Extraction solvent, stability constant

A knowledge of stability constant values is of considerable importance in analytical chemistry, since they provide information about the concentrations of the various complexes formed by a metal in specified equilibrium mixtures this is invaluable in the study of complexometry, and of various analytical separation procedures such as solvent extraction, ion exchange, and chromatography.2,3... 

The physical nature of the sulfate complexes formed by plutonium(III) and plutonium(IV) in 1 M acid 2 M ionic strength perchlorate media has been inferred from thermodynamic parameters for complexation reactions and acid dependence of stability constants. The stability constants of 1 1 and 1 2 complexes were determined by solvent extraction and ion-exchange techniques, and the thermodynamic parameters calculated from the temperature dependence of the stability constants. The data are consistent with the formation of complexes of the form PuSOi,(n-2)+ for the 1 1 complexes of both plutonium(III) and plutonium(IV). The second HSO4 ligand appears to be added without deprotonation in both systems to form complexes of the form PuSOifHSOit(n"3) +. ... 

The study of carbonate complexes of Pu is complicated by various experimental difficulties. The low solubility of many carbonates (7), leaving a very dilute Pu concentration in solution, results in difficulties to the experiments with electrochemical or spectrophotometric methods. However, the radiometric method with solvent extraction or solubility measurement is easily applicable for the purpose. Unlike the solution with anions, like Cl, N03 etc., the concentration of which can be varied at a constant pH, the preparation of solutions with varying carbonate concentration accompanies indispensably the change of pH of the solution. As a result, the formation of carbonate complexes involves accordingly the hydrolysis reactions of Pu ions in solutions under investigation. It is therefore prerequisite to know the stability constants of Pu(IV) hydroxides prior to the study of its carbonate complexation. 

The following physico-chemical properties of the analyte(s) are important in method development considerations vapor pressure, ultraviolet (UV) absorption spectrum, solubility in water and in solvents, dissociation constant(s), n-octanol/water partition coefficient, stability vs hydrolysis and possible thermal, photo- or chemical degradation. These valuable data enable the analytical chemist to develop the most promising analytical approach, drawing from the literature and from his or her experience with related analytical problems, as exemplified below. Gas chromatography (GC) methods, for example, require a measurable vapor pressure and a certain thermal stability as the analytes move as vaporized molecules within the mobile phase. On the other hand, compounds that have a high vapor pressure will require careful extract concentration by evaporation of volatile solvents. 

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

The free energy is calculated from the stability constant, which can be determined by a number of experimental methods that measure some quantity sensitive to a change in concentration of one of the reactants. Measurement of pH, spectroscopic absorption, redox potential, and distribution coefficient in a solvent extraction system are all common techniques. 

The stepwise stability constants for zirconium and hafnium thiocyanato-complexes have been determined by solvent-extraction techniques. The values ) i = 12.1 2.2, = 215 11, P4. = 205 + 20 for zirconium, and Pi =... 

Owing to the inadequacy of the mathematical model available for analysis of the amine extraction system (7), accurate values of the stability constants could not be evaluated for the Hs Pbn1-" system in the presence of NaCl. However, using the values of stability constants obtained by Bertazzi for the system (C2H5)3PbCln1 n in LiCl at 8.0 m (10), viz. 0 = 3.5, 02 = 1.0, 0s = 0.1, the neutral species Hs PbCl0 (n = 1) is seen to be dominant. Therefore a simple solvent extraction would be expected to remove a certain amount of triethyl lead from solution. As shown in Table II, this is seen to be so. However,... 

We have included in this chapter stability constants for these systems in di-chloromethane, although this is a low permittivity medium and therefore the reliability of these data may be reduced by the formation of ion pairs. However, these data may be useful in solvent extraction processes due to the immiscibility of this solvent with water which allows the direct partition of electrolytes in the dichloromethane solvent system [59,60],... 

Solvent extraction, potentiometry, and calorimetry have been used to determine the thermodynamic parameters of the formation of the monofluoride complex of the trivalent lanthanide ions at 25°C. and an ionic strength of IM (NaClOj ). The enthalpies were all endothermic, ranging from 4.0 to 9.5 Kcal./mole consequently, the large, positive entropies, ranging from 25 to 48 cal./°C./mole, explain the high stability constants. This large entropy results from the decrease in overall water structure when the fluoride ion is complexed. The difference in the enthalpies of formation of LnF and LnAc " can possibly be explained by a difference in covalence for Ln-F and Ln-O bonds. 

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. ... 

Stability constants for the interaction between Th", U02, Np02, and Am and the complete polytungstates, [Na(H20)P5W3oOiio] " and [P2Wi8062] " have been measured in 0.1 M NaCl, using solvent extraction. The log P values range from 3 to 6 and the order Th > Am U > Np for the P5 anion has been interpreted in terms of steric interactions of the yl oxygens with the... 

It should be noted that some data on complexation of transuranium elements with various N-donor ligands, such as azide ion, isothiocyanate ion, amine-N-polycarboxylic and heterocyclic acids, 8-hydroxyquinoline and its derivatives, were published as early as 1950-1970 [42-77], The quantitative parameters of complexation (number and composition of complexes, stability constants, thermodynamic parameters) were determined using accessible at that time methods (spectrophotometry, potentiometry, ion exchange, solvent extraction). It was shown that transuranium elements in different oxidation states can form complexes with N-donor ligands. However, only presumptive conclusions regarding structure of complexes were drawn at that time,... 

Interaction of An(III) with azide and isothiocyanate ions in solution was studied extensively [10,54,56,58,108]. Complexation of Am(III) with azide ion was studied by means of HTTA solvent extraction in the work [108]. Stability constant for 1 1 complex was found to be y0 =18 that is significantly lower than... 

Complex formation of Np(lV) with NCS ion has been extensively studied using TTA and HDNNS solvent extraction with in different conditions [58]. Table 6 presents values of stability constants measured in 2M NaC104 at 298K. 

Neptunium and plutonium complexes [AnL4] with 8-hydroxyquinoline and its derivatives were obtained in the form of solid compounds (L=Ox, MOx, COx, DCOx) [62], In all cases the coordination number of metal atom is equal to 8, while Th(IV) and U(IV) are known to form complexes H[M(Ox)5] [114], Stability constants were determined by solvent extraction and are equal to logy04=45.28 and logy04=46.O5 for Np complexes with Ox and DCOx, respectively [62],... 

The complexation of pentavalent neptunium with pyridinecarboxylic acids was investigated by different methods including spectrophotometry, potentiometry and solvent extraction [18,42,101,127-130]. Depending on the technique, complexation model used in calculations and experimental conditions, different values of stability constants were obtained (Table 8). 

Complexation of Np with NCS has been extensively studied using different experimental techniques. In the work [57], stability constants were calculated from spectrophotometry and potentiometry data for ionic strength 1=9 (Table 12). The stability constant logy0i=O.32 was obtained using solvent extraction technique 1=2 M) [60], Since the isothiocyanate ion is able to reduce Np(V), the special attention was given in this study to maintain the Np oxidation state. 

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