Thorium complexes hydrolysis

Alkoxide gels, 23 60 Alkoxide gels, in optical fiber manufacturing, 11 145 Alkoxide initiators, 14 259 Alkoxide ligands, thorium, 24 770 Alkoxides, 12 190 25 72-86 controlled hydrolysis of, 23 56 iron, 14 533 mixed-metal, 25 100 titanium, 25 82 uranium complexation with,... 

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. 

A different type of bridging occurs in hydrolysis complexes of tho-rium(IV) (219) and uranium(IV) (130). Here a distinct peak at 3.94(2) A in the hydrolyzed solutions can be ascribed to the metal-metal distances in the hydrolysis complexes. Discrete dinuclear complexes with a very similar metal-metal distance, 3.988(2) A, in which the metal atoms are joined by double hydroxo bridges have been found in crystals ofTh2(OH2)(N03)6(H20)8 (229). The same type of bridging, therefore, must occur in solution. When hydrolysis is increased, however, the number of metal-metal distances per metal atom increases beyond a value of 0.5, valid for a dinuclear complex, and larger hydrolysis complexes are obviously formed. These structures are unknown but an extensive X-ray investigation of highly hydrolyzed thorium(IV) solutions has shown that there is probably no close relation between the structures of the hydrolysis complexes in solution and the structure of thorium dioxide, which is the ultimate product of the hydrolysis process (230). 

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. 

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. ... 

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

The release of uranium and thorium from minerals into natural waters will depend upon the formation of stable soluble complexes. In aqueous media only Th is known but uranium may exist in one of several oxidation states. The standard potential for the oxidation of U in water according to equation (2) has been re-evaluated as E° - 0.273 0.005 V and a potential diagram for uranium in water at pH 8 is given in Scheme 3. This indicates that will reduce water, while U is unstable with respect to disproportionation to U and U Since the Earth s atmosphere prior to about 2 x 10 y ago was anoxic, and mildly reducing, U " would remain the preferred oxidation state in natural waters at this time. A consequence of this was that uranium and thorium would have exhibited similar chemistry in natural waters, and have been subject to broadly similar redistribution processes early in the Earth s history. Both U " and Th are readily hydrolyzed in aqueous solutions of low acidity. A semiquantitative summary of the equilibrium constants for the hydrolysis of actinide ions in dilute solutions of zero ionic strength has been... 

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. 

Experimental studies on the hydrolysis of the Th" ion, its complexes with strong inorganic hgands, and the solubility of thorium oxides or hydroxides ate usually performed with low concentrations of thorium in perchlorate, chloride, and nitrate media. There is no evidence for complex formation between Th" and CIO4 however, chloride and nitrate form weak Th(lV) complexes as discussed in Sections Vlll.2.2.1 and X. 1.3.3, respectively. For the evaluation of equihbrium constants at zero ionic strength from data in chloride and nitrate media we have therefore the general problem to decide if the activity of Th", , should be calculated using a... 

The enthalpy of solution of ThCl4(cr) in ca. 16000 H2O was given at 288 K as -Till kJmol by Chauvenet [1911CHA]. As discussed in Appendix A, use of the constants for the hydrolysis of the thorium ion selected in this review (Table VIl-15) and of that for the formation of the first thorium chloride complex (Section Vlll.2.2) leads to a dissolution reaction that can be written as ... 

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. 

In this paper on the formation of acetate complexes of scandium and thorium in water and aqueous ethanol mixtures at / = 0.05 M and 25°C, Usherenko and Skorik [1971USH/SKO] used preliminary hydrolysis constants of logn, Vn (Th(OH) = -3.15 (water, / = 0.05 M), -2.77 (25 vol.%/19.8 mass.% ethanol) and -2.17 (50 vol.%/40.3 mass% ethanol). The experimental determination of the hydrolysis constants is reported in a later paper of these authors [1972USH/SKO]. 

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 ... 

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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