The Extraction Equilibrium

Usually the description of the phase transfer behavior is presented in a simplified form based on the overall extraction equilibrium. This is illustrated schematically for a simple ion-pair extraction of a metal ion by a neutral macrocyclic extractant in Fig. 4.4. 

The related extraction equilibrium is represented by Eq. (4.2) and the accompanying extraction constant Kex by Eq. (4.3). The three individual equilibria for complex formation and partitioning of ligand and metal complex are given by  

Equation (4.3) shows a direct proportionality of Kex with the stability constant Kmi + and the distribution constant of the extracted complex KmlsA In contrast the distribution constant of the ligand possesses a reverse dependence on Kex. 

Provided constant ionic strength of both phases is maintained (adjusted by salt or acid addition to the aqueous phase and ligand excess in the organic phase) the extraction constant can be formulated as given by Eq. (4.4)  

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The present description pertaining to copper refers to solvent extraction of copper at the Bluebird Mine, Miami. When the plant became operational in the first quarter of 1968 it used L1X 64, but L1X 64N was introduced in to its operation from late 1968. The ore consists of the oxidized minerals, chrysocolla and lesser amounts of azurite and malachite. A heap leaching process is adopted for this copper resource. Heap-leached copper solution is subjected to solvent extraction operation, the extractant being a solution of 7-8% L1X 64N incorporated in kerosene diluent. The extraction process flowsheet is shown in Figure 5.20. The extraction equilibrium diagram portrayed in Figure 5.21 (A) shows the condi-... 

The extraction equilibrium in the CUPREX process is dependent on the activity of CU in the feed solutions which in turn is dependent on the stability of chlorometallate complexes [MCIJ2 and [MC1X]3 X for the di- and tri-valent metals present149,150, rather than the simple stoichiometry represented by the equation shown in Figure 6.151,152... 

This reaction is suppressed by using a large excess of Cl ion in the stripping phase. Furthermore, the extraction equilibrium constant of the above reaction is much lower than that of reaction given by Eq. (4). The overall distribution coefficient of CPC, mg (defined as Cqp/Cp) is given by [50]... 

Conversely, it was shown that the composition of the polar core depends only on the polar heads of the malonamide and on the initial salt concentration in the aqueous phase in equilibrium. The extraction efficiencies of HN03 and H20 are independent of the diluent and extractant chain lengths (34). Diluent-extractant interactions and variations in the amphiphilic balance of the extractant thus do not influence the extraction equilibrium of HN03 and H20. Here again, this conclusion is consistent with published results for other extractant systems. 

The prevailing extraction-chromatographic method for 90Sr separation entails the use of Sr-Resin.116"119 This material is prepared by impregnating a porous polymer with a 1-octanol solution of the crown ether 4,4 (5 )-bis(t-butylcyclohexano)-18-crown-6 (DtBuCH18C6). This material will extract and retain Sr from 2-8 M HN03 solutions, while most of the matrix constituents are not retained and are removed with a column wash. The separated Sr is released by elution with water or weak nitric acid. The extraction equilibrium is shown in Equation 9.2, where the bar above a species indicates that it is immobilized on the resin.4... 

A typical kinetic synergism in the extraction of metal ions was reported for the Ni(II)-dithizone(Hdz)-phen chloroform system [47]. The extraction equilibrium constant (log Xex) was enhanced from —0.7 to 5.3 upon the addition of phen ... 

In these complicated extraction systems containing the polymeric and/or hydroxo species, one would expect the solvent used as a diluent to exert a considerable effect on the extraction equilibrium. In the extraction of gallium (III) with decanoic acid it has been found that the less polar the solvent, the more polymerized the extracted species (150). More recently, the solvent effect on the extraction (156) and dimerization (151, 153) of copper(II) decanoate has been interpreted according to regular solution theory (141,142). 

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)]. 

When a y-merized metal carboxylate of the composition MjAaHA (nj = a — h) is responsible for the extraction of a metal ion MB + with a carboxylic acid HA, the extraction equilibrium is written as ... 

In most inert solvents the extraction equilibrium of copper(II) is given by Eq. (15), with the extraction constant formulated as Eq. (16). 

This manifold has been used for the USALLE of paracetamol from suppositories [17]. Hydrolysis of the analyte prior to reaction with o-cresol in the alkaline extractant medium was also favoured by US (the entire sample plug was irradiated in EC). Hydrolysis and formation of the reaction product displaced the extraction equilibrium, thus favouring extraction into the aqueous phase. The influence of the variables related to the dynamic manifold (namely, flow rate and sample volume), chemical variables (namely, NaOH and o-cresol concentrations) and temperature was studied using the univariate method on account of their independence on the other hand, those related to US (namely, probe position, radiation amplitude and pulse duration) were the subject of a multivariate study in which the latter two exhibited an insignificant but positive effect. Positioning the probe closest to the extraction coil was found to maximize extraction efficiency. The positive effect of US on extraction and analyte hydrolysis provides the overall enhancement shown in Fig. 6.4A, which shows the results obtained in the presence and absence of US. The time required for the development of the method was significantly shorter than that required by the United States Pharmacopoeia (USP) method. In addition, the latter produces emulsions that need about 30 min for phase separation after extraction. 

In perchlorate media, HTTA extracts Th according to the extraction equilibrium equation (Equation (22)).This arguably makes HTTA a potentially useful extractant for Th by itself2 ... 

In many cases, synergists are added to HTTA extraction systems to enhance the separation of actinide ions. One example is the addition of the crown ethers (CE) dibenzo-18-crown-6, dicyclo-hexyl-18-crown-6, dibenzyl 24-crown-8, and benzyl-15-crown-5. These crown ethers have been shown to synergistically enhance extraction into benzene and the increase follows Eu > 1102 " > Th". The extraction equilibrium for crown ether/HTTA systems for the separation of Th is shown in Equation (23). The binding of the crown ether in the extracted complex seems to be a function of crown ether basicity and steric effects2 ... 

The preference of the SIPE for the trivalent actinides is due to the stabilization of the crown ether/ bonds due to a size-fitting effect where the diameter of the metal ion matches that of the crown ether cavity, leading to significantly greater extraction efficiency due to this synergistic effect. Another widely studied /3-diketone for the extraction of trivalent actinides has been l-phenyl-3-methyl-4-benzoylpyrazolone-5 (HPMBP), illustrated in Figure 93. The extraction of Am , Cm , Bk , and Cf by HPMBP may be expressed in two ways. First, the extraction of Cm has been observed to follow the extraction equilibrium shown in Equation (67) ... 

Trioctylphosphine oxide (TOPO) is an organophos-phorus compound and is a stable and inexpensive extractant. TOPO, as we have shown [3-5], is unique in that it can function as a monodentate ligand and as a cation for ion-pair extraction when protonated and the extraction equilibrium for the neutral ligand is shown in Eq. (5), indicating the extraction of the neutral complex MCl2(TOPO)2 [M = Pd(II), Pt(II) n = 2-4] ... 

In order to verify the differences observed in 4f-5f complex stability, we used an additional solvent extraction method based on the competition between a soluble organic chelatant (TTA] and the aqueous soluble azide ions for binding the metal ions. The extraction equilibrium is shown by equation (2] and the distribution coefficients CD] of the metal at constant pH are correlated with the formation constants by equation (3]. 

As a rule, the rates of ion-exchange reactions, or rates of complex formation and destruction on the interfaces or within the IBM, are fast compared to diffusion rates. Thus, corresponding concentration gradients of the counterions are related through the extraction equilibrium constants [46, 53, 56]. The averaged sums of the IBM and LMF potentials can be experimentally realized through distribution coefficients at membrane-based solvent forward and backward extraction Ep = Mfl/Mei on the feed side and Ep = Me/M i on the strip side (see Fig. 6.2B). 

Next, Qi, the raffinate product point is plotted on the raffinate equilibrium curve at Xq, = 0.12, as specified. The extract product point is located by joining Q, to M and extending the line to its intersection with the extract equilibrium curve at The product compositions are read from the diagram ... 

The extract composition on stage 2 L2) is found from the intersection of the operating line through (X j, X,J and (X q, X j,), and the extract equilibrium curve. 

The relatively large valons found for the extraction equilibrium constant of copper with Kelex 100 (3 and 90) indicate (hat shipping of copper from (his rcngenl should he difficult. It is fonnd, however, that copper does strip reedily into sulfuric acid solutions because Kalex 100 reacts with sulhiric acid in preference to copper. Fitting the extinction of sulforic acid by Kelex 100 by a chemical-reaction equilibrium constant. 

Pertechnetate is efficiently extracted from aqueous solutions by rhodamine-B hydrochloride (RH Cl ) in nitrobenzene. At pH 4.7 and 28 "C an extraction coefficient of 7.96-10 was reported when 10 ml of the organic phase contained 5 mg of RH CL. The extraction equilibrium was reached within 2 min and the separation of the two phases was rapid. The extraction process may be formulated as... 

The absorption and IR spectra in both the aqueous and the organic phase were consistent with the extraction equilibrium (A.67). There is no quantitative information on the equilibrium constant for Reaction (A.67) or for the complex formation reactions in the aqueous phase. 

The extraction equilibrium is therefore dictated by the concentration of the neutral salt, ML2, in the aqueous phase. That concentration can be calculated from the known initial concentrations of the metal salt and of the ligand and from the stepwise stability constants of ail metal complexes present, by using Bjerrum s method [6]. 

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