Comparison of numerical and physicochemical models for spectrophotometric monitoring of uranium concentration

On-line spectrophotometric monitoring of nuclear-fuel reprocessing streams requires a physicochemical model suitable for predicting uranium and nitric acid concentrations in a uranyl nitrate/nitric acid system. The effects of uranium, nitrate and hydrogen ion concentrations and ionic strength on the complexation equilibria of uranium(IV) with nitrate are described. Molar absorptivities for the uranium mononitrate and dinitrate complexes between 410 and 440 nm are given. The apparent equilibrium constants are evaluated as a function of the ionic strength. The limitations of this predictive model are emphasized and comparisons with numerical models are discussed.     

Binding constant determination of uranyl–citrate complex by ACE using a multi‐injection method

The binding constant determination of uranyl with small‐molecule ligands such as citric acid could provide fundamental knowledge for a better understanding of the study of uranyl complexation, which is of considerable importance for multiple purposes. In this work, the binding constant of uranyl–citrate complex was determined by ACE. Besides the common single‐injection method, a multi‐injection method to measure the electrophoretic mobility was also applied. The BGEs used contained HClO₄ and NaClO₄, with a pH of 1.98 ± 0.02 and ionic strength of 0.050 mol/L, then citric acid was added to reach different concentrations. The electrophoretic mobilities of the uranyl–citrate complex measured by both of the two methods were consistent, and then the binding constant was calculated by nonlinear fitting assuming that the reaction had a 1:1 stoichiometry and the complex was [(UO₂)(Cit)]^?. The binding constant obtained by the multi‐injection method was log K = 9.68 ± 0.07, and that obtained by the single‐injection method was log K = 9.73 ± 0.02. The results provided additional knowledge of the uranyl–citrate system, and they demonstrated that compared with other methods, ACE using the multi‐injection method could be an efficient, fast, and simple way to determine electrophoretic mobilities and to calculate binding constants.     

Modelling and Optimization of Stability Constants of Cadmium or Zinc with Biological Buffers (DIPSO or TAPS) in Aqueous Solutions by Electrochemical Techniques

The complexation behavior of four systems involving cadmium(II) or zinc(II) in aqueous solutions with the biological buffers 3-[N,N-bis(2-hydroxyethyl)amino]-2-hydroxypropanesulfonic acid (DIPSO), and [(2-hydroxy-1,1-bis(hydroxymethyl)ethyl)amino]-1-propanesulfonic acid (TAPS) was studied by direct current polarography (DCP) and glass electrode potentiometry (GEP), at 25.0 ± 0.1 °C and ionic strength 0.1 mol·dm^?3 KNO₃. Except for the Cd–TAPS system, for which full characterization of the system was possible either by DCP or GEP, full characterization of the other metal-buffer systems (Zn–DIPSO, Zn–TAPS and Cd–DIPSO) was only possible using DCP. For Zn-buffers systems, ZnL⁺ and $ {\text{ZnL(OH)}}_{2}^{ - } $ ZnL(OH) 2 ? (where L stands for buffer) were identified. For the Zn–DIPSO system, the overall stability constant values (as log₁₀ β) are 2.1 ± 0.2 and 13.4 ± 0.2, respectively. For the Zn–TAPS system, the overall stability constants values (as log₁₀ β) are 2.4 ± 0.1 and 12.9 ± 0.3, respectively. For the Cd–DIPSO system, the overall stability constants values (as log₁₀ β) of CdL⁺ and CdL(OH) are 2.9 ± 0.1 and 6.9 ± 0.3, respectively. For the Cd–TAPS system, only the species CdL⁺ was identified with log₁₀ β = 2.5 ± 0.1.     

A Spectrophotometric Study on Uranyl Nitrate Complexation to 150 °C

The formation constant of the mononitratouranyl complex was studied spectrophotometrically at temperatures of 25, 40, 55, 70, 100 and 150 °C (298, 313, 328, 343, 373 and 423 K). The uranyl ion concentration was fixed at approximately 0.008 mol⋅kg⁻¹ and the ligand concentration was varied from 0.05 to 3.14 mol⋅kg⁻¹. The uranyl nitrate complex, UO₂NO₃⁺, is weak at 298 K but its equilibrium constant (at zero ionic strength) increases with temperature from log ₁₀ β ₁=−0.19±0.02 (298 K) to 0.78±0.04 (423 K).     

Structural correspondence between uranyl chloride complexes in solution and their stability constants

Pair-distribution functions (PDF)s were obtained from high-energy X-ray scattering (HEXS) data on a series of uranyl solutions as a function of chloride ion concentration. Analyses reveal that chloride forms only inner-sphere complexes with the uranyl, replacing inner-sphere waters such that the total uranyl coordination number decreases from 4.7 waters at [Cl(-)] = 0 m to 4.4 (1.7 water and 2.7 Cl(-)) at [Cl(-)] = 6.8 m. Some of the second-coordination sphere waters reorient upon uranyl inner-sphere chloride complexation in order to hydrogen bond with the bound anion. Similar data obtained on a series of solutions maintained at constant ionic strength are used to confirm structural assignments through determining stability constants for the addition of chloride to uranyl and comparison with published values. The stability constants, β(1) = 1.5(10) m(-1), β(2) = 0.8(4) m(-2), and β(3) = 0.4(1) m(-3), obtained in a series of solutions with constant ionic strength of 5.3 m, are in reasonable agreement with previously published results determined by solvent extraction. The agreement of stability constants supports our peak assignments for the PDF and thus our structural model for uranyl chloride complexes in solution. Using coordination numbers and speciation determined here as a function of chloride ion concentration, the monochloro species is found to have four coordinating waters in the uranyl equatorial plane, the dichoro species is found to be an equilibrium of three and two coordinating waters, and the trichloro species has only a single water in the equatorial plane. These values correspond to total average coordination numbers of 5, 4.3, and 4 for the mono-, di-, and trichlorouranyl complexes. From the equilibrium value of the dichloro species, it can be further estimated that ΔG = -0.5 kcal/mol for the conversion of five to four coordinate species. Overall, the HEXS data support the assertion that uranyl chloride correlations do exist and the results are not simply the result of solvent-ion effects.     

Paper electrophoretic technique in the study of metal complexes

A new method, paper electrophoresis, involving the use of a ionophoretic technique is described for the study of equilibria in binary complex system in solution. The stability constants of ML and ML₂ complex species of metal(II)-α-aminobutyric acid and metal(II) — homoserine were found to be [(8.09 ± 0.03, 6.89 ± 0.09) (3.58 ± 0.07, 2.67 ± 0.11) (7.66 ± 0.06, 6.13 ± 0.02)]; [(7.80 ± 0.07, 6.45 ± 0.02) (2.93 ± 0.04, 1.97 ± 0.01) (7.41 ± 0.011, 4.67 ± 0.06)] for copper(II), manganese(II) and uranyl(II) complexes, respectively at an ionic strength 0.1 M and 35°C. Published in Russian in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1196–1199. The article is published in the original.     

Electrophoretic studies on the mixed nitrilotriacetate-penicillamine complexes of Pb2+ and UO 2 2+

A technique involving the use of paper electrophoresis is described for the study of binary and mixed-complex systems in solution. This technique is based on the movement of a spot of metal ion in an electric field with the complexants added in the background electrolyte at pH 8.5. The concentration of primary ligand (nitrilotriacetate) was kept constant, while that secondary ligand (penicillamine) was varied. A graph of log[penicillamine] versus mobility was used to obtain information on the mixed complexes and to calculate stability constants. The stability constants of mixed-ligand complexes lead(II)-nitrilotriacetate-penicillamine and uranyl(II)-nitrilotriacetate-penicillamine have been found to be 5.68 ± 0.09 and 6.56 ± 0.05 (logarithm stability constant values), respectively, at ionic strength 0.1 M and a temperature of 35°C. The text was submitted by the author in English.     

Composition and stability constants of copper(II) complexes with succinic acid determined by capillary electrophoresis

The advantage of capillary electrophoresis was demonstrated for studying a complicated system owing to the dependence of direction and velocity of the electrophoretic movement on the charge of complex species. The stability constants of copper(II) complexes with ions of succinic acid were determined by capillary electrophoresis, including the 1?:?2 metal to ligand complexes which are rarely mentioned. The measurements were carried out at 25 °C and ionic strength of 0.1, obtained by mixing the solutions of succinic acid and lithium hydroxide up to pH 4.2–6.2. It was shown that while pH was more than 4.5 the zone of copper(II) complexes with succinate moves as an anion. It is impossible to treat this fact using only the complexes with a metal-ligand ratio of 1?:?1 (CuL⁰, CuHL⁺). The following values of stability constants were obtained: log β(CuL) = 2.89 ± 0.02, log β(CuHL⁺) = 5.4 ± 0.5, log β(CuL₂^2?) = 3.88 ± 0.05, log β(CuHL₂^?) = 7.2 ± 0.3.     

Solvent extraction of uranyl by N,N,N′,N′-tetraoctylsuccinylamide from nitric acid solution

Synthesis and characterization of N,N,N′,N′-tetraoctylsuccinylamide (TOSA) was carried out and used for extraction of U(VI) from nitric acid solutions. The effect of different factors affecting the extraction distribution ratio (TOSA concentration, concentrations of nitric acid, salting-out agent LiNO₃ concentration, equilibration time, temperature and effect of diluents) have been investigated. The results obtained indicated that TOSA have a great capability to extract uranyl with kerosene-1,3,5-trimethylbenzene than other diluents, it have a high extraction distribution ratios when the concentration of TOSA is lower and not found the third matter. It was found that the main extracted species is UO₂(NO₃)₂·TOSA. The apparent equilibrium constant of extraction determined is (2.32 ± 0.31) L³/mol³ at (298 ± 1) K. The enthalpy of extraction is ?35.20 ± 0.352 kJ/mol.     

Some Thermodynamic Properties of Aqueous 2-Mercaptopyridine-N-Oxide (Pyrithione) Solutions

The thermodynamic properties of 2-mercaptopyridine-N-oxide (pyrithione, PT) were studied potentiometrically in NaCl aqueous solutions at different ionic strengths and temperatures. A set of protonation constants is provided, together with distribution (water/2-methyl-1-propanol) and solubility data. The total and the specific solubility (solubility of neutral species) values of pyrithione were determined and, for example, are 0.0561 and 0.0518 mol·dm^?3 at c _NaCl = 0.244 mol·dm^?3 and T = 298.15 K. By fitting the distribution and solubility results against the ionic strength, the Setschenow coefficient and the activity coefficients of the neutral species were determined. In pure water, the specific solubility is log₁₀ \( S_{m 0}^{0} = \, {-} 1. 20 \, \pm \, 0.0 4 \) . To determine the activity coefficient of the charged species and the protonation constant at infinite dilution, the data were analyzed by different models, namely the Debye–Hückel type equation, the SIT (Specific ion Interaction Theory) and the Pitzer approach. The interaction coefficient of the deprotonated pyrithione species was determined [ε(Na⁺, PT^?) = ?0.105 ± 0.002]. The protonation enthalpy was also determined, is slightly positive, and the protonation process is entropic in nature. At infinite dilution and T = 298.15 K, log₁₀ K ^H0 = 4.620 ± 0.002, ΔG ⁰ = –26.4 ± 0.1 kJ·mol^?1, ΔH ⁰ = 2.1 ± 0.5 kJ·mol^?1 and TΔS ⁰ = 28.5 ± 0.5 kJ·mol^?1. The electrochemical behavior of pyrithione was studied in NaCl solutions at T = 298.15 K. It was found that voltammetry can be used to study the binding ability of pyrithione towards metal cations. The results of this work are in agreement with literature findings and improve the knowledge of the chemistry of pyrithione in aqueous solutions.     

Interaction of uranyl with Se(IV) and Se(VI) in aqueous acid solutions by means of capillary electrophoresis

The uranyl–selenium(IV) and uranyl–selenium(VI) interactions were studied by CE in aqueous acid solutions, containing U(VI) and Se(IV) or Se(VI) at different concentrations, at pH 1.5, 2.0 and 2.5. The method proposed in this paper allows one with the use of CE data on metal ion mobilities at different pHs to establish the ligand species interacting with metal ion and complex species formed. In the case of Se(VI) a selenate, as demonstrated, interacts with uranyl ions, in the case of Se(IV) this is a hydroselenite. It was also shown that the equilibria for the U(VI)–Se(VI) and U(VI)–Se(IV) systems can be established from CE data. The formation of UO₂SeO₄, UO₂(SeO₄), UO₂HSeO and UO₂(HSeO₃)₂ species is demonstrated. The stability constant values were measured at different ionic strengths (from 0.02 to 0.2 mol/L). The logarithms of the stability constant values (β°) extrapolated to ionic strength 0 by the specific ion interaction theory (SIT) are found to be log β°₁=2.93±0.06 for UO₂SeO₄ formation, log β°₂=4.030.18 for UO₂(SeO₄) formation, log β°₁=3.270.15 for UO₂HSeO formation and log β°₂=5.510.11 for UO₂(HSeO₃)₂ at 25°C. The results for the first constant values for each of systems are consistent with the published values. For UO₂(SeO₄) formation, a new constant stability value is given. The existence of UO₂(HSeO₃)₂ complex species is demonstrated and its constant stability value is given for the first time.     

Thermodynamics of uranyl complexes with threonine and hydroxyproline

The stability constants of uranyl complexes with threonine and hydroxyproline were studied in aqueous solution at 25 and 45°C by the Calvin-Bjerrum technique. Thermodynamic stability constants have been obtained by extrapolation of the values at various ionic strengths. The values of stepwise changes in ΔG, ΔH and ΔS have been reported.     

Fluorescence spectroscopic study on complexation of uranium(VI) by glucose: a comparison of room and low temperature measurements

Cryogenic techniques are currently used in scanning tunnelling microscopy (STM) and single molecule spectroscopy. Recently such cryogenic devices have also been adapted to time resolved laser-induced fluorescence spectroscopy (TRLFS) systems applied to uranium(VI). In our study, we interpret TRLFS results obtained for the uranyl(VI) glucose system at room temperature (RT) and under cryogenic conditions of 153 K (cryo-TRLFS). A uranyl(VI) glucose complex was only identified by cryo-TRLFS measurements at pH 5 and not by RT measurements. The uranyl(VI) glucose complex was characterized by five emission bands at 499.0, 512.1, 525.2, 541.7, and 559.3 nm and a fluorescence lifetime of 20.9 ± 2.9 μs. The uranyl(VI) glucose complex formation constant was calculated for the first time to be logß_I=0.1 M = 15.25 ± 0.96. Cryo-TRLFS investigation opens up new possibilities for the determination of complex formation constants since interfering quenching effects often encounter at RT are suppressed by measurements at cryogenic conditions.     

Complexation of Molybdenum(VI) with GLDA at Different Ionic Strengths

The interaction of the biodegradable ligand, l-glutamic acid N,N-diacetic acid tetrasodium salt (GLDA) with molybdenum(VI) was studied by determining stability constants at pH 6.00, T?=?298.15 K, and ionic strength 0.0992?<?I/mol·dm^?3?<?2.5689 of sodium chloride. The ionic strength dependence of the stability constants was fitted to both extended Debye–Hückel and specific ion interaction models. Job’s method confirmed the formation of one species, MoO₃GLDA^4?. The values of the stability constants are in agreement with the other data in the literature for the complex formation of aminopolycarboxylic acids with molybdenum(VI). Experimental data were obtained by using UV spectrophotometric method. The formation constant in pure water is 18.96?±?0.08 on the molal concentration scale.     

Complexation of Am(III) and Nd(III) by 1,10-Phenanthroline-2,9-Dicarboxylic Acid

The complexant 1,10-phenanthroline-2,9-dicarboxylic acid (PDA) is a planar tetradentate ligand that is more preorganized for metal complexation than its unconstrained analogue ethylendiiminodiacetic acid (EDDA). Furthermore, the backbone nitrogen atoms of PDA are aromatic, hence are softer than the aliphatic amines of EDDA. It has been hypothesized that PDA will selectively bond to trivalent actinides over lanthanides. In this report, the results of spectrophotometric studies of the complexation of Nd(III) and Am(III) by PDA are reported. Because the complexes are moderately stable, it was necessary to conduct these titrations using competitive equilibrium methods, competitive cation complexing between PDA and diethylenetriaminepentaacetic acid, and competition between ligand protonation and complex formation. Stability constants and ligand protonation constants were determined at 0.1 mol·L^?1 ionic strength and at 0.5 mol·L^?1 ionic strength nitrate media at 21 ± 1 °C. The stability constants are lower than those predicted from first principles and speciation calculations indicate that Am³⁺ selectivity over Nd³⁺ is less than that exhibited by 1,10-phenanthroline.     

Stability Constants and Spectroscopic Properties of Thorium(IV)–Arsenazo III Complexes in Aqueous Hydrochloric Medium

The complexation characteristics of thorium–arsenazo III in the range of 1–6 mol·L^?1 hydrochloric acid media were investigated by UV–Vis absorption spectroscopy and computational analysis. The chemical equilibrium model of thorium–arsenazo III complexation was established including the species distribution of arsenazo III, the formation of thorium chloride species, and the release of protons from thorium–arsenazo III complexes. In the spectra of thorium–arsenazo III complexes, two characteristic absorption peaks were observed at 610 and 660 nm, and the latter peak showed a tendency to shift about 4 nm to higher wavelength as the acidity of the hydrochloric acid media increased from 1 to 6 mol·L^?1. Analysis of the experimental data indicates that the molar absorptivities of both 1:1 and 1:2 complexes (thorium to arsenazo III) steadily increase as the acidity of medium increases. The determined stability constants of 1:1 and 1:2 complexes at various concentrations of hydrochloric acid were extrapolated to zero ionic strength, based on the specific ion interaction theory (SIT) approach. The limiting stability constants were determined to be \( { \log }_{10} \beta_{11}^{\text{o}} \) = 8.56 ± 0.13 and \( {\log}_{10} \beta_{12}^{\text{o}} \) = 15.17 ± 0.18 with ion interaction coefficients of Δε ₁₁ = –0.57 ± 0.02 kg·mol^?1 and Δε ₁₂ = –0.60 ± 0.04 kg·mol^?1, respectively.     

Silicate complexation of NpO<Subscript>2</Subscript><Superscript>+</Superscript> ion in perchlorate media

Complexation behavior of NpO₂ ⁺ with ortho-silicic acid (o-SA) has been studied using solvent extraction at ionic strengths varying from 0.10 to 1.00M (NaClO₄) at pcH 3.68±0.08 and 25 °C with bis-(2-ethylhexyl) phosphoric acid (HDEHP) as the extractant. The stability constant value (log β₁) for the 1:1 complex, NpO₂(OSi(OH)₃), was found to decrease with increase in ionic strength of the aqueous phase [6.83±0.01 at I=0.10M to 6.51±0.02 at I = 1.00M]. These values have been fitted in the SIT model expression and compared with similar values of complexation of the metal ions Am³⁺, Eu³⁺, UO₂ ²⁺, PuO₂ ²⁺, Np⁴⁺, Ni²⁺ and Co²⁺. The speciation of NpO₂ ⁺-o-silicate/carbonate system has been calculated as a function of pcH under ground water conditions. On leave from Radiochemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400 085, India.     

31P-NMR study of the organophosphorous complexes of uranium(VI)

The intermolecular ligand exchange in uranyl nitrate complexes with TBP and TOPO is studied by³¹P-NMR. The constant rates at 25°C in CCl₄ are: (8.47±1.86)·10³ s^?1 for U-TBP and (1.3±0.04)·10⁴M^?1·s^?1 for U-TOPO system. The very similar activation parameters values of the ligand exchange suggest the same mechanism for both systems, namely an one-step interchange mechanism. The differences between the systems regarding the rate equations and the extraction properties are discussed.     

Paper electrophoretic determination of the stability constants of biologically significant binary complexes of beryllium(II) and cobalt(II) with sarcosine

Quantitative indication of the process of forming a complex comes from the evaluation of the stability constants or formation constant, which characterize the equilibria corresponding to the successive addition of ligands. Paper electrophoretic technique is described for the study of beryllium(II) and cobalt(II) biologically significant binary complexes with sarcosine. The stability constants of ML and ML₂ complex species of Be(II)/Co(II)—sarcosine have been found to be (6.17 ± 0.09, 4.06 ± 0.04) and (4.27 ± 0.07, 2.98 ± 0.11) (log-arithm stability constant values), respectively at ionic strength 0.1 Mol L⁻¹ and a temperature of 35°C.     

Efficient uranous nitrate production using membrane electrolysis

Electrochemical reduction of uranyl nitrate is a green, simple way to make uranous ion. In order to improve the ratio of uranous ion to the total uranium and maintain high current efficiency, an electrolyser with very thin cathodic and anodic compartment, which were separated by a cation exchange membrane, was setup, and its performance was tested. The effects of various parameters on the reduction were also evaluated. The results show that the apparatus is quite positive. It runs well with 120 mA/cm² current density (72 cm² cathode, constant current batch operation). U(IV) yield can achieve 93.1 % (500 mL feed, total uranium 199 g/L) after 180 min electrolysis. It was also shown that when U(IV) yield was below 80 %, very high current efficiency was maintained, and there was almost a linear relationship between uranous ion yield and electrolysis time; under the range of experimental conditions, the concentration of uranyl nitrate, hydrazine, and nitric acid had little effect on the reduction.