Metals organic extraction

The low solubility in the aqneons phase of metal organic extractants reduces the possible locations of the rate controlling reaction to (1) the diffnsional aqueous film and (2) the interfacial plane itself. Fignre 6.1 gives a representation of the concentration profiles corresponding to an interfacial reaction. A model that considers a reaction region of variable thickness has been proposed by Hnghes and Rod [13]. 

The choice between the use of solid-state supported extractants and solvent extraction is often made on the basis of the concentration of the desired metal in the aqueous feed. Solvent extraction is usually not effective for treating very dilute feeds because an impracticably large volume of the aqueous phase must be contacted with an organic extractant to achieve concentration of the materials across the circuit. However, solvent extraction is preferred for treating moderately concentrated feeds because most ion-exchange resins and related materials have relatively low metal capacities and very large quantities of resin are required. In this review we will focus on reagents used in solvent extraction because, in the main, the nature of the complexes formed are better understood. 

Armannsson [659] has described a procedure involving dithizone extraction and flame atomic absorption spectrometry for the determination of cadmium, zinc, lead, copper, nickel, cobalt, and silver in seawater. In this procedure 500 ml of seawater taken in a plastic container is exposed to a 1000 W mercury arc lamp for 5-15 h to break down metal organic complexes. The solution is adjusted to pH 8, and 10 ml of 0.2% dithizone in chloroform added. The 10 ml of chloroform is run off and after adjustment to pH 9.5 the aqueous phase is extracted with a further 10 ml of dithizone. The combined extracts are washed with 50 ml of dilute ammonia. To the organic phases is added 50 ml of 0.2 M-hydrochloric acid. The phases are separated and the aqueous portion washed with 5 ml of chloroform. The aqueous portion is evaporated to dryness and the residue dissolved in 5 ml of 2 M hydrochloric acid (solution A). Perchloric acid (3 ml) is added to the organic portion, evaporated to dryness, and a further 2 ml of 60% perchloric acid added to ensure that all organic matter has been... 

Bruland et al. [122] have shown that seawater samples collected by a variety of clean sampling techniques yielded consistent results for copper, cadmium, zinc, and nickel, which implies that representative uncontaminated samples were obtained. A dithiocarbamate extraction method coupled with atomic absorption spectrometry and flameless graphite furnace electrothermal atomisation is described which is essentially 100% quantitative for each of the four metals studied, has lower blanks and detection Emits, and yields better precision than previously published techniques. A more precise and accurate determination of these metals in seawater at their natural ng/1 concentration levels is therefore possible. Samples analysed by this procedure and by concentration on Chelex 100 showed similar results for cadmium and zinc. Both copper and nickel appeared to be inefficiently removed from seawater by Chelex 100. Comparison of the organic extraction results with other pertinent investigations showed excellent agreement. 

Class B Type MA, . Neutral coordinatively saturated complexes formed between the metal ion and a lipophihc organic acid. This class contains the large group of metal-organic chelate compounds. For monbasic acids forming bifunctional chelates, z = N/2. They belong to the extraction Type in-B, treated in section 4.8. 

This example illustrates a case of considerable analytical importance, especially for the determination of complex formation constants for hydrophilic complexes, as discussed in section 4.12, when the equilibrium constants for the stepwise metal-organic complexes are of secondary interest. values are tabulated in several reference works. is a conditional constant and only valid provided no other species are formed besides the extracted one. 

Solvent extraction has become a common technique for the determination of formation constants, P , of aqneons hydrophilic metal complexes of type MX , particularly in the case when the metal is only available in trace concentrations, as the distribntion can easily be measnred with radioactive techniques (see also section 4.15). The method reqnires the formation of an extractable complex of the metal ion, which, in the simplest and most commonly used case, is an nn-charged lipophilic complex of type MA. The metal-organic complex MA serves as a probe for the concentration of metal ions in the aqueous phase through its equilibrium with the free section 4.8.2. This same principle is used in the design of metal selective electrodes (see Chapter 15). Extractants typically used for this purpose are P-diketones like acetylacetone (HAA) or thenoyltrifluoroacteone (TTA), and weak large organic acids like dinonyl naph-talene sulphonic acid (DNNA). 

Few solubility parameters are available for the metal-organic complexes discussed in this chapter. Another approach is then necessary. The distribution constant for the reagent (extractant), R, can be expressed as ... 

In Chapters 2 and 3 several physicochemical factors of importance to solvent extraction have been described, and many of their effects have been illustrated in this chapter. Here, we summarize some observed regularities. The effect of various stractures on the bond strengths in metal organic complexes has been extensively treated in other publications [47-49]. 

The extractabilities of metal-organic complexes depend on whether inner or outer sphere complexes are formed. Case 1, section 4.2.1, the extraction of ura-nyl nitrate by TBP, is a good example. The free uranyl ion is surrounded by water of hydration, forming U02(H20)f, which from nitric acid solutions can be crystallized out as the salt U02(H20)6 (N03), though it commonly is written U02(N03)2(H20)6. Thus, in solution as well as in the solid salt, the UOf is surrounded by 6 HjO in an inner coordination sphere. In the solid nitrate salt, the distance du.o(nitrate) between the closest oxygen atoms of the nitrate anions, (0)2N0, and the U-atom is longer than the corresponding distance, du-o(water), to the water molecules, OH2, i.e., du.o(nitrate) > 4u.o(water) thus the nitrate anions are in an outer coordination sphere. 

Metal ion Extractant Complex in organic phase Extraction constant Ref. 

Tungsten carbonyl may be dissolved in an organic solvent and analyzed by GC/MS. The compound should form mass spectra corresponding to the masses for W(CO)6, CO and W. The compound may be decomposed thermally and product carbon monoxide transported with helium onto a GC column to be analyzed by GC-TCD or GC/MS. Residue tungsten metal is extracted with nitric acid-hydrofluoric acid, diluted with water, and analyzed (See Tungsten). 

Organic extractants can be used to complex metal ions and to increase lipophilic-ity. The traditional metal extractants l-(2-pyridylazo)naphthol (PAN) and l-(2-thia-zolyl)-2-naphthol (TAN) have been used in polymer-based aqueous biphasic systems [43] and traditional solvent extraction systems [44]. These are conventional metal extractants widely used in solvent extraction appHcations. When the aqueous phase is basic, both molecules are ionized, yet they quantitatively partition into [HMIM][PF6] over the pH range 1-13. The distribution ratios for Fe, Co, and Cd (Figure 3.3-4) show that the coordinating and complexing abihties of the extractants are dependent on pH and that metal ions can be extracted from the... 

Given all these attractive features, it is easy to understand the increasing interest in the application of ILs in solvent extraction. The review gives an introduction into the rapidly growing area it focuses on the extraction of organic compounds, metal ion extraction being considered in Chapter 10 of this book. 

Metabolism studies in dogs and rats with radiolabeled tetracycline showed that with the exception of metal-chelate formation, tetracycline was chemically unaltered by the rat (247). Organ extracts from dosed animals were not found to contain metabolic products of tetracycline. Dog urine also contained unchanged drug, indicating that metabolic transformation of tetracycline had not occurred. 

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