for solvent extraction. In the case of anions, the number of voltammetric studies reported has been much smaller [155-163], although the field of supramolecular chemistry for anion recognition is developing fast as recently reviewed [164]. 

Due to their relevance in solvent extraction and metal recovery, the facilitated transfer of transition metal ions has been investigated by the type of electrochemical methodology described above. This electrochemical approach has in fact proven to be a valuable tool for the study of the mechanism of solvent extraction processes. Wendt et al studied the transfer of Fe, Ni, and Zn assisted by bidentate nitrogen bases, such as phenanthroline and bipyridine. Similar studies were carried out by Wang et al and by Doe and Freiser, who also studied the transfer of cadmium assisted by diphenylthiocarbazone, a weak monobasic acid. Solomon et studied terpyridine as an extractant for transition metals. An interesting result from this type of work is... 

Molecular modelling of transition metal complexes (TMC), reproducing characteristic features of their stereochemistry and electronic structure, is in high demand in relation with studies and development of various processes of complex formation with an accent on ion extraction, ion exchange, isotope separation, neutralization of nuclear waste, and also when studying structure and reactivity of metal-containing enzymes. Solving these techno-... 

The ion exchange resin selected for this study was a copolymer of styrene with divinylben ne with a weakly basic iminodiacetic function group. The reason for this choice is that it has been studied in the extraction of transition metal and other ions (8-10), is commercially available, and is being used in industrial applications. At first it would appear that the prevalent complexation of iminodiacetic acid with most metallic elements would preclude the type of selectivity sought for the Sc extraction. But such separations are possible by the exploitation of specific chemical behavior and complexation characteristics. 

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