Stability constants sulfate complexes, acid

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 low TTA dependence at 35.0°C probably is attributable to dissolution of TTA in the aqueous phase. Observation of fourth-power dependence on acidity argues against any change in the extraction mechanism (e.g., Pu(IV) reduction or NO3 involvement). An aqueous Pu(TTA)3+ complex has been reported (14, 15) and this possibility has been considered in the error analysis of the Pu(IV)-sulfate stability constants. 

As was observed in the case of the extraction of zirconium and hafnium from nitrate media, it is probable that the different tendencies of the metals towards hydrolysis has some effect on the selectivity observed,298 313 expecially in view of the proved extraction of hydroxo complexes. The extraction of both metals decreases markedly in the presence of sulfate ions in the aqueous phase (a feature that is utilized in the stripping of the loaded hafnium with sulfuric acid), although the selectivity for hafnium over zirconium is simultaneously increased on account of the higher stability constants of the inextractable sulfato complexes of zirconium.298... 

The way in which conditional stability constants are used to calculate the distribution of chemical species can be illustrated by consideration of the forms of dissolved Cu(II) in a dilute, acidic soil solution. Suppose that the pH of a soil solution is 6.0 and that the total concentration of Cu is 0.1 mmol m 3. The concentrations of the complex-forming ligands sulfate and fulvic acid have the values 50 and 10 mmol m 3, respectively. The important complexes between these ligands and Cu are CuS04 and CuL where L refers to fulvic acid ligands (see Section 2.3). These illustrative complexes are not the only ones formed among Cu, S04, or L, nor are the three ligands the only ones that form Cu complexes in soil solution.29 Under the conditions assumed, the equation of mole balance for Cu is (cf. Eqs. 2.11 and 2.30)... 

Previous investigations of the Pu(III)-sulfate system in 1 -2 M acid media have produced results in which the authors do not agree on either the nature of the complexes or the magnitude of the stability constants. Both Fardy and Buchanan W and... 

Type IV includes chiral phases that usually interact with the enantiomeric analytes through the formation of metal complexes. There are usually used to separate amino acid enantiomers. These types of phases are also called ligand exchange phases. The transient diastereomeric complexes are ternary metal complexes between a transitional metal (usually Cu +), an amino acid enantiomeric analyte, and another compound immobilized on the CSP which is able to undergo complexation with the transitional metal (see also the ligand exchange section. Section 22.5). The two enantiomers are separated based on the difference in the stability constant of the two diastereomeric species. The mobile phases used to separate such enantiomeric analytes are usually aqueous solutions of copper (II) salts such as copper sulfate or copper acetate. To modulate the retention, several parameters—such as the pH of the mobile phase, the concentration of the copper ion, or the addition of an organic modifier such as acetonitrile or methanol in the mobile phase—can be varied. 

Differences in the speciation of other ions at the surface can be noted. Using an analysis similar to that above for metal ions, one finds that adsorbed anions are less acidic than in bulk solution. For example, it was shown in Figure 6 that protolysis of adsorbed sulfate ions becomes significant in the pH range 4-5, whereas in solution bisulfate is formed at much more acidic conditions (pH 2). Complexes formed by supporting electrolyte, e.g. Na" ,. with oxide surface sites have greater stability constants (logK 0.5-1.7) than observed for complex formation with oxyanions in solution (log K 0.0) (J.). ... 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

The presence of vanadium(III) complexes in the very acidic medium of the vacuoles of the signet ring cells in Ascidia ceratodes, containing one or two sulfato/hydrogensulfato ligands, has focused interest on sulfatovanadium complexes as model systems. " ] Whereas readily forms a 1 1 complex with sulfate K = 300 m ), the affinity of V to sulfate is less pronounced. Selected stability constants and pA a values obtained from... 

If precursor complexes are formed in sulfuric acid solutions, they will almost certainly be mixed sulfate-carboxylate complexes, and the thermodynamic data will be of only peripheral use. In nitrate and especially perchlorate media, the thermodynamic data should be directly comparable as Ce(IV)-N03 complexes are weak and perchlorate complexes are unknown in aqueous solutions. It should be possible to compare the calculated constants with the kinetically derived data to gain further insight into the nature of the precursor complexes, and to assess the validity of authors claims with respect to complex stability and stoichiometry. 

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