Behavior of Am(V) in concentrated HNO<Subscript>3</Subscript> solutions containing potassium phosphotungstate K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript>

The Am(V) disproportionation in (2–10) × 10^?3 M K₁₀P₂W₁₇O₆₁ (KPW) solutions in 1–7 M HNO₃ was studied spectrophotometrically. Under the experimental conditions, Am(VI) and Am(III) are the final products of Am(V) transformation; the process is described by the rate equation of a reversible reaction. The direct reaction follows the first-order rate equation with respect to Am(V) concentration, and the reverse reaction, the zero-order equation.     

Behavior of Np(VI) in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The behavior of Np(VI) in HNO₃ solutions containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) at various concentrations of HNO₃ (1–3 M) and KPW [(1–3) × 10⁻³ M] was studied by spectrophotometry. The rate law and the Np(VI) transformation scheme were determined, and the effective rate constants of the Np(VI) reduction and Np(V) reproportionation were calculated.     

Formal oxidation potentials of the Np(VI)-Np(V) couple and schemes of formal oxidation potentials of Np and am ions in HNO3 solutions containing potassium phosphotungstate K10P2W17O61

The formal oxidation potentials E _f⁰ of the Np(VI)-Np(V) couple in 1–3 M HNO₃ solutions containing (1–3) × 10⁻³ M potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) were measured by the potentiometric method with spectrophotometric identification of the valence states. The potentials slightly decrease with an increase in the HNO₃ and KPW concentrations, and the shift of the Np(VI)-Np(V) potential toward negative values relative to its value in 1 M HClO₄ reaches 0.08 V. The formal oxidation potentials of the Np(V)-Np(IV) couple were calculated from the experimentally determined equilibrium constants (K _eq) of the redox reaction of the Np(V) disproportionation. Under the examined conditions, these potentials also vary insignificantly, and the shift of E _f⁰ toward positive values relative to its value in 1 M HClO₄ is 0.53 V. The schemes of potentials of the Np and Am couples in 1 M HNO₃ in the presence of KPW are suggested.     

Behavior of Np(VI) and Np(V) Ions in NaHCO<Subscript>3</Subscript> Solutions Containing H<Subscript>2</Subscript>O<Subscript>2</Subscript>

The behavior of Np(VI) and Np(V) in NaHCO₃ and NaHCO₃ + Na₂CO₃ solutions containing H₂O₂ was studied spectrophotometrically. In 0.75–1.0 M NaHCO₃, hydrogen peroxide oxidizes Np(V) to Np(VI). The kinetics curves of Np(V) oxidation into Np(VI) have a complex shape and are characterized either by an induction period of up to tens of minutes or by a period of steady-state Np(VI) concentration, followed by an increase in the Np(VI) concentration. When Np(VI) initially exists in the solution, the induction period is lacking. The process character changes when the bicarbonate concentration decreases, or when Na₂CO₃ is added. In 1.0 M Na₂CO₃, 0.5 M NaHCO₃ + 0.5 M Na₂CO₃, or 0.01–0.5 M NaHCO₃, hydrogen peroxide completely reduces Np(VI) into Np(V). The probable mechanisms of this process were discussed. Accumulation of Np(VI) in NaHCO₃ solutions can be accounted for by assuming that Np(VI) itself participates in the transformations. Initially, the reaction of Np(VI) with H₂O₂ yields the excited *Np(V) ion. Then it reacts with another H₂O₂ molecule and forms a carbonate-peroxide complex. In the collision of the latter with unexcited Np(V), two electrons from two Np(V) ions are transferred onto the O ₂ ^2? ligand with formation of two Np(VI) ions.     

Oxidation of Np(V) with Xenon Trioxide in an HClO<Subscript>4</Subscript> Solution

Oxidation of Np(V) to Np(VI) with xenon trioxide in a 0.5–1.4 M HClO₄ solution was studied by spectrophotometry. The reaction rate is described by the equation–d[Np(V)]/dt = k[Np(V)][XeO₃], where k = 4.6 × 10^–3 L mol^–1 s^–1 in 1 M HClO₄ at 92°С. The activation energy is close to 92 kJ mol^–1. The activated complex is formed in contact of NpO ₂ ⁺ and ХеО₃ without participation of Н⁺ ions. The activated complex transforms into NpO ₂ ²⁺ and the products: ОН, Хе, and О₂. The ОН radical oxidizes Np(V). Admixtures of Со²⁺ and especially Fe³⁺ accelerate the Np(V) oxidation.     

Formal oxidation potentials of actinide couples in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

The formal oxidation potentials of the M(VI)/M(V), M(V)/M(IV), and M(IV)/M(III) couples for actinides from U to No and of the M(IV)/M(III) couples for some actinides in 1 M H⁺ or 1 M Na⁺ (pH ～5–5.5) solutions containing K₁₀P₂W₁₇O₆₁ were calculated from the data on stability of complexes of f element ions with the unsaturated heteropolytungstate anion P₂W₁₇O ₆₁ ^10? . In some cases, the previously accepted values were subjected to major revision, especially the potentials of the An(V)/An(IV) couples. Problems arising in measuring the potentials of the couples involving Np(III) and Pu(III) which react with the heteropolyanion to form a heteropoly blue are discussed. The potentials of some M(III)/M(II) couples are estimated.     

Electrochemical properties and dissolution of URu<Subscript>3</Subscript> in nitric acid solutions

The electrochemical properties of URu₃ intermetallic compound (IMC) in 0.5–8 M HNO₃ solutions were studied by linear voltammetry and galvanostatic electrolysis. In 0.5–2 M HNO₃, URu₃ occurs in the passive state at potentials lower than +1.3 V (here and hereinafter, vs. SHE), and in 4–8 M HNO₃, an anodic oxidation peak is observed at potentials from +1.0 to +1.2 V. This process, however, leads to IMC passivation and not to its dissolution. At potentials higher than +1.4 V, URu₃ passes into the transpassive state and starts to actively dissolve. The principal possibility of electrochemical dissolution of IMC at potentials exceeding the transpassivation potential was demonstrated by galvanostatic electrolysis. The rate of uranium leaching during electrolysis depends to a greater extent on the current density than on the HNO₃ concentration and reaches 35 mg cm^–2 h^–1 in 6 M HNO₃ at a current density of 182 mA cm^–2.     

Reaction of Np(VI) with cyclohexanediaminetetraacetic acid in HClO<Subscript>4</Subscript> solutions

The stoichiometry of the reaction of Np(VI) with cis-cyclohexanediaminetetraacetic acid (CHDTA, H4chdta) in 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of the complexone converts 4 mol of Np(VI) into Np(V). In 0.115–0.98 M HClO₄ solutions (the ionic strength of 1.0 was supported with LiClO₄) containing 3–29 mM CHDTA at 20–45°С, Np(VI) at a concentration of 0.2–3.3 mM is consumed in accordance with the first-order rate law until less than 40% of Np(VI) remains. After that, the reaction decelerates. The reaction rate has first order with respect to [CHDTA] and the order of–1.2 with respect to [H⁺]. The activated complex is formed with the loss of one and two Н⁺ ions. The activation energy is 82.3 ± 3.8 kJ mol^–1.     

Reaction of Np(VI) with H<Subscript>2</Subscript>O<Subscript>2</Subscript> in carbonate solutions

The kinetics and stoichiometry of the reaction of Np(VI) with H₂O₂ in carbonate solutions were studied by spectrophotometry. In the range 1–0.02 M Na₂CO₃, the reaction 2Np(VI) + H₂O₂ = 2Np(V) + O₂ occurs, as Δ[Np(VI)]/Δ[H₂O₂] ≈ 2. In Na₂CO₃ + NaHCO₃ solutions, the stoichiometric coefficient decreases, which is caused by side reactions. The reduction at low (1 mM) concentrations of Np(VI) and H₂O₂ follows the first-order rate law with respect to Np(VI), which suggests the formation of a Np(VI) peroxide-carbonate complex, followed by intramolecular charge transfer. Addition of Np(V) in advance decreases the reaction rate. An increase in the H₂O₂ concentration leads to the reaction deceleration owing to formation of a complex with two peroxy groups. In a 1 M Na₂CO₃ solution containing 1 mM H₂O₂, the first-order rate constant k increases with a decrease in [Np(VI)] from 2 to 0.1 mM. For solutions with [Np(VI)] = [H₂O₂] = 1 mM, k passes through a minimum at [Na₂CO₃] = 0.5–0.1 M. The activation energy in a 0.5 M Na₂CO₃ solution is 48 kJ mol⁻¹.     

Crystal structure of double acetates SrAnO<Subscript>2</Subscript>(OOCCH<Subscript>3</Subscript>)<Subscript>3</Subscript>·3H<Subscript>2</Subscript>O [An = Np(V), Pu(V)]

The crystal structure of isostructural Pu(V) and Np(V) acetates of the general composition SrAnO₂Ac₃ · 3H₂O (Ac = CH₃COO^?) was determined. The structures are based on complex anions [AnO₂Ac₃]^2? and Sr²⁺ cations combined into a three-dimensional framework with water molecules located in framework cavities. The An(V) atoms are characterized by the hexagonal-bipyramidal oxygen surrounding; the equatorial plane is formed by the O atoms of three acetate groups. The coordination surrounding of the Sr atom is a tetragonal antiprism formed by the O atoms of acetate ions and water molecules. The bond lengths within the coordination sphere decrease in passing from Np(V) to Pu(V): the average An=O and An-O bond lengths are 1.828(5) and 2.549(6) Å for Np and 1.811(4) and 2.530(4) Å for Pu, respectively.     

Kinetics of electrochemical oxidation of neptunium ions in nitric acid solutions containing potassium phosphotungstate K10P2W17O61

The kinetics of electrochemical oxidation of Np(IV) and Np(V) ions on a smooth platinum electrode in a 1 M HNO₃ solution containing potassium phosphotungstate K₁₀P₂W₁₇O₆₁ (KPW) was studied by spectrophotometry. For the redox reactions under consideration, the rate laws were determined, the rate constants were calculated, and the schemes of transformations of Np ions were suggested. The electrochemical oxidation of Np(IV) and Np(V) is described by the first-order rate law with respect to the Np(IV) and Np(V) concentrations, respectively. The rate constants of these reactions are close and comparable with the rate constant of the Np(V) disproportionation.     

Electrochemical properties and dissolution of UPd<Subscript>3</Subscript> in nitric acid solutions

Electrochemical properties of the intermetallic compound UPd₃ in 0.5–8 M HNO₃ solutions were studied by linear voltammetry. In 0.5–2 M HNO₃ solutions, the UPd₃ surface is in the passive state. At HNO₃ concentrations exceeding 4 M, the alloy passivation was not observed. The previously unknown electrochemical characteristics of UPd₃ in nitric acid solutions were obtained using the Tafel equation. The values of Е(i = 0) and v_corr increased from 39 mV and 38 μg cm^–2 h^–1 in 0.5 M HNO₃ to 821 mV and 11 mg cm^–2 h^–1 in 8 M HNO₃, respectively. Dissolution experiments have shown that UPd₃ can dissolve in HNO₃ solutions of concentration exceeding 4 M at room temperature. In 8 M HNO₃, the dissolution rate can reach 17 mg cm^–2 h^–1 at 25°С, with the dissolution being virtually equimolar and accelerating with time.     

Disproportionation of Pu(V) in K<Subscript>10</Subscript>P<Subscript>2</Subscript>W<Subscript>17</Subscript>O<Subscript>61</Subscript> solutions

In solutions of unsaturated heteropolytungstate K₁₀P₂W₁₇O₆₁, Pu(V) disproportionates in a wide pH range; it is a first-order reaction with respect to Pu(V), and its rate only slightly changes in the pH range from 0.7 to 4.0. The activation energy E _a of Pu(V) disproportionation was determined as 78.6±2.0 and 64.2±3 kJ mol^?1 at pH 2.0±0.1 and 4.0±0.2, respectively. The thermodynamic parameters of activation ΔH ^≠ and ΔS ^≠ were evaluated. Published data on disproportionation of Np(V) and Am(V) in K₁₀P₂W₁₇O₆₁ solutions were analyzed.     

Reduction of Np(V) with Fe(II) in weakly acidic solutions containing H<Subscript>2</Subscript>C<Subscript>2</Subscript>O<Subscript>4</Subscript>

The kinetics of the reaction of Np(V) with Fe(II) in dilute perchloric and nitric acid solutions containing H₂C₂O₄ was studied by spectrophotometry. In the range pH 1–2, the reaction rate is described by the equation d[Np(V)]/dt = k[Np(V)][Fe(II)][H₂C₂O₄]²[H⁺]^−1.6, k = 182 mol^−1.4 l^1.4 s⁻¹. The activation energy in the range 25–45°C is 26 kJ mol⁻¹. The reaction mechanism involves formation of Fe(II) and Np(V) oxalate complexes, followed by their reaction with the participation of the H⁺ ion.     

Synthesis,crystal structure,and absorption spectra of a complex neptunium(v) chromate,Na<Subscript>3</Subscript>[NpO<Subscript>2</Subscript>(CrO<Subscript>4</Subscript>)<Subscript>2</Subscript>](H<Subscript>2</Subscript>O)<Subscript>5</Subscript>

A new Np(V) chromate complex with outer-sphere sodium cations, Na₃[NpO₂(CrO₄)₂](H₂O)₅ (I), was synthesized from aqueous solution. Its composition and structure were determined by single crystal X-ray diffraction. The structure of I is based on anionic chains of the composition [NpO₂(CrO4)2] _n ³ⁿ⁻, running along [010] and forming layers parallel to the (101) plane. The Na⁺ ions and water molecules of crystallization are arranged between the layers. The coordination polyhedra of the Np atoms (pentagonal bipyramids) are combined pairwise by sharing common equatorial edges formed by two bridging oxygen atoms of bidentate chelate-bridging CrO₄ groups. The absorption spectra of I in the IR and visible ranges are presented.     

Synthesis of New Crystalline Pu(V) Compounds from Solutions: II. Double Pu(V) Oxalates with Co(NH<Subscript>3</Subscript>)<Subscript>6</Subscript><Superscript>3+</Superscript> Ions in the Outer Sphere

Crystalline compounds of the general composition Co(NH³)⁶PuO₂(C₂O₄)²·nH₂O (n = 2, 3, 5) were isolated from freshly prepared neutral oxalate solutions of Pu(V) by addition of Co(NH₃)₆³⁺ ions. These compounds are fairly stable in storage in air and poorly soluble in water. Previously unknown double Np(V) oxalates Co(NH₃)₆NpO₂(C₂O₄)·nH₂O (n = 2, 5) were also synthesized and studied. All the compounds of Pu(V) and Np(V) of the same composition are mutually isostructural. The behavior of these compounds at heating was studied, and their IR spectra were measured. The optical spectra of new Np(V) compounds were measured.     

Structure of the double formate KNpO<Subscript>2</Subscript>(OOCH)<Subscript>2</Subscript>

KNpO₂(OOCH)₂ was isolated from neutral Np(V) solutions with a high concentration of potassium formate. The crystal structure of this compound was determined. The structure consists of infinite anionic chains [NpO₂(OOCH)₂] _n ^n? . Potassium cations are located between these chains. The Np coordination polyhedron is a hexagonal bipyramid whose equatorial plane is formed by the oxygen atoms of four HCOO^? ions. The Np bipyramids in the chains are bound via common equatorial edges. The anionic chains in the structures of KNpO₂(OOCH)₂ and NH₄NpO₂(OOCH)₂ studied previously have similar composition but different structure.     

Reduction of Pu(IV) and Np(VI) with hydroxylamine in solutions of low acidity with high uranium content

Decomposition of hydroxylamine in HNO₃ solutions containing 350 to 920 g l^?1 U(VI) was studied. In the absence of fission and corrosion products (Zr, Pd, Tc, Mo, Fe, etc.), hydroxylamine is stable for no less than 6 h at [HNO₃] < 1 M and 60°C. In the presence of these products, the stability of hydroxylamine appreciably decreases. The reduction of Pu(IV) and Np(VI) with hydroxylamine in aqueous 0.33 and 0.5 M HNO₃ solutions containing 850 g l^?1 U(VI) and fission and corrosion products at 60°C was studied. Np(VI) is rapidly reduced to Np(V), after which Np(V) is partially reduced to Np(IV). The rate of the latter reaction in such solutions is considerably higher than the rate of the Np(V) reduction with hydroxylamine in HNO₃ solutions without U(VI). At [HNO₃] = 0.33 M, the use of hydroxylamine results in the conversion of Pu to Pu(III) and of Np to a Np(IV,V) mixture, whereas at [HNO₃] = 0.5 M the final products are Pu(IV) and Np(V).     

Disproportionation of Pu(V) in the CH<Subscript>3</Subscript>COOH-H<Subscript>2</Subscript>O system

The behavior of Pu(VI) and Pu(V) in CH₃COOH (HAc)-H₂O solutions was studied by spectrophotometry. The absorption spectrum of Pu(VI) does not change on adding HAc to a concentration of 5 M in the presence of 0.5–1.0 M HClO₄, but in solutions containing less than 0.001 M mineral acid, changes in the spectrum are observed at HAc concentration of 0.6 M.he major absorption band of PuO ₂ ²⁺ ions, caused by an f-f transition, with increasing [HAc] is shifted from 830.6 to 836 nm, with a simultaneous decrease in the absorption intensity, which is due to formation of 1: 1 complexes of Pu(VI) with Ac^? ions. In anhydrous HAc, the peak intensity increases again, owing to total change in the composition of the solvation shell. Pu(V) is unstable in 1–17 M HAc solutions and disproportionates to form Pu(VI) and Pu(IV). The Pu(V) loss follows a second-order rate law with respect to [Pu(V)] and accelerates with increasing HAc concentration. The reaction products exert opposite effects on the reaction rate: Pu(IV) accelerates the consumption of Pu(V), whereas Pu(VI) does not affect the process in dilute HAc solutions but decelerates the disproportionation in concentrated solutions owing to formation of a cation-cation complex with Pu(V).     

Reaction of Np(VI) with <Emphasis Type="Italic">N</Emphasis>-(2-Hydroxyethyl)ethylenediaminetriacetic acid in perchloric acid solutions

The stoichiometry of the reaction Np(VI) + H₃hedta [hedta is N-(2-hydroxyethyl)ethylenediaminetriacetate, HEDTA, anion] in a 0.05 M HClO₄ solution was studied by spectrophotometry. With Np(VI) in excess, 1 mol of HEDTA reduces 4 mol of Np(VI) to Np(V). In 0.125–1.0 M HClO₄ solutions (the ionic strength of 1.0 was maintained constant by adding LiClO₄), containing 3–29.2 mM HEDTA, at 20–45°С Np(VI) at a concentration of 0.4–3.5 mM is consumed in accordance with a first-order rate law until approximately 40% of Np(VI) remains. Then, the reaction decelerates. The reaction rate has first order with respect to [HEDTA] and the order of–1.6 with respect to [Н⁺]. The activated complex arises with the loss of one and two Н⁺ ions. The activation energy is 88.4 ± 5.3 kJ mol^–1.