Extraction Chemistry and Reagents

The extraction of a metal ion from an aqueous solution into an organic solvent is accomplished by the chemical formation of an uncharged species that is soluble in the organic phase. Because metal salts usually 

FIGURE 8.1-1 Solvent extraction portion of the Kennecott copper-nickel carbonate process for deep-sea manganese nodules. Adapted from U.S. Patent 3,907,966 in Ref. 11. 

The extractants that are used to form oiganic-soluble metal species from aqueous metal salts usually are grouped into three classes according to the type of reaction that occurs. Solvating extractants compete with water in coordination bonds around a metal ion and cany neutral salts into the organic phase. Cation 

I Return chloride or sulfate electrolyte adjusted to 3 iVin H  

FIGURE 8.1 2 Deepsea Ventures solvent extraction flowsheet for separation of metals in a manganese nodule leach liquor. From Ref. 11, with permission. 

Extractant Trade Name Chemical Stmctuie Molecular Weight 

TABLE S.2 1 Examples of Some Common Metal Extractants  

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Prechromatographic derivatization reactions re usually favored when it is desired to modify the properties of the sample to enhance stability during measurement (i.e., minimize oxidative and catalytic degradation, etc.), to improve the extraction efficiency of the substance during sample cleanup, to improve the chromatographic resolution, or to simplify the optimization of the reaction conditions [698-702]. As both pre- and postchromatographic methods enhance the sensitivity and selectivity of the detection process a choice between the two methods will usually depend on the chemistry involved, ease of optimization, and which method best overcomes matrix and reagent interferences. 

In addition to the presence of these elements in ores, they are also available from recycled feeds, such as catalyst wastes, and as an intermediate bulk palladium platinum product from some refineries. The processes that have been devised to separate these elements rely on two general routes selective extraction with different reagents or coextraction of the elements followed by selective stripping. To understand these alternatives, it is necessary to consider the basic solution chemistry of these elements. The two common oxidation states and stereochemistries are square planar palladium(II) and octahedral platinum(IV). Of these, palladium(II) has the faster substitution kinetics, with platinum(IV) virtually inert. However even for palladium, substitution is much slower than for the base metals so long as contact times are required to achieve extraction equilibrium. 

Oximes, hydroxamic acids and related species are often used as reagents in inorganic analytical chemistry for precipitation, gravimetric and volumetric determinations as well as for preconcentration, extraction, derivatizations and subsequent determination of analyte using instrumental techniques. A brief review of analytical chemistry in general and of these species in particular follows. 

Practically motivated, the aim was to develop methods for recovery and determination of amino acids in the context of analytical chemistry and biotechnology. Amino acids are hydrophilic compounds, which therefore are difficult targets for conventional solvent extraction. Extraction to an organic solvent may be enhanced by the addition of lipophilic cationic or anionic extractants, forming extractable complexes with amino acids, or by the use of macrocyclic compounds, which form stable hydrophobic host-guest complexes. The most popular reagents from the latter group are crown... 

The exigencies of research current around that time in the chemistry and separation of radionuclides led Calvin and his associates (1950) to introduce trifluorothenoylacetone (TTA), a reagent systematically and specifically designed for the solvent extraction of highly charged cations which had to be carried out from acidic solutions in order to avoid hydrolysis. 

Whilst such a classification [1, 9] is useful because it indicates the different types of chemical changes which can occur at the metal centres during phase transfer, it oversimplifies the situation in many cases and fails to indicate the importance of the outer sphere coordination chemistry and of solvation effects in general on the free energies of extraction. Alternative classifications of extraction processes which take better account of these have been presented recently by the Moyer Group. [22] The importance of such supramolecular effects in the design of reagents will be stressed in the examples below. 

The extraction chemistry of the more important reagents will be examined, and their characteristics compared, in the following section. 

In the field of calibration the concept of an interferent is very important.1 A useful terminology is as follows an analyte (sometimes also called analyte of interest) is a compound in the sample for which quantification is needed, and an interferent is another compound in the sample that gives a contribution to the instrumental response but for which quantification is not needed. For a zeroth order instrument an interferent makes calibration impossible. The traditional way of dealing with interferents in analytical chemistry is to pretreat the sample such that only the analyte contributes to the signal of the zeroth order instrument. Extraction, separation, and or selective reagents are often used for this purpose. 

An unavoidable by-product of the Swem reaction is the volatile dimethylsulphide which, on account of its unpleasant smell, is a reagent regulated by offensive odour control laws. This makes large scale chemistry problematic, especially in industry. To overcome this, several methods exist to perform the Swem oxidation under odourless conditions. For example, Node et al. outline a protocol for the Swem oxidation which uses dodecyl methyl sulfoxide in place of methyl sulfoxide,12 while Crich and co-workers have developed a fluorous Swem oxidation reaction that uses tridecafluorooctylmethyl sulfoxide 17,l3a,b This reagent can be recovered via a continuous fluorous extraction procedure and recycled by reoxidation with hydrogen peroxide. Additionally, the fluorous DMSO is crystalline, odourless and soluble in CH2CI2 to —45 °C. 

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