Acetylacetone, solvent extraction metals

If a neutral chelate formed from a ligand such as acetylacetone is sufficiently soluble in water not to precipitate, it may stiH be extracted into an immiscible solvent and thus separated from the other constituents of the water phase. Metal recovery processes (see Mineral recovery and processing), such as from dilute leach dump Hquors, and analytical procedures are based on this phase-transfer process, as with precipitation. Solvent extraction theory and many separation systems have been reviewed (42). 

Other fluorinated derivatives of acetylacetone are trifluoroacetylacetone (CF3COCH2COCH3) and hexafluoroacetylacetone (CF3COCH2COCF3), which form stable volatile chelates with aluminium, beryllium, chromium(III) and a number of other metal ions. These reagents have consequently been used for the solvent extraction of such metal ions, with subsequent separation and analysis by gas chromatography [see Section 9.2(2)]. 

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). 

The solvent extraction of metal-chelate complexes for trace analysis of water samples has been briefly reviewed (473). Concentration factors of less than 1000 are to be expected because of the need to handle reasonable quantities of organic solvent. However, since not all of the organic solvent sample is used in the analysis, this results in a less than optimal use of the system. An excellent batch-extraction apparatus recently has been described (474) for the extraction of organic substances from water. Concentration factors of more than 10,000 were easily obtained if organic volumes of 100 /w, or less, which were used to extract 1 liter of solution, could be fully analyzed (eg., by AAS). A useful system has been described (475) which combines three chelating agents (dithizone, 8-hydroxyquinoline, and acetylacetone) and quantitatively collects Al, Be, Cd, Co, Cu, Fe, Pb, Ni, Ag, and Zn in one extraction at a pH of 6. 

Group IIB. As, Sb, and Sn The separation of a mixture of the three elements is a difficult operation. The metals are present as their lower chlorides in dilute (2-4m) hydrochloric acid. The solution is spotted on paper and allowed to dry in air for 15 minutes. The solvent consists of 7-5 ml acetylacetone (b.p. 137°-141°) saturated with water and treated with 0 05 ml concentrated hydrochloric acid and 2-5 ml acetone (sufficient of the last-named to give a clear solution). The separation is allowed to proceed for 1 hour in an atmosphere saturated with respect to a saturated solution of acetylacetone in water the solvent movement is about 15 cm. The complexes formed are very stable, particularly that with tin (Rf = 1). The strip is removed from the extraction vessel, the solvent is allowed to evaporate for several minutes, and the strip is sprayed (before it is completely dry) with a chloroform solution of dithizone (0-005 per cent w/v), and then allowed to dry thoroughly. The tin is found in the solvent front. 

An = Th, U, Np, and Pu. In complexing with metal ions, the / -diketones form planar six-member chelate rings with elimination of the enol proton. The simpler / -diketones, such as acetylacetone (HAA), are fairly water soluble, but form complexes that may be soluble in organic solvents. This is especially true for the An ions which form strong complexes with HAA and can be effectively sequestered to the organic phase, making HAA a potentially useful extractant (See Table 27). The four stability constants in Table 27 for tetravalent actinides imply that four HAA ligands coordinate with each metal ion in the formation of the extracted neutral ML4 complexes. ... 

The extraction of chelates is usually applied to preconcentration and separation of small amounts of metals. Owing to their low solubility in organic solvents, most chelates can not be used for the extraction of macrocomponents. Cupferronates and acetylacetonates are exceptions. 

The pH value for which 50% of metal as its chelate is extracted, i.e., D = 1, and for [H2L] = 1, is equal to log Kex)/n- This is termed pH 1/2, and has a value characteristic for the particular reagent, solvent, and the metal ion being determined. For example, in the case of metal extraction in the form of dithizonates using carbon tetrachloride as a solvent, the pHj/2 values for Pb(II), Zn(II), In(III), and Bi(III) are 8.0, 5.0, 3.8, and 2.0, respectively. Similar considerations apply to other chelating reagents as, e.g., 8-hydro-xyquinoline, acetylacetone, and thenoyltrifluoroace-tone. Such relationships are often presented in graphical form (Figure 9). 

Aoetylacetone can be used as both solvent and complexlng agent In extraction of numerous metal Ions. Lead extracts completely at pH 7.4 (F1). Lead has been separated from copper, uranium and bismuth by extraction at pH 7.4 (K9) from chloride solutions. Copper and uranlvim are both extracted at pH 2.3-5 and bismuth comes out of solution as the oxychloride above pH 1 and does not extract. Extraction of lead acetylacetonate Into chloroform or butyl acetate Is Incomplete and Is not useful for lead separations (t4). 

This compound is solid at ordinary temperature (m.p. 43°C) and cannot, therefore, be used as a pure extract. For all solvents investigated, however, the distribution constants are much higher than those found in the case of acetylacetone (see Table 21.18). On account of the low solubility in water of both HTTA and the metal chelates, complex formation in water is incompletely known, and anyhow of limited interest. The extraction equilibria are, on the other hand, very important and will be discussed more fully in Section 21.5.2. Other /J-diketones used for the extraction of actinides are benzoylacetone, PhC(0)CH2C(0)CH3, and dibenzoylmethane, these have properties between those of HAA and HTTA [134] (cf. also Table 21.18). 

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