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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
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Figure 6.2 Some common metal chelates used for solvent extraction. Note that in all cases, the hydrogen ion of the parent chelating agent has been replaced by a metal.
Some other common chelating agents used in solvent extraction are shown in Figure 6.2. Of these, 8-hydroxyquinoline is particularly useful for extracting Al, Mg, Sr, V and W, diethyldithiocarbamates (e.g. the sodium derivative) for As(iii), Bi, Sb(m), Se(IV), Sn(IV), Te(IV), Tl(m) and V(v), and dithiozone for Ag, Bi, Cu, Hg, Pb, Pd, Pt and Zn.
6.3 Ion-Exchange
In this case, cation- or anion-exchange chromatographic techniques are commonly used. As their names suggest, cation-exchange is used to separate metal ions
Methods for Environmental Trace Analysis
(positively charged species), while anion-exchange is employed to separate negatively charged species. At first hand, it may seem that the only useful form of ion-exchange chromatography for metal separation/determination is cationexchange, but this is not always the case.
DQ 6.1
Can you suggest any cases where anion-exchange chromatography may be used in metal determination?
Some elements are found as their anions. For example, SO42 - can be used for sulfur determination, while both arsenite (AsO2 -) and arsenate (AsO43-) can be used for arsenic determination.
As an example, in the use of a strong cation-exchange resin, the following general equations can be written:
1. Metal ion (M"+) pre-concentrated on cation exchange resin
nRSO3-H+ + Mn + = (RSO3-)n Mn+ + H+
2. Desorption of metal ion using acid
(RSO3-)n Mn+ + H+ = n RSO3-H+ + Mn+
An alternative approach which allows the separation of an excess of alkali metal ions from other cations uses a chelating ion-exchange resin. This type of resin forms chelates with the metal ions. The most common of these is ‘Chelex-100’. This resin contains iminodiacetic acid functional groups which behave in a similar way to ethylenediaminetetraacetic acid (EDTA). It has been found that ‘Chelex-100’, in acetate buffer at pH 5-6, can retain Al, Bi, Cd, Co, Cu, Fe, Ni, Pb, Mn, Mo, Sc, Sn, Th, U, V, W, Zn and Y, plus various rare-earth metals, while at the same time it does not retain alkali metals (e.g. Li, Na, Rb and Cs), alkali-earth metals (Be, Ca, Mg, Sr and Ba) and anions (F-, Cl-, Br- and I-).
By using ion-exchange resins, a selected metal ion can be isolated (separated) and pre-concentrated from its matrix. This process can be carried out in two ways, i.e. by (i) batch, or (ii) column processes. In the batch process, the ion-exchange resin is added to the aqueous sample, whereas in the second process the resin is packed into a chromatographic column. The former would be always carried out off-line, while the latter could be carried out in either on-line or off-line mode.
6.4 Co-Precipitation
While other techniques, such as co-precipitation, can be used for pre-concentration they are not as common as those discussed above. Co-precipitation allows the quantitative precipitation of the metal ion of interest by the addition of a co-precipitant.
Liquids - Natural and Waste Waters 105
Co-precipitation of metal ions on collectors can be attributed to several mechanisms, as follows:
• adsorption - the charge on the surface can attract ions in solution of opposite charge
• occlusion - ions are embedded within the forming precipitate
• cocrystallization - the metal ion can become incorporated in the crystal structure of the precipitate
The major disadvantage of co-precipitation is that the precipitate, which is present at a high mass-to-analyte ratio, can be a major source of contamination. In addition, further sample preparation, e.g. dissolution, is required prior to analysis for the analyte. This can also increase the risk of contamination and analyte losses. One of the most common co-precipitants is iron, e.g. as Fe(OH)3.
6.5 Summary
The presence of trace metals in natural and waste waters can often cause a problem in terms of the selected analytical technique. In order to be able to quantify the concentration of trace metals in aqueous samples, appropriate methods of preconcentration therefore need to be selected. This present chapter has summarized the main methods available for such pre-concentration procedures.
1. Cresser, M. S., Solvent Extraction in Flame Spectroscopic Analysis, Butterworths, London, 1978.
2. Majors, R. E., LC-GC Int., 10, 93-101 (1997).
3. Kirkbright, G. F. and Sargent, M., Atomic Absorption and Fluorescence Spectroscopy, Academic Press, London, 1974.
Methods for Environmental Trace Analysis. John R. Dean
Copyright © 2003 John Wiley & Sons, Ltd.
ISBNs: 0-470-84421-3 (HB); 0-470-84422-1 (PB)
Sample Preparation for Organic Analysis
Methods for Environmental Trace Analysis. John R. Dean
Copyright © 2003 John Wiley & Sons, Ltd.
ISBNs: 0-470-84421-3 (HB); 0-470-84422-1 (PB)
Chapter 7
Learning Objectives
• To appreciate the different approaches available for the preparation of solid samples for organic analysis.
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