Chelate extraction systems

2 Kinetic Extraction Mechanism and Interfacial Reactions 2.5.2.1 Chelate Extraction Systems 

The extraction systan that was measured by the HSS method for the first time was the extraction kinetics of Ni(II) and Zn(II) with n-alkyl substituted dithizone (HL) [28]. The observed extraction rate constants linearly depended on both concentrations of the metal ion [M +] and the dissociated form of the ligand [L ]. This seemed to suggest that the rate-determining reaction was the aqueous phase complexation that formed 1 1 complex. However, the observed extraction rate constant k was not 

When the extraction kinetics is governed by the aqueous phase reaction 

the extraction rate constant k is related to k, and a dissociation constant 

FIGURE 2.22 Dependence of the extraction rate constants, of Zn(II) and Ni(II) on the number of carbon atoms in the substitution of n-alkyl dithizones. The solid lines trace the observed values and the broken lines the values predicted from the aqueous mechanism. 

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From these results, one can understand that the liquid-liquid interface can assist effectively in the interfacial reaction through the adsorption of extractants like a solid catalyst. The whole extraction scheme of the chelate extraction system is represented in Scheme 1. 

When an auxiliary ligand is added to a chelate extraction system and it enhances the extractability and extraction rates, such a phenomenon is termed synergism. A typical... 

Chelate extraction systems have also had very extensive application in rapid radiochemical separations. 

The carboxylate extraction system has been recognized as more advantageous than the chelate extraction system, since one can deal with more concentrated metal solutions in the former than in the latter. Thus the application of the carboxylate extraction to hydrometallurgy has been attempted, and in this connection extensive studies have been carried out in the Soviet Union and the United Kingdom. 

As stated earlier the polymeric species are often involved in the extraction of metal carboxylates. Therefore, the extraction equilibrium is sometimes more complicated than in the chelate extraction system. As is evident from the following treatment, it is advantageous and often indispensable to study the total metal concentration in the organic phase [Eq. (8)] instead of the conventionally utilized distribution ratio of the metal [Eq. (7)]. 

The plot according to Eq. (10) is of general use in studies not only on the carboxylate extraction system but also on the chelate extraction system. For instance, vanadium (V) 8-quinolinolate was found to be extracted as an oxo-bridged dimeric form (158), whose structure was later established by X-ray crystallography (157). 

Other authors propose the APDC/MIBK chelation/extraction system for the vanadium determination in urine by GFAAS (Buratti et al., 1985 White et al., 1987). A detection limit of 0.4 /[Pg.534]

Kinetics of Extraction. Kinetic factors may be important in all types of extraction, but they are most frequently observed with chelate-extraction systems. Extraction equilibrium can usually be achieved in one or two minutes of shaking because mass-transfer rates are reasonably rapid however, the formation of an extractable complex... 

For chelate extraction systems involving additional ligands characterized by m"" + nM + aS J MA aS + nH" ... 

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