Examples of Metal-Ion Separation

Reiffenstuhl and Jandik were able to separate a number of divalent metal ions effectively in a single run using an IDA resin [17]. A silica based material (Nucleosil 300-7 of 7 pm diameter. 300 A average pore size) was derivatized with y-glycidoxypropyltrimethoxy silane, then iminodiacetic acid was covalently coupled to the epoxy activated surface. The final material was slurry packed into a 100 x 4.6 mm stainless steel column. A complexing eluent containing 10 mM citric acid plus 

04 mM 2,6-pyridine dicarboxylic acid (PAD) gave a good separation of low ppm concentrations of Mg +, Fe +, Co, Cd and Zn using conductivity detection. Traces of Co, Zn + and Cd + were concentrated and separated with 10 mM tartaric acid at pH 2.54. 

In another example a silica gel-based sorbent with chemically bonded amidoxime groups was used for chromatographic separation of transition and heavy metals [18]. The resin known as Amidoxim was obtained from Elsik, Russian Federation and had the structure  

Separation of five transition metals is shovm in Fig. 7.15 using 5 mM sodium at pH 

6 as the eluent. Post-column detection was employed with 0.5 mM 4-(2-pyridylazo) resorcinol (PAR) in 3 M ammonia and 1 M acetic add as the color-forming reagent. 

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Let us consider the selectivity parameter on the example of metal ions separation. According to the transport model equations the selectivity of two solutes, for example, two metal species, Sm,/M2> is determined by relation ... 

In this chapter selected examples of metal ion separations with polymeric macrocycles such as crown ethers, calixarenes, resorcinarenes, calixcrowns and cyclodextrins, reported in recent literature, are presented. Particularly, the use of those polymers in separation processes such as ion flotation, solvent extraction as well as transport across liquid and polymer membranes is shown. First, selected examples of crown ether polymers variety cross linked as metal ion carriers are described, then selectivity species by donor sites bonding and coordination are characterized. 

Some of the more obvious sources of contamination of solvents arise from storage in metal drums and plastic containers, and from contact with grease and screw caps. Many solvents contain water. Others have traces of acidic materials such as hydrochloric acid in chloroform. In both cases this leads to corrosion of the drum and contamination of the solvent by traces of metal ions, especially Fe. Grease, for example on stopcocks of separating funnels and other apparatus, e.g. greased ground joints, is also likely to contaminate solvents during extractions and chemical manipulation. 

Table 5.8 gives an indication of the range of elements that may be determined. Most procedures will require an analyte concentration of 10-3 mol dm 3 or more, although with special conditions, notably potentiometric end-point detection, the sensitivity may be extended to 1(H mol dm 3. The analysis of mixtures of metal ions necessitates masking and demasking, pH adjustments and selective separation procedures. Areas of application are spread throughout the chemical field from water treatment and the analysis of refined food and petroleum products to the assay of minerals and alloys. Table 5.10 gives some selected examples. 

Competing amines such as triethylamine and di-rc-butylamine have been added to the mobile phase in reversed-phase separations of basic compounds. Acetic acid can serve a similar purpose for acidic compounds. These modifiers, by competing with the analyte for residual active sites, cause retention time and peak tailing to be reduced. Other examples are the addition of silver ions to separate geometric isomers and the inclusion of metal ions with chelating agents to separate racemic mixtures. 

Let us continue with the example of copper ions in contact with copper metal and zinc ions in contact with zinc metal. This combination is usually referred to as the Darnell cell or zinc/copper couple(Fig. 6.5a). For this electrochemical cell the reduction and oxidation processes responsible for the overall reaction are separated in space one half reaction taking place in one electrode compartment and the other takes place in the other compartment. 

Two examples where metal ions confer stability or increased activity in proteins are human deoxyribonuclease (rhDNase, Pulmozyme ), and Factor VIII. In the case of rhDNase, Ca2+ ions (up to 100 mM) increased the stability of the enzyme through a specific binding site (64). In fact, the removal of calcium ions from the solution with EGTA caused an increase in deamidation and aggregation. However, this effect was observed only with Ca+2 ions other divalent cations, Mg2+, Mn2+, and Zn2+, were observed to destabilize rhDNase. Similar effects were observed in Factor VIII. Ca2+ and Sr2+ ions stabilized the protein, whereas others such as Mg2+, Mn2+ and Zn2+, Cu2+, and Fe2+ destabilized the enzyme (65). In a separate study with Factor VIII, a significant increase in the aggregation rate was observed in the presence of Al3+ ions (66). The authors note that other excipients like buffer salts are often contaminated with Al3+ ions and illustrate the need to use excipients of appropriate quality in formulated products. 

The use of metal ions as kinetic synthetic templates is extremely widespread, and is an excellent way in which to bring about the organisation of a number of reacting components in order to direct the geometry of the product. Because some metal ions, such as the transition metals, often have preferred coordination geometries (e.g. tetrahedral, square planar, octahedral etc), changes in metal ion may have a profound effect on the nature of the templated product. Metal-ion-templated syntheses may be classified more generally as examples of self-assembly with covalent postmodification. For example, the synthesis of the artificial siderophore 10.2 is effected by the use of an octahedral Fe3+ template.8 In this case, the macrobicyclic product is obtained as the Fe3+ complex from which it is difficult to separate. 

More importantly, the use of heavy metal anionic micellar media has been shown to allow for observation of analytically useful room-temperature liquid phosphorescence (RTLP) (7.484.487). There are several examples in which phosphorescence has been employed as a LC detector with the required micellar assembly being present as part of the LC mobile phase (482) or added post column (485). More recently, metal ions have been determined in a coacervate scum by utilizing the micellar-stabilized RTLP approach (498). Thus, the future should see further development in RTLP detection of metal ions in separation science applications. 

Examples of the use of synthetic mixed donor macrocycles in heavy metal ion separations are found in the discrimination of silver from lead. A number of studies indicate that the inclusion of sulfur in macrocyclic sequestering agents shifts the discrimination to silver. An example of this is seen with (57) and (58). For the aza-oxa macrocycle (57) the log K is 5.9 for both silver and lead ions, while the thia-incorporated ligand (58) complexes silver more efficiently (log K = 9.9) compared to lead (log K = 5.7). ... 

Mixtures of metal ions in aqueous solution are often separated by selective precipitation—that is, by using a reagent whose anion forms a precipitate with only one of the metal ions in the mixture. For example, suppose we have a solution containing both Ba2+ and Ag+ ions. If NaCl is added to the solution, AgCl precipitates as a white solid but since BaCl2 is soluble, the Ba2+ ions remain in solution. 

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