Thermoanalytical study on the oxidation of uranium dioxides derived from uranyl nitrate,uranyl acetate and ammonium diuranate

The oxidation of UO₂ was investigated by TG, DSC and X-ray diffraction . UO₂ samples were prepared by the reduction of UO₃ at P_H2 + P_N2 = 100 + 50 mm Hg and 5°C min^?1 up to 800°C. In order to obtain six UO₂ samples with different preparative histories, UNH, UAH and ADU were used as starting materials and their thermal decomposition was carried out at 450–625°C for 0–9 h at an air flow rate of 100 ml min^?1. α-UO₃, γ-UO₃, UO₃ - 2 H₂O, and their mixtures were obtained. The reduction of UO₃ gave β-UO_2+x with different x values from 0.030 to 0.055. The oxidation carried out at P_O2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO₂ - U₃O₇ (Step I) and U₃O₇ → U₃O₈ (Step II). TG and DSC curves of the oxidation showed two plateaus and two exothermic peaks corresponding to Steps I and II. In the case of two of the samples, the DSC peak of Step II split into two substeps, which were assumed to be due to the different reactivities of U₃O- formed from α-CO₃ and that from other types of UO₃. The increase in O/U ratio due to the oxygen uptake at room temperature changed from 0.010 to 0.042 except for a sample prepared from ADU which showed an extraordinarily large value of 0.445. TG curves showed an increase in O/U from room temperature to near 250°C for Step I and the plateau at 250–350°C where O/U was about 2.42, and showed a sharp increase in O/U above 350°C for Step II and the plateau above 100°C where O/U was 2.72–2.75. It is thought that the prepared UO₂ had a defective structure with a large interstitial volume to accommodate the excess oxygen.     

Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solution with ammonium hydroxide

Thermal decomposition of ammonium uranates precipitated from uranyl nitrate solutions on addition of aqueous ammonium hydroxide under various conditions has been examined by thermogravimetry (TG), differential thermal analysis (DTA), infrared spectroscopy and X-ray diffraction study. The TG curves of all precipitates show the weight-loss corresponding to the calculated value as UO₃·NH₃·H₂O. The DTA curves of the precipitates give the endotherms at about 130, 210 and 590 °C and the exotherms at 340–420 °C. As a result, it is found that ammonium uranates thermally decompose to amorphous UO₃ at about 400 °C, and transform to U₃O₈ via β-UO₃.     

Thermal decomposition of uranyl nitrate hexahydrate

TG-DTA-EGA studies have shown that anhydrous uranyl nitrate cannot be obtained by thermal decomposition of uranyl nitrate hexahydrate. Hydrolysis and polymerization of the salt during dehydration resulted in hydroxynitrates which decomposed in multiple steps with the evolution of oxides of nitrogen and water. The extent of hydrolysis dependend on the sample size, heating rate and nature of sample containment. Large samples on decomposition at relatively high heating rates showed evolution of nitric oxide even above 500°C. Infrared studies on the residues prepared at various temperatures supported the conclusions.     

Complexation of the carbonate, nitrate, and acetate anions with the uranyl dication: density functional studies with relativistic effective core potentials

The structures and vibrational frequencies of uranyl carbonates, [UO2(CO3)n](2-2n) and [(UO2)3(CO3)6]6-, uranyl nitrates, [UO2(NO3)n](2-n), and uranyl acetates, [UO2(CH3COO)n](2-n) (n = 1,2,3) have been calculated by using local density functional theory (LDFT). Only bidentate ligand coordination modes to the uranyl dication have been modeled. The calculated structures and frequencies are compared to available experimental data, including IR, Raman, X-ray diffraction, and EXAFS solution and crystal structure data. The energetics of ligand binding have been calculated using the B3LYP hybrid functional. In general, the structural and vibrational results at the LDFT level are in good agreement with experimental results and provide realistic pictures of solution phase and solid-state behavior. For the [UO2(CO3)3]6- anion, calculations suggest that complexity in the CO3(2-) stretching signature upon complexation is due to the formation of C=O and C-O domains, the latter of which can split by as much as 300 cm(-1). Assessment of the binding energies indicate that the [UO2(CO3)2]2- anion is more stable than the [UO2(CO3)3]4- anion due to the accumulation of excess charge, whereas the tri-ligand species are the most stable in the nitrate and acetate anions.     

Complexes of alkylenediphosphine dioxides with uranyl nitrate

Conclusions A number of complexes of tetraaryl (alkyl) alkylenediphosphine dioxides with uranyl nitrate have been synthesized. Coordination of both P=O groups with the uranyl ion is observed in the complexes of methylenediphosphine and cis-tetraarylvinylenediphosphine dioxides; in complexes of trans-tetraaryl (alkyl) vinylenediphosphines, only one P=O group is involved in complexing. Tetraphenylethylenediphosphine dioxide forms both types of complexes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1714–1721, August, 1978.We thank A. M. Rozen for discussing the results.     

The heat of dissolution of uranyl nitrate in aqueous nitrate solutions

Conclusions A model has been suggested to explain the observed relationship between the measured heats of dissolution of uranyl nitrate in aqueous nitrate solutions and the concentration of the salting-out agent. The model describes the change in the structure of water in the solution with change in its concentration. On the one hand, a destruction of the water structure by ions occurs, which is weakened with increase in the distance from the ion, and leads to such irregularity in the distribution of water molecules in the solution that the mean number of molecules of water in unit volume is increased with increase in the distance from the ions. In experiments on the heat of dissolution this increase leads to increased hydration of the uranyl cation and reduction in the endothermicity of the dissolution with increase in the concentration of the solution. On the other hand, an interaction occurs between the ions of the salting-out agent and the water molecules in the solution, leading to the opposite result: There is an increase in the mean number of water molecules of the solution in unit volume in the direction of these ions. In experiments on the heat of dissolution this is revealed in the dehydration of the uranyl cation, and correspondingly in an increase in the endothermicity of the dissolution with increase in the concentration of the solution. The proposed model is in harmony with data on vapor pressure above the solutions (the relationship between the activity coefficient of the water and the concentration of the solution).Translated from Zhurnal Strukturnoi Khimii. Vol. 3, No. 2, pp. 143–150, March–April. 1962     

DFT studies of uranyl acetate,carbonate, and malonate,complexes in solution

The aim of this work is to demonstrate that theoretical chemistry can be used as a complementary tool in determining geometric parameters of a number of uranyl complexes in solution, which are not observable by experimental methods. In addition, we propose plausible structures with partial geometric data from experimental results. A gradient corrected DFT methodology with relativistic effects is used employing a COSMO solvation model. The theoretical calculations show good agreement with experimental X-ray and EXAFS data for the triacetato-dioxo-uranium(VI) and tricarbonato-dioxo-uranium(VI) complexes and are used to assign possible geometries for dicalcium-tricarbonato-dioxo-uranium(VI) and malonato-dioxo-uranium(VI) complexes. The results of this exercise indicate that carbonate bonding in these complexes is mainly bidentate and that hydroxo bridging plays a critical role in the stabilization of the polynuclear uranyl complexes.     

Spectrophotometric determination of tetracyclines in pharmaceutical preparations, with uranyl acetate

Uranyl acetate is proposed as a reagent for the spectrophotometric determination of the tetracycline group of antibiotics. The reagent forms orange-red 1:1 complexes with the drugs in N,N-dimethylformamide medium. The complexes show absorption maxima at 414, 406, 419, 405 and 402 nm for tetracycline hydrochloride (TCH), oxytetracycline hydrochloride (OTCH), chlortetracycline hydrochloride (CTCH), doxycycline hydrochloride (DCH) and methacycline hydrochloride (MCH), respectively. Beer's law is valid over the concentration ranges 0–115, 0–120, 0–125, 0–135 and 0–110 μg/ml for TCH, OTCH, CTCH, DCH and MCH, respectively.     

Gas-phase uranyl, neptunyl, and plutonyl: hydration and oxidation studied by experiment and theory

The following monopositive actinyl ions were produced by electrospray ionization of aqueous solutions of An(VI)O(2)(ClO(4))(2) (An = U, Np, Pu): U(V)O(2)(+), Np(V)O(2)(+), Pu(V)O(2)(+), U(VI)O(2)(OH)(+), and Pu(VI)O(2)(OH)(+); abundances of the actinyl ions reflect the relative stabilities of the An(VI) and An(V) oxidation states. Gas-phase reactions with water in an ion trap revealed that water addition terminates at AnO(2)(+)·(H(2)O)(4) (An = U, Np, Pu) and AnO(2)(OH)(+)·(H(2)O)(3) (An = U, Pu), each with four equatorial ligands. These terminal hydrates evidently correspond to the maximum inner-sphere water coordination in the gas phase, as substantiated by density functional theory (DFT) computations of the hydrate structures and energetics. Measured hydration rates for the AnO(2)(OH)(+) were substantially faster than for the AnO(2)(+), reflecting additional vibrational degrees of freedom in the hydroxide ions for stabilization of hot adducts. Dioxygen addition resulted in UO(2)(+)(O(2))(H(2)O)(n) (n = 2, 3), whereas O(2) addition was not observed for NpO(2)(+) or PuO(2)(+) hydrates. DFT suggests that two-electron three-centered bonds form between UO(2)(+) and O(2), but not between NpO(2)(+) and O(2). As formation of the UO(2)(+)-O(2) bonds formally corresponds to the oxidation of U(V) to U(VI), the absence of this bonding with NpO(2)(+) can be considered a manifestation of the lower relative stability of Np(VI).     

Complexes of uranyl formate and acetate with mycobactin S

Interaction of solutions of uranyl formate or acetate with mycobactin S in anhydrous organic solvents leads to the formation of 11 complexes which can be isolated as dark orange, vitreous, non-hygroscopic solids. Aqueous extraction measurements with the²³²U-labelled complexes show that extensive dissociation occurs in acidic medium. On the other hand, the compounds are highly resistant to photolytic decomposition. Spectrophotometric characterization and structural considerations suggest that coordination geometry of the ionophore with UO ₂ ²⁺ is practically identical in both derivatives, and may impose sterical requirements different from those of other metalmycobactin complexes.     

Determination of free acidity in uranyl nitrate solutions

Two modifications of the existing method of determining free acidity in highly concentrated uranyl nitrate solutions by alkalimetric titrations in neutral potassium oxalate medium have been carried out to improve the reliability of the method. Free acidities of several synthetic solutions containing uranium and nitric acid in a wide range of concentration ratios were determined by all the three methods and compared with the results obtained by a more accurate ion exchange method. Interference from other hydrolyzable ions and the precision and accuracy were studied and compared.     

The gravimetric determination of uranium in uranyl nitrate

A study of the factors which affect the gravimetric determination of uranium in uranyl nitrate is described. In the gravimetry of uranium, the U₃O₈ (weighing form) produced by ignition is usually assumed to deviate <0.02% from theoretical composition; and elemental impurities are assumed to form common oxides within the U₃0₈ matrix. It is shown that these assumptions are incorrect. Ignition temperature and time affect U₃O₈ stoichiometry. Ignitions of uranyl nitrate for 1–3 h at 850° produce U₃O₈ that deviates as much as 0.15% from stoichiometric U₃O₈; deviations are negligible when uranyl nitrate is ignited at 1000° for 2 h. Elemental impurities, particularly calcium and phosphorus, affect the composition of U₃O₈ formed in the ignition of uranyl nitrate. A variety of impurity complexes such as uranates and phosphates are found within the U₃O₈ matrix. Formation of these impurity complexes depends on the elements present, their concentration, and ignition temperature. Therefore, in the gravimetric determination of uranium in uranyl nitrate, the effects of ignition parameters and nonvolatile impurities must be considered in order to obtain accurate uranium determinations.     

Complexes of uranium and thorium tetrachlorides and uranyl nitrate withN-methyl-N-ethylO-ethylcarbamate

Summary TheN-methyl-N-ethylO-ethylcarbamate ligand (L) has been synthesized and its UCl₄4·2 L, UO₂(NO₃)₂ · 2 L and ThCl₄·3L complexes isolated. In addition, the UCl₄4-2L complex (where L=N,N-diethylO-ethylcarbamate) is described. From the i.r. spectra, it is clear that the ligands bind to the metal through the carbonyl oxygen atom. The¹H n.m.r. spectra of ligands and complexes are reported and discussed.     

Raman studies of uranyl nitrate and its hydroxy bridged dimer

In this paper, the authors report Raman spectra obtained on imidazolium di-μ-hydroxybis[dioxobis-(nitrato)uranium(VI)], (UO₂(NO₃)₂(OH))₂.2C₃H₅N₂ (IUNH). An assignment of the Raman bands is made by comparing the spectrum of IUNH with those of uranyl nitrate hexahydrate (UNH) and imidazole (IMID). The electron charge transfer from the imidazole ring to the uranyl ion has been empirically determined.     

Synthetic strategy of new powerful tris-bisphosphonic ligands for chelation of uranyl, iron, and cobalt cations

New tripodal uranyl ion chelators containing gem-bisphosphonic units have been synthesized. All bisphosphonic units present a side chain with 0, 1, or 2 methylene group terminated by -NH₂ or -CO₂H group. These units were respectively coupled with a -CO₂H or -NH₂ functions of a suitable tri-functional platform. The shape and size of the new designed ligands were selected and validated through computer molecular modelization.