Forms of Rh(III) in Nitric Acid Solutions

The forms of Rh(III) in nitric acid solutions are studied by ¹⁴N, ¹⁵N, ¹⁷O, and ¹⁰³Rh NMR and electronic absorption spectra. At HNO₃concentrations below 5 mol/l, rhodium mainly exists as low-nuclear oligomers with a bridging fragment Rh(-OH)(-ONO₂)Rh; terminal positions of the central atoms are occupied by water molecules. It was found that the isotopic equilibrium ¹⁴NO^– ₃/¹⁵NO^– ₃of the bridging ligand sets in at an abnormally high rate, at which the isotopic exchange H₂ ¹⁷O/H₂ ¹⁶O of the terminal ligands also occurs in the dimers and trimers. The formation of low-nuclear oligomers is a common feature of Rh(III) complexes in aqueous solutions with ligand deficiency in the system. The possibility of isolating rhodium from nitric acid solutions using different methods is predicted.     

Pyrrhotite Electrooxidation in Acid Solutions

The electrooxidation of pyrrhotite, including that preliminarily etched in acid solutions, is studied by voltammetry, scanning electron microscopy, and X-ray electron spectroscopy. The oxidation mechanism includes the formation; reversible oxidation (which predominantly involves an increase in the amount of polysulfide ions) of a near-surface nonequilibrium metal-deficient layer (NL); and the layer destruction. The pyrrhotite passivation is caused not by a thick NL, but rather by a thin layer that contains oxidized iron, probably by a top, oxygen-containing zone of NL.     

Isolation of Rare Earth Element's Radionuclides from Phosphoric Acid Solutions on Cerium (III) Phosphate

Abstract

The isolation of the lanthanide phosphates by crystallization from the solutions of phosphoric acid with the concentration of 2–5 mol·dm^?3, produced during destruction of the apatite, was investigated. The kinetic parameters of crystallization of the lanthanide phosphates and the values of their solubility in phosphoric acid with various impurities under temperatures between 60–90°C have been obtained from the data on distribution of Ce¹⁴⁴ and Eu^152–154 radio- nuclides between the solution and the solid phase. The wide region of supersaturated solution metastability has been determined. The possibility to remove supersaturation in the metastable region by introduction of the cerium (III) phosphate seeds has been proved. Separation of the lanthan-ides from calcium and other accompanying elements in the apatites by crystallization of the phosphates on the CePO₄ ·0.5H₂O seed in the region of small supersaturation of strongly acid solutions has been studied. Calcium phosphate demonstrates the “salting out” effect on the lanthanide phosphates. By thermodynamic computations the ionic compositions of the produced solutions from breaking down the apatite and the solubilities of the lanthanide phosphates were obtained, which agrees with the experimental data. The distribution coefficients for the solid phase and the liquid phase are 1.1.10⁴ for cerium and 4–10³ for europeum. The lanthanides/calcium separation coefficient is 1.3·10³.     

Solubility of Lead Nitrate in Aqueous Solutions of Nitric Acid and Iron(III) Nitrate

Solubility of lead(II) nitrate in the system H₂O-Fe(NO₃)₃-HNO₃ was studied using the simplex-lattice method of experiment design.     

Electrodeposition of Rhodium on Cathodes of Carbon-Fiber Material from Rh(III) Complexes in Nitric Acid Solutions

Rhodium electrodeposition on cathodes of a carbon-fiber material from nitric acid solutions containing various complex forms of Rh(III) is studied. Depending on the Rh(III) form, the deposition degree varies by several times, all other factors being the same. The charge of a complex species prevailing in solution is presumably a factor determining the electrochemical behavior of Rh(III).     

Reduction of Np(VI) in Irradiated Solutions of Nitric Acid

The reliable separation of neptunium from dissolved nuclear fuel assumes the ability to maintain a preferred oxidation state. However, regardless of its initial redox speciation, a series of reactions occurs in nitric acid to create a mixture of oxidation states including Np(V), Np(VI) and sometimes Np(IV). To further complicate the situation, irradiated solutions such as fuel dissolution contain both transient and long-lived radiolysis products which may be strongly oxidizing or reducing. Thus, irradiation may be expected to impact the equilibrium distributions of the various neptunium valences.We have irradiated nitric acid solutions of neptunium with ⁶⁰Co gamma-rays, and measured radiolytically-induced changes in neptunium valences, as well as the nitrous acid concentration, by UV/Vis spectroscopy. It was found that in 4 M HNO3 at low absorbed doses, the oxidizing radicals oxidized Np(V) to Np(VI). However, as the irradiation proceeded the concentration of nitrous acid became sufficient to reduce Np(VI) to Np(V), and then continued irradiation favored this reduction until an equilibrium was achieved in balance with the oxidation of Np(V) by nitric acid itself. The starting abundances of the two neptunium valences did not affect the final equilibrium concentrations of Np(V) and Np(VI), and no Np(IV) was detected.     

State of Rh(III) in Hydrofluoric Acid Solutions

A simple method is developed for synthesizing [Rh(H₂O)₆]F₃. 3H₂O with a yield of 80–90%. ¹⁹F, ¹⁰³Rh, and ¹⁷О NMR spectroscopic studies show that the following three processes simultaneously run in the Rh(III)–HF/K–H₂O system via parallel routes: the formation of mononuclear aquafluoro complexes [Rh(H₂O)₆]³⁺ + F–→ [Rh(H₂O)₅F]²⁺ + H₂O; the formation of aquahydrofluoro complexes [Rh(H₂O)₆]³⁺ + HF₂^-→ [Rh(H₂O)₅HF₂]²⁺ + H₂O; and hydrolysis of the aqua ion followed by coordination of fluoride ion and condensation of the hydroxo species [Rh(H₂O)₆]³⁺ + 2F^– → [Rh(H₂O)₄(OH)F]⁺ + HF → condensation. [Rh(H₂O)₆]³⁺ and [Rh(H2O)5F]²⁺ are the two species making a major contribution to the material balance at high acidity under equilibrium conditions. Parameters of the ¹⁹F NMR spectra of individual complex species are presented.     

Sorption Recovery of Transplutonium Elements from Nitric Acid Solutions with Arsenazo Group Reagents

Sorption recovery of actinides and lanthanides from nitric acid solution with arsenazo group reagents, activated carbon modified with these reagents, and chelating sorbents containing arsenazo functional groups was studied.     

Spectrophotometric Studies of Cerium(III) and Thorium(IV) Chelates of Diethylenetriaminepentaacetic Acid (DTPA)

The interaction between diethylenetriaminepentaacetic acid (DTPA or H_sZ) and Ce(III) and Th(IV) ions has been investigated spectrophotometrically in aqueous solution at an ionic strength of 0.1 and for various temperatures. It has been found that the Ce(III)-DTPA chelate (1:1) exhibited a characteristic absorption maximum at 297 nm, and the optimum pH range is between 3.4 to 4.4. The absorption of Ce(III)-DTPA chelate is considerably diminished by adding small amounts of Th(IV) ions. This phenomenon was used to evaluate the formation constant of Th(IV)-DTPA chelate (1:1). The formation constants and the thermodynamic properties characterizing the formation of the chelates have been calculated at 25°. The results are as follows:      

The Thermokinetics of the Formation Reactions of Cerium(III) n-dodecylbenzene Sulfonate and Cerium(III) Stearate

The thermokinetics of the formation reactions of cerium(III) n-dodecylbenzene sulfonate and cerium(III) stearate are studied by using a microcalorimeter. On the basis of experimental and calculated results, three thermodynamics parameters (the activation enthalpies, the activation entropies, the activation free energies), the rate constant, three kinetic parameters (the activation energies, the pre-exponential constant and the reaction order) and the enthalpies of the reaction of preparing cerium(III) n-dodecylbenzene sulfonate in the temperature range of 20–35°C and cerium(III) stearate in the temperature range of44.6–62.8°C are obtained. The results showed that the title reactions easily took place in the studied temperature. This revised version was published online in July 2006 with corrections to the Cover Date.     

14N, 17O, 103Rh NMR Studies of Rh(III) State in Diluted Nitric Acid Solutions

The state of rhodium(III) in nitric acid solutions diluted with 3M HNO₃ or water was studied by ¹⁴N, ¹⁷O, and ¹⁰³Rh NMR methods. The ¹⁰³Rh NMR chemical shifts significantly depend on the ionic background of the solution. Concentrated nitric acid solutions (c _Rh = 0.5–1 mol/l) diluted 100 to 200 times retain the polynuclear Rh(III) forms with double (-NO₃, -OH) bridges. The predominant form in the diluted solutions is a dimer.     

Solvoluminescence of Cerium(III) Thiocyanate Complex

A cerium(III) thiocyanate complex shows bright-blue emission at approximately 450 nm in acetonitrile, the quantum yield of which reaches more than 40 % at 298 K. Non-coordinating solvents such as acetonitrile give blue emission whereas oxygen-coordinating and nitrogen-coordinating solvents afford near UV and green emissions, respectively.     

Spectrophotometric Studies of Cerium(III) Chelate of 7-Iodo-8-Hydroxyquinoline-5-Sulphonic Acid

Cerium(III) chelate of 7-iodo-8-hydroxyquinoline-5-sulphonic acid(ferron) has been studied spectrophotometrically in aqueous solution at 25°C and at an ionic strength of 0.1M. The formation of this chelate was pH dependent, and the optimum pH range was between 5.5 to 6.0. Its mole ratio of ligand to cerium(III) ion was found to be 2 to 1 stoichiometry and the formation constant, logK, was determined as 10.15. Enthalpy and entropy changes characterizing the formation of the chelate have been calculated as follows: ΔG° =-13.85kcal mole^-1, ΔH°= ?5.03 kcal mole^-1, ΔS°=29.56 eu mole^-1. By using the wavelength of 370 nm, determination of trace amount of cerium(III) ion with the sensitivity of 0.056 was possible.     

Extraction of Nitric Acid from Chloride-Nitrate Solutions

The extraction of nitric acid from chloride-nitrate solutions simulating solutions that result from eudialyte breakdown with hydrochloric acid after extractive recovery of rare-earth elements was studied. A general flowsheet was suggested for the extraction of rare-earth elements from chloride solutions and utilization of nitrate ions as concentrated solutions of nitrates used in recycling.     

Thermodynamic Analysis of Cerium(III) Extraction from Sulfate Solutions with Salts of Quaternary Ammonium Bases

Cerium(III) distribution between an aqueous sulfate solution and a solution of trialkylbenzylammonium sulfate in xylene was studied. The Ce(III) distribution coefficient grows with increasing pH. The extraction is provided by the formation of cerium mono- and disulfate complexes in the aqueous phase. Schemes of extraction equilibria are proposed and Gibbs energies and equilibrium constants are found.     

Cerium(III/IV) Formamidinate Chemistry,and a Stable Cerium(IV) Diolate

Four new cerium(III) formamidinate complexes comprising [Ce(p‐TolForm)₃], [Ce(DFForm)₃(thf)₂], [Ce(DFForm)₃], and [Ce(EtForm)₃] were synthesized by protonolysis reactions using [Ce{N(SiMe₃)₂}₃] and formamidines of varying functionality, namely N,N′‐bis(4‐methylphenyl)formamidine (p‐TolFormH), N,N′‐bis(2,6‐difluorophenyl)formamidine (DFFormH), and the sterically more demanding N,N′‐bis(2,6‐diethylphenyl)formamidine (EtFormH). The bimetallic cerium lithium complex [LiCe(DFForm)₄] was synthesized by treating a mixture of [Ce{N(SiHMe₂)₂}₃(thf)₂] and [Li{N(SiHMe₂)₂}] with four equivalents of DFFormH in toluene. Oxidation of the trivalent cerium(III) formamidinate complexes by trityl chloride (Ph₃CCl) caused dramatic color changes, although the cerium(IV) species appeared transient and reformed cerium(III) complexes and N′‐trityl‐N,N′‐diarylformamidines shortly after oxidation. The first structurally characterized homoleptic cerium(IV) formamidinate complex [Ce(p‐TolForm)₄] was obtained through a protonolysis reaction between [Ce{N(SiHMe₂)₂}₄] and four equivalents of p‐TolFormH. [Ce{N(SiHMe₂)₂}₄] was also treated with DFFormH and EtFormH, but the resulting cerium(IV) complexes decomposed before isolation was possible. The new cerium(IV) silylamide complex [Ce{N(SiMe₃)₂}₃(bda)_0.5]₂ (bda=1,4‐benzenediolato) was synthesized by treatment of [Ce{N(SiMe₃)₂}₃] with half an equivalent of 1,4‐benzoquinone, and showed remarkable resistance towards protonolysis or reduction.     

Cerium(III) chloride-mediated reactions of sulfonamide dianions

Presented here is the first report on the ability of cerium(III) chloride to mediate high-yielding and, oftentimes, highly diastereoselective additions of N-benzyl-alpha, N-dilithio methanesulfonamide to aldehydes and ketones of biological importance. Smooth addition was effected to base-sensitive substrates such as Fmoc-protected alaninal, citral, 5-cholesten-3-one, uridine 5'-aldehyde, 3'-ketouridine, and 3'-ketothymidine. The reaction was chemoselective for aldehydes in the presence of nitriles. Acetoxy groups are labile and thus not suitable protecting groups for alcohols under these conditions. N-Benzyl-alpha, N-dilithio methanesulfonamide was found to be of sufficient basicity to cause enolate formation with sensitive substrates, such as 1-phenylacetone. However, the addition of cerium(III) chloride mediated the basicity of the dianion and suppressed enolate formation in these cases. Further, cerium(III) has general utility for the addition of various N-aliphatic/aromatic methanesulfonamide dianions to 3'-ketouridine.     

Delayed Doublet Emission in a Cerium(III) Complex

Doublet emission from open-shell molecules has demonstrated its research and application value in recent years. However, understandings of the photoluminescence mechanism of open-shell molecules are far less than that of closed-shell molecules, leading to challenges in molecular design of efficient doublet emission systems. Here we report a cerium(III) 4-(9H-carbozol-9-yl)phenyl-tris(pyrazolyl)borate complex Ce(CzPhTp)₃ with a new luminescence mechanism of delayed doublet emission, which also represents the first example with metal-centered delayed photoluminescence. The energy gap between the doublet and triplet excited states of Ce(CzPhTp)₃ is reduced by the management of the inner and outer coordination spheres, thereby promoting efficient energy transfer between the two excited states and activating the delayed emission. The photoluminescence mechanism discovered may provide a new way for the design of efficient doublet emission and bring insights into rational molecular design and energy level regulation in open-shell molecules.     

Kinetics and Mechanism of Iridium（Ⅲ）-Catalyzed Oxidation of Ethanol Amine by Cerium（ Ⅳ ） in Sulfuric Acid Media

In this study, the kinetics and mechanism of the iridium(Ⅲ)-catalyzed oxidation of ethanol amine(EAN) by cerium(Ⅳ) in a sulfuric acid medium was investigated using titrimetric technique of redox in a temperature range of 298—313 K. It was found that the reaction is of first order with respect to Ce(Ⅳ) and Ir(Ⅲ), and a positive fractional order with respect to EAN. It was also found that the pseudo-first-order([EAN][Ce(Ⅳ)]) rate constant kobs decreases with the increase of [H ] and [HSO-4]. Under the protection of nitrogen gas, the reaction system can initiate the polymerization of acrylonitrile, indicating the generation of free radicals. On the basis of the experimental results, a suitable mechanism was proposed. From the dependence of kobs on the concentration of hydrogen sulfate, Ce(SO4)2 was found to be the kinetically active species. The rate constants of the rate-determining step together with the activation parameters were evaluated.