Thorium hydrolysis species

MOU/AME] Moulin, C., Ameraz, B., Hubert, S., Moulin, V., Study of thorium hydrolysis species by electrospray-ionization mass spectroscopy, Anal. Chim. Acta, 441, (2001), 269-279. Cited on pages 156, 157, 158, 159, 664. 

Early work" established that Cu", Ni" and Co" promote the hydrolysis of glycinamide in the pH range 9.35 to 10.35 at temperatures of 6.5 to 75 °C. Bamann and his collaborators carried out an extensive series of studies on the metal ion-promoted hydrolysis of peptides and related compounds" and a review of this early work is available." Highly charged ions such as thorium(IV) were found to promote the hydrolysis of leucylglycylglycine at pH values as low as 5. The thorium(IV) species is very extensively hydrolyzed at this pH and the reaction is presumably heterogeneous. Gel hydrolysis is effective at relatively low temperatures (37 °C), whereas observable effects were only obtained with such ions as copper(II) at temperatures of ca. 70 °C. 

Finally, complexes of even higher nuclearity are no doubt formed. There is no need, however, to assume any complexes containing two, three, four or five uranium atoms. The hexamer is a very important species also in thorium hydrolysis, but the dimer and tetramer which are also prominent in that system do not seem to be favored at all by uranium(iv). In Table 21.1, the value of p)9j, u calculated originally [42] is given, as well as the value calculated on the assumption that the hexamer is the only polymer of low nuclearity formed [25]. 

The formation of the hydrolysis species of thorium can be described by reaction (2.5) (M = Th ). The polymeric hydrolysis species that have been reported for thorium are somewhat different to those identified for zirconium and hafnium, although thorium does form the Th (OH)8 species. The other polymeric species that have been identified include two dimeric species, Th2(OH)2 + and Th2(OH)3 , and two hexameric species, Thg(OH)i4 and Thg(OH)45, and a second tetramer, Th4(OH)i2 - Four monomeric species have also been found, ThOH to Th(OH)4(aq). No anionic monomeric species have been identified. 

Stabihty constant data have been reported for a number of the thorium polymeric hydrolysis species at more than one temperature (Baes, Meyer and Roberts, 1965 Ekberg et al., 2000). The experimental medium used in both of these studies was the same (1.0 mol 1 NaClO ). The three species Th2(OH)3 , Th (OH)j2 and Thg(OH) 4 + do not have stabihty constant data at temperatures other than 25 "C. In this review, it is assumed that for each of the species where data are available at more than one temperature, the stabihty constants reported are a linear function of the reciprocal of absolute temperature. 

Table 10.13 Data for the stability constant of the second monomeric hydrolysis species of thorium(IV), Th(OH)2 + (reaction (2.5), M = Th , p= 1, j = 2).

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Several attempts have been made to correlate the adsorptivity of hydrolyzable cations to the composition of the species in aqueous solution (1, 2, 20). In particular, the adsorption of thorium on silver halides indicated a very close relationship between the change in the amount of thorium adsorbed and the concentration of the hydrolyzed soluble species in solution (19). The major difficulty in this type of work is the lack of quantitative data on the hydrolysis of various metal ions. The other uncertainty is with regard to the knowledge of the true surface area of the adsorbent in aqueous solution. This latter information is needed if surface coverages are to be evaluated. 

These two mechanisms, the one emphasizing the adsorption of specific often polynuclear hydrolysis products, the other emphasizing the role of polymeric species, are clearly not mutually exclusive an earlier study on thorium (IV) adsorption suggested a combined mechanism (J). 

Tetravalent. The hydrolysis of tetravalent actinide ions can begin to occur in solutions with pH levels < 2. Under dilute conditions, species of the form An(OH) " (n = 1 4) are predicted however, most hydrolysis studies have only been able to identily the first hydrolysis product, An(OH) +. It should be noted that in all of these compounds the remainder of the coordination sphere is made up of bound H2O molecules. The end member of the speciation is the neutral An(OH)4 or An02-2H20. This complex has low solubihty but has been postulated to exist in solutions from solubihty experiments when using the isolated solid as the starting material. Under more concentrated conditions, polymeric materials have been postulated. In modeling the hydrolysis of thorium at concentrations greater than mM, polynuclear species of the form Th2(OH)2 +, Th2(OH)4 +, Th4(OH)g +, Th6(OH)i4 +, and so on, have been included. 

Hydroxides. Pure and mixed metal actinide hydroxides have been studied for their potential utility in nuclear fuel processing. At the other end of the nuclear cycle, the hydroxides are important in spent fuel aging and dissolution, and environmental contamination. Tetravalent actinides hydrolyze readily, with Th more resistant and Pu more likely to undergo hydrolysis than and Np. All of these ions hydrolyze in a stepwise marmer to yield monomeric products of formula An(OH) with = 1,2,3 and 4, in addition to a number of polymeric species. The most prevalent and well characterized are the mono- and tetra-hydroxides, An(OH) and An(OH)4. Characterization of isolated bis and tri-hydroxides is frustrated by the propensity of hydroxide to bridge actinide centers to yield polymers. For example, for thorium, other hydroxides include the dimers. 

The thorium ion, Th4+, is more resistant to hydrolysis than other 4+ ions but undergoes extensive hydrolysis in aqueous solution at pH higher than 3 the species formed are complex and dependent on the conditions of pH, nature of anions, concentration, etc. In perchlorate solutions the main ions appear to be Th(OH)3 +, Th(OH)2+, Th2(OH) +, Th4(OH) +, while the final product is the hexamer Th6(OH)95 of course, all these species carry additional water.19 Hexameric ions exist also for Nbv and for Ce1 v and Ulv [M604(0H)J12 + ions are found in crystals of the sulfates. The metal atoms are linked by hydroxo or oxo bridges. In crystals of the hydroxide, Th(OH)4, or the compound Th(0H)2Cr04 HzO, chain-like structures have been identified, the repeating unit being Th(OH)2+ in solution, the polymers may have similar form (28-1) or may additionally be cross-linked. 

The present review therefore puts much weight on the assessment of the thermodynamics of thorium in aqueous solution at ambient temperatures and makes independent analyses of the available hterature in this area. As discussed in Chapter Vtl, the hydrolysis of the Th(lV) ion in aqueous solutions is particularly complex at least nine Th-OH complexes (including polymeric species of high ionic charge) are fairly firmly established, and a number of additional species have been proposed in the literature. 

As discussed in Appendix A, use of the adopted constants for the hydrolysis of the thorium ion (Table VII-15) indicates that, in the resulting solution, the largest part of the thorium is found as polymeric hydroxide species, with appearance of a very small amount of precipitate of Th(OH)4(am). Under these circumstances, these results will not be considered further. 

Souchay has studied hydrolysis and the formation of polynuelear eomplexes of a number of different metal ions, including Th(lV), using ciyoscopy. The method requires high total concentrations of the solute (in this case 0.4 M thorium nitrate) to which different amounts of NaOH were added. Souchay interpreted his result as the formation of a single tetranuclear complex Th (OH). There is no information on the pH in the system and as a result of later investigations discussed in the present review, it is well estabhshed that no single species can describe the hydrolysis of Th. There are no thermodynamic data reported and the data do not provide supporting evidence for the formation of a tetranuclear Th hydroxide complex. 

Baes and Mesmer s [1976BAE/MES] comprehensive survey and critical review of the hydrolysis of cations is the most frequently cited standard book on metal ion hydrolysis and widely accepted to represent the state of the art for long time after its publication. The authors have in most cases made their own analysis of previously published data and tested a number of different equilibrium models. The choice of models is based on the standard deviation of the experimental average number of coordinated hydroxide ions 0H However, one should complement this method by a calculation of the speci-ation in the various test solutions as done in the present review. Species that occur in low concentrations should be looked upon with suspicion as gradual changes in diffusion potentials and changes in the ionic medium may be erroneously interpreted as minor complexes. The discussion of the hydrolytic behaviour of thorium(IV) is based on the following potentiometric titration studies which are also included in the data evaluation of the present review ... 

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