Polynuclear complexes thorium

Although the exact extentis not known accurately, hydrolysis of various salts is known to occur. Since the hydroxide is not precipitated it is assumed that the hydrolysis product is some ion on the form Th(OH)2++ orThOHJ+. The solution chemistry of thonum is made more complicated because of the hydrolytic phenomena observed and the polynuclear complex ions that are formed at low acidities and higher thorium concentrations. 

Similarly, the hydrolysis of thorium(IV) can be accounted for on the assumption of a series of polynuclear complexes Th[(OOH)Th] ", where n has values exceeding 6 in certain cases. ... 

This is a potentiometric study made at 25°C and an ionic strength of 0.50 M. The total concentration of thorium varied from 5 to 0.25 mM and the value of from 0.01 to 2.58. The authors demonstrate the formation of polynuclear complex but they have preferred to analyse their data using the core and link model described in [1964H1E/S1L]. The experimental data are consistent with this model, but as this does not provide a proper chemical description of the constitution of the complexes, as discussed in the present review, these data are not accepted by this review. 

The hydrolysis of Th" involves extensive formation of polynuclear complexes. In the earlier stages of the hydrolysis in perchlorate media, when the number of hydroxide ions per thorium atom in the complexes (i.e. the ligand number n) is n 2, the hydrolytic reactions are fully reversible and equilibrium is quickly reached [30-32]. The first extensive measurements [30] were interpreted as indicating the formation of an infinite series of core-I-links complexes, ThffOHjjTh). Other measurements at widely varying pH and central-ion concentration, C, could, however, be satisfactorily fitted by only three polymers, viz. ThjfOH) , Th4(OH), and Th6(OH)i5, besides the two monomers ThOH and Th(OH)2 [31, 32]. In Table 21.1 the constants are listed, referring to the equilibrium (asterisk refers to hydrolysis reaction) ... 

The formation of very large polymers, and the ensuing precipitation, takes place at much lower ligand numbers for uranium(iv) than for thorium(iv). It is therefore much more difficult to study the structures of the hydroxo complexes of uranium(iv) in solution. Polynuclear complexes are formed, with U-U distances... 

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. 

The equilibrium constants logi A °4= -(8.5+ 1.0) and log,o A°i = - (17.4 + 0.7) can be used as "operational values" for geochemical modelling. However it is by no means ascertained that the thorium concentration measured in solubility studies at pH 6-14 is actually due to the mononuclear complex Th(OH)4(aq). This review finds it more likely that the soluble uncharged complex is one or more small (< 1.5 nm) polynuclear species Th O K(OH) 4 2i)(aq), more simply written as... 

The results reported by Usherenko and Skorik [1972USH/SKO] are of a reasonable order of magnitude. However, the possible formation of polynuclear thorium hydroxide complexes is not taken into account by the authors. This would have required additional studies at different total thorium concentrations. Therefore the reported results are not considered by the present review in the selection of data for thorium hydroxide complexes. 

There is strong evidence from other investigations discussed in the present review that polynuclear thorium complexes are formed in the [Th]tot-pH range investigated, complexes that have not been found by Sawant et al. This review considers this as an indication of unidentified errors in [2003SAW/SHA] and does therefore not accept the equilibrium constants proposed. 

Davydov, Yu. P., Toropov, I. G., Hydrolysis of thorium(4+) cations with formation of polynuclear hydroxo complexes in solutions, Dokl. Akad. Nauk Belarusi, 36, (1992), 229-233, in Russian. Cited on page 593. 

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