Extraction equilibria equilibrium constant data

Stary, J. Freiser, H., Compilers, Part IV "Chelating Extractants" in "Equilibrium Constants of Liquid-Liquid Distribution Reactions" IUPAC Chemical Data Series No. 

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extract-able complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. 

Table 7. Equilibrium constant data for the extraction of U(VI) by mixtures of HTTA and neutral donors...

The hexavalent state of neptunium and plutonium are identical to that of U(VI), in the sense that both exist as M02+ in aqueous solutions. Whereas U(VI) is quite stable in aqueous solutions Np(VI) and Pu(VI) are unstable, especially when in contact with organic solutions, and need to be stabilized by using holding oxidants. Only a few studies have been reported for these systems. In the extraction with HTTA-B mixtures, the species M02(TTA)2 B was reported6, to be responsible for the observed synergism with both these ions, just like U(VI), and the equilibrium constant data are included in Table 7. It appears that the stabilities of the M02(TTA)2 B adducts of U(VI), Np(VI) and Pu(VI) are of, more or less, similar magnitude. 

The extraction of some trivalent actinide ions by HPMBP-B mixtures was reported143"145. The extraction constants obtained for different diluents143 are given in Table 19. The constants were shown to decrease with increasing dielectric constant of the diluent. An additional mixed adduct, M(N03)(PMBP)2 2 TOPO participation was also considered when the neutral donor used was TOPO. The involvement of two ternary adducts in these systems was reported in the subsequent work144,145 and the equilibrium constant data are given in Table 20. 

Another approach that has been used to extract rate constants from ultrasonic relaxation data involves using independently determined equilibrium constants to determine concentrations of each species at equilibrium163 or, for special cases where molecule-specific electrodes exist, direct determination of equilibrium concentrations.167,168... 

Equilibrium constants involving each compound were evaluated using the partial pressures by the third law method. Accepting the heats of formation of WF5 and WF obtained from bomb calorimetry, the values for WF (n = 1 to 4) could be extracted by iterative fitting to partial pressure data. The W/02/F2 and W/S/F2 systems were also examined to give heats of formation of tungsten oxo- and thiofluorides. This experimentally simple technique yields thermodynamic data on high-temperature species inaccessible to conventional calorimetry. 

From plots of the distribution ratio against the variables of the system— [M], pH, [HA] , [B], etc.—an indication of the species involved in the solvent extraction process can be obtained from a comparison with the extraction curves presented in this chapter see Fig. 4.3. Sometimes this may not be sufficient, and some additional methods are required for identifying the species in solvent extraction. These and a summary of various methods for calculating equilibrium constants from the experimental data, using graphical as well as numerical techniques is discussed in the following sections. Calculation of equilibrium constants from solvent extraction is described in several monographs [60-64]. 

An attempt has been made to summarize the available literature for comparison of adsorption constants and forms of the equations used. Table XV presents a number of parameters reported by different authors for several model compounds on CoMo/A1203 in the temperature range 235-350°C (5,33,104,122,123,125-127). The data presented include the adsorption equilibrium constants at the temperatures employed in the studies and the exponential term (n) of the denominator function of the 0 parameter that was used in the calculation. The numbers shown in parentheses, relating to the value of n, indicate that the hydrogen adsorption term (Xh[H2]) is expressed as the square root of this product in the denominator. Data are presented for both the direct sulfur extraction site (cr) and the hydrogenation site (t). 

Fortunately, the initial rotation of our S(+)-BN was high ([oi]d +220°) and allowed us to follow the kinetTcs over at least two half-lives. Bate constants, k, could be extracted from raw data using a standard first-order treatment (equation 2). Since the equilibrium constants between R- and S-BN are very near one in optically-active media and are exactly one in achiral solvents, we can take kRg = k = kgjj (see equation 1). The a0, at, and doo represent, respectively, rotations at an arbitrarily defined ini-... 

An INTERNET compatible database for solvent extraction of metal ions (SEDATA), developed by H. Watarai et al., contains about 9,600 equilibrium constants, including distribution constants, extraction constants, and adduct formation constants, for more than 1,400 ligands and 82 metal ions.50 Raw data points of extraction curves are also incorporated to be reconstructed as a figure. However, SEDATA contains no fields for 2D chemical structures of extractants and allows one to perform a search using only eight fields (classification, metal, valence, reagent, solvent, title of the paper, author, and year). 

Discussions of results of rate studies permeate thisbookbecause kinetics investigations are the single most important group of techniques in mechanistic determinations. However, kinetics results have to be derived from measurements which are the outcome of experiments. Chapter 3 on conventional kinetics methods includes techniques which are generally applicable, and also current procedures for extracting rate constants (and, in some cases, equilibrium constants) from raw experimental data. 

We have looked at four KD equilibrium resources laboratory data, triangular equilibrium data from publications, Henry s law constants, and operating field data. All four are reliable data sources if applied with limitations as described. All can be applied to the rectangular equilibrium curve (the distribution coefficient equilibrium curve) as shown in Fig. 7.2, which includes a listing of laboratory data for the extraction process shown. Please notice the data table in the boxed area of columns... 

In some systems a series of adducts ML(n )A aB, ML(n 2)A2 bB,..., MA yB are involved in the extraction. An attempt was recently made24,251 to determine their composition and the corresponding equilibrium constants by obtaining the metal ion distribution ratio data using a fixed [B] and varying [HA]. 

The partition data for the dilute solution of the extractants TBP, TBPO, TOPO, TEHPO, TPPO and some betadiketones were reported39-44) and some of these data are given in Table 1. The distribution of TPrP between water and several diluents was studied45) and the data were used for calculation of the equilibrium constant for the reaction represented by... 

The various equilibrium constants for the equilibria involved in the extraction of U(VI) by HTTA-B combinations are summarized in Table 7. A comparison of the data is rather difficult in view of the different experimental conditions employed. The deviation of 2,6-DMPO and 2 MPO from the basicity order of the series was attributed92 to the occurence of steric effects with these donors. 

The effect of diluents on the extraction of U02(TTA)2 B was studied10,83 87 and the adduct composition was reported to remain unaltered with change in diluent. However, the diluents have marked influence on the equilibrium constants as can be seen from the data given in Table 8. Attempts were made83 to determine diluent-free equilibrium constants by determining the activities of the solutes in different diluents. 

The stoichiometry shown in Equation (1) is similar to that followed during extraction of trivalent americium by TBP and other monofunctional neutral organophosphorus extractants. Distribution ratio data plotted in Figure 2 show that DBBP extracts Am(III) more strongly than TBP from HNO3 media. The equilibrium constant for Reaction (1) (at zero ionic strength) is 7.4 compared to a value of only 0.4 for the similar reaction with TBP.(5)... 

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