Thorium nitrate complexes

6 Complexes of the Actinide(iv) Nitrates and Halides 11.6.1 Thorium Nitrate Complexes 

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Relatively few complexes with A-alkylacetamides have been reported. No structural information is available for any of them, but the thorium nitrate complexes,... 

A number of thorium nitrate complexes have been synthesized and studied. Hydrated thorium nitrate, Th(N03)4.5H20, contains [Th(N03)4.(H20)3] molecules and was one of the first 11-coordinate compounds to be recognized (Figure 11.6). 

As in the case of the dissolution of the anhydrous thorium nitrate in water by the same authors, these results caimot be used as such for the determination of the enthalpy of formation of the compound, due to the lack of experimental data on the enthalpies of formation of the thorium nitrate complexes. This is also the case for the enthalpy of solution of the pentahydrate in water to reach the same final concentration, also given by the same authors as (Th(N03)4-5H20, cr, 298.15 K) =... 

FOM/MAI], The equilibrium constants proposed for the thorium nitrate complexes are therefore considerably overestimated and not accepted by this review. 

Danon, J., Determination of the stability constants of thorium nitrate complexes with anion-exchange resins, J. Inorg. Nucl. Chem., 13, (1960), 112-118. Cited on pages 317, 318, 462. 

The most popular device for fluoride analysis is the ion-selective electrode (see Electro analytical techniques). Analysis usiag the electrode is rapid and this is especially useful for dilute solutions and water analysis. Because the electrode responds only to free fluoride ion, care must be taken to convert complexed fluoride ions to free fluoride to obtain the total fluoride value (8). The fluoride electrode also can be used as an end poiat detector ia titration of fluoride usiag lanthanum nitrate [10099-59-9]. Often volumetric analysis by titration with thorium nitrate [13823-29-5] or lanthanum nitrate is the method of choice. The fluoride is preferably steam distilled from perchloric or sulfuric acid to prevent iaterference (9,10). Fusion with a sodium carbonate—sodium hydroxide mixture or sodium maybe required if the samples are covalent or iasoluble. 

Upon coordination via oxygen, as in uranyl sulfoxide complexes and thorium nitrate sulfoxide complexes, the positive charge on sulfur is virtually unaltered (19), whereas coordination via sulfur, as in palla-dium(II) sulfoxide complexes, causes an increase in the positive charge, as a result of transfer of electron density from the sulfur atom to the metal center (19, 373). 

In the system Th(N03)4-HN03 H20, five thorium(iv) nitrate hydrates, previously unknown, have been identifiedfrom some of the solutions H2Th(N03)g,3H20 was obtained, which probably contains either H30 or H5O," or both, stabilized by [Th(N03)g]. Raman spectral data obtained from thorium nitrate solutions could be interpreted in terms of an equilibrium between complexed and free NO . 

In the second separation, uranium is extracted from thorium. Ethyl acetate is the extractant for uranium, which is bound in a nitrate complex. Thorium remains in the aqueous phase. Thorium is then co-precipitated with Nd(OH)3 to avoid absorption of the beta particles emitted by 234Th and 234mPa by the large amount of NH4N03 if the solution were simply evaporated and counted. The filter with Nd(OH)3 is mounted on a planchet for counting beta and alpha particles. 

Principle of Separation. Uranium forms a nitrate complex that is extractable into ethyl acetate (as well as other organic extractants). Thorium does not readily form an extractable nitrate complex. When ethyl acetate is contacted with an aqueous solution, the uranium-nitrate complex is partitioned favorably into the ethyl acetate whereas thorium nitrate is not. The distribution of the metal ion between the two phases is expressed as D = Corganic/Caqueous where C is the concentration in moles or dps per unit volume in the respective phases. The thorium remains in the aqueous phase and is precipitated as the hydroxide for counting. 

Uranium(vi) nitrate complexes have been discussed in Section 11.5.4, but uranium forms complexes in the 4-4 state that are generally similar to those of thorium. 

A similar scheme has been used for example, for the recovery on anion exchanger of uranium as a nitrate complex from its mixture with thorium [17, p. 317], or for the purification of nickel from calcium on cation exchanger[18]. 

Plutonium Purification. The same purification approach is used for plutonium separated from sediments or seawater. In case reduction may have occurred, the plutonium is oxidized to the quadrivalent state with either hydrogen peroxide or sodium nitrite and adsorbed on an anion exchange resin from 8M nitric acid as the nitrate complex. Americium, curium, transcurium elements, and lanthanides pass through this column unadsorbed and are collected for subsequent radiochemical purification. Thorium is also adsorbed on this column and is eluted with 12M hydrochloric acid. Plutonium is then eluted from the column with 12M hydrochloric acid containing ammonium iodide to reduce plutonium to the non-adsorbed tervalent state. For seawater samples, adequate cleanup from natural-series isotopes is obtained with this single column step so the plutonium fraction is electroplated on a stainless steel plate and stored for a-spectrometry measurement. Further purification, especially from thorium, is usually needed for sediment samples. Two additional column cycles of this type using fresh resin are usually required to reduce the thorium content of the separated plutonium fraction to insignificant levels. 

Thorium has been sorbed on strongly basic anion-exchangers as the nitrate complex most other metals are not retained [15-19]. Other media employed include chloride [20], sulphate [2], thiocyanate [21], and citrate [22]. In numerous cases, mixed aqueous-organic... 

Thorium is one of the few multivalent metals [others are Au(III), Ce(IV), U(VI), and Cr(VI)] which are extractable as nitrate complexes from nitric acid solutions [25-28]. The extractants used include TBP in CCI4 [26,29], TOPO in cyclohexane, toluene or xylene [25,30,31], and triphenylarsine oxide in CHCI3 [27]. Other reagents used for extraction of the nitrate complex of thorium include dibutyl dithiophosphate in various organic solvents [32], dibutyl sulphoxide in xylene [28], and bis(2-butoxyethyl) ether [33].The liquid anion-exchanger Aliquat 336 in xylene [34] and a solution of tertiary ammonium salt (Hyamine) in dichloroethane [35] have been also proposed for extraction of Th. The presence of Li, Na, or A1 nitrate improves the extraction of thorium. Sulphate, phosphate, and tartrate do not interfere, but fluoride must be masked, e.g., by aluminium. Thorium has been separated from U and Pu with the use of Alamine 336 and TOPO (in xylene or cyclohexane) [36]. 

Thorium-Arsenazo 111 complex can be extracted with butanol from 1 M HCl (e = 6.41 O at 670 nm) [57]. Thorium has been determined in the organic phase after extraction with HDEHP in cyclohexane and addition of aqueous Arsenazo 111 and isopropyl alcohol (e = 8.8-10 at 660 nm) [41]. In another method Arsenazo III was added to the xylene extract of the ion-associate of the nitrate complex of thorium with Aliquat 336 [34]. Thorium has been determined after froth flotation separation of the ternary compound of Th with Arsenazo III and surfactants [58,59]. 

For the analysis of molybdenum, the sample is decomposed by fuming with a few drops of nitric acid and sulfuric acid in a platinum crucible and the molybdenum is determined gravimetrically7 as the 8-quinolinol complex. From the filtrate, potassium is determined gravimetrically as K2S04. Fluoride is determined by titration with a standard solution of thorium nitrate using sodium alizarinsulfonate as indicator, after steam distillation of fluorosilicic acid.8 The determination of the oxidation state of molybdenum is carried out by oxidizing a known amount of the compound with a known amount of potassium dichromate in hot 2 N sulfuric acid and titrating the excess dichromate with standard Fe2+ solution. 

Hydrated thorium fluoride is precipitated when a soluble fluoride is added to a solution of thorium nitrate. Precipitation can be prevented by addition of aluminum nitrate to complex the fluoride ion, an expedient used in the Thorex process (Chap. 10, Sec. 5). 

Similarly to the activity of U(IV) in nitrate solution, shown in Figure VI-2, that of Th" in chloride and nitrate solution can be described equally well with either the complexation model or the ion interaction approach cf. Appendix A reviews of [1952WAG/STO] and [2006NEC/ALT]). The application of the two models to thorium chloride and nitrate complexes is discussed in Sections VIII.2.2.1 and X.1.3.3, respectively. 

In the case of the non-hydrolysed thorium species we have the choice of using either the ion interaction model (with s(Th", NO3) = (0.31+0.12) kg-mol ) or calculating the free Th" concentration using the complex formation model and the corresponding equilibrium constants logm (VII.15) selected for Th(IV) nitrate complexes. However, there is no quantitative information on the ternary Th(lV)-hydroxide-nitrate complexes formed by Reactions (VII. 16). Therefore it is more convenient to use the ion interaction model for both the Th" ion and the hydroxide complexes, i.e., to use ion interaction coefficients e(Th", NOj) and... 

As can be seen, the nitrate complexes are, in the main, the dominant thorium species in solution. As there are no experimental values for the enthalpy of formation of these complexes (neither for thorium nor for uratuum), we will not attempt to use further the experimental results of [1956FER/KAT] which are given here for information only. 

It has to be emphasised that these equilibrium constants must not be combined with 8(ThAN03) = (0.31 +0.12) kg-mol used in the Sections VII.3 and VIII. 1.2 on aqueous thorium hydroxide and fluoride complexes this value refers to a strict ion interaction approach where the effect of nitrate complexation is included in the interaction coefficient. The equilibrium constants for the formation of nitrate complexes must be combined with s(ThA NOj) = e(ThA CIO ,) = (0.70 + 0.10) kg-mol, ... 

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. 

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