Transition metal cation separations

Transition Metal Cation Separations by Organophosphorus Compounds in Liquid Membrane Processes... 

WALKOWIAK GEGA Transition Metal Cation Separations... 

Separation is based on the reversible chelate-complex formation between the chiral selector covalently bonded to the chromatographic support, and the chiral solute with transition metal cations. Chelation properties of both the chiral selector and the chiral solute are required. Compounds therefore need to have two polar functional groups in a favorable arrangement to each other, like a )3-amino acids, amino alcohols and a-hydroxy acids, which can form rings membered with central chelating metal ions, like Cu(II), Zn(II), Cyclic... 

Pranfois, C., Morin, P., and Dreux, M. (1995). Separation of transition metal cations by capillary electrophoresis, optimization of complexing agent concentrations (lactic acid and 18-crown-6).. Chromatogr. A 717, 393—408. 

Figure 9.15. Separation of nine transition metal cations by ion chromatography. Reprinted by permission of Dionex Corporation, Sunnyvale, CA.

For a solid-state chemist, it is possible to synthesize compounds with polyhedra containing transition-metal ions separated by diamagnetic elements. By cationic substitution, the following arrangements can be obtained ... 

Cation exchanged zeolites are successfully applied as catalysts or selective sorbents in separation technologies. " For both catalytic and sorption processes a concerted action of polarizing cations and basic oxygen atoms is important. In addition, transition metal cation embedded in zeolites exhibit peculiar redox properties because of the lower coordination in zeolite cavities compared to other supports." " Therefore, it is important to establish the strength and properties of active centers and their positions in the zeolite structure. Various experimental methods and simulation techniques have been applied to study the positions of cations in the zeolite framework and the interaction of the cations with guest molecules.Here, some of the most recent theoretical studies of cation exchanged zeolites are summarized. 

The structure and distorted variants of it are also found for a number of mixed transition metal oxides. In the trirutile structure, the tetragonal c axis of the simple unit cell is tripled and the titanium sites are no longer equivalent. The mineral tapiolite, FeTa206, has this structure in which the Fe and Ta occupy separate crystallographic positions. The ferrous ion can be replaced by Mg+ and other divalent transition metal cations such as Co+ and Ni+. Antimonates of the form MSb206 (M+ = Mg, Fe, Co, Ni, Zn) also form with the trirutile structure. Examples are known in which the transition metals are mixed on the two sites. This occurs for WCr206, which might more correctly be formulated Cr(Cro.5 Wo.5)206. 

The autunite-type compounds with Mg or divalent transition metal cations (Mn, Fe, Co, Ni, Cu, Zn, Cd) in their interlayers are listed in Table 14 (phosphates) and Table 15 (arsenates). In the known structures of these compounds, the divalent interlayer cations are all in sixfold coordination appearing as nearregular octahedra with the exception of Cu, which shows a typically Jahn-Teller distorted sixfold coordination environment appearing as tetragonal dipyramids, and whose structures will be discussed separately. Three states of hydration are observed among these (Mn, Fe, Co, Ni) structures the triclinic dodecahydrates, the monoclinic decahydrates and the triclinic octahydrates [136, 138]. With progressive dehydration, the interlayer spacing decreases, the sheets shift in relative position, and the interlayer octahedra shift positions relative to the sheets (Fig. 34). 

Sodium diethyldithiocarbamate (DDTC) was used to separate transition metal cation with a CTAB micellar phase and a CIS column [32]. The limits of detection obtained with atomic absorption spectroscopy were in the tens of picograms injected. Since a high concentration of i-propanol (45% v/v) was added to the 0.03 M CTAB mobile phase, the presence of micelles may be discussed. Simple ion-pairing between CTAB and the DDTC metal species may explain the observed selectivity. Tartaric acid was also used as a ligand for transition metal cations with a SDS micellar mobile phase and a CIS column [33]. 

Selectivity of transition metal cations is not as straightforward as for the alkali and alkaline earth metals. Metal ions which have different valencies, or which have the same valency but are in different rows, can be separated by cation exchange. However, many transition metal cations have the same valency and therefore cannot be separated by cation exchange. One common solution to increase selectivity among transition metal cations is to form coordination complexes which form strong ionic species. In aqueous solutions some transition metals can hydrolize to form coordination complexes in which water is covalently bound. The water can be replaced by an ion or molecule which can donate one or more electron pairs to the metal. Such molecules are referred to as ligands and are classified by how many electron pairs they donate. Several anionic chelating molecules are shown in Table 5 which coordinate with many of the transition metal cations to form coordination complexes. 

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