Mendeleev Communications Electronic Version, Issue 6, 1998 (pp. 207–248) The formation of neptunium peroxo complexes upon reduction of neptunium(VI) by hydrogen peroxide in concentrated solutions of alkalis Vladimir P. Shilov, Andrei V. Gogolev and Alexei K. Pikaev* Institute of Physical Chemistry, Russian Academy of Sciences, 117915 Moscow, Russian Federation. Fax: +7 095 335 1778 Kinetic studies have shown that the formation of NpV peroxo complexes upon reduction of NpVI by hydrogen peroxide in concentrated solutions of alkalis occurs via the intermediate appearance of NpVI peroxo complexes.In our previous work,1 it was shown that for reactions of hydrogen peroxide with NpVI and AmVI in 0.1 mol dm–3 solutions of HClO4, or with NpVII within the pH range 9–14, there is a linear dependence of lgk (k is the reaction rate constant) on potential difference DE = E(Ann + 1/Ann) – E(O2/H2O2) (An = = actinide).These reactions proceed via an outer sphere mechanism. However, rate constants for reactions of hydrogen peroxide with NpVI in solution at pH 5 or in alkaline medium are higher by 4 orders of magnitude than those expected from the respective DE values. Such behaviour indicates an intrasphere reaction mechanism, i.e., the formation of an NpVI peroxo complex.Note that peroxo complexes were also described in the case of UVI.2 Earlier3 we investigated the kinetics of the reaction between NpVI and hydrogen peroxide in slightly alkaline solutions (pH 9.2–13.7). It was found that the reaction rate decreased with increasing pH value.Continuing the study with concentrated solutions of alkalis (1–8.4 mol dm–3), we observed that in these solutions, the product of reduction is NpV peroxo complex. The respective data are briefly described in the present paper. A solution of 237NpO2(ClO4)2 in perchloric acid prepared via a standard procedure was used as a stock solution. A solution of NpV was also utilized.The neptunium concentration in the solutions was determined by a complexonometric method with its preliminary reduction to tetravalent state.4 Hydrogen peroxide was produced by decomposition of BaO2 (high-purity grade) by 1 mol dm–3 perchloric acid solution. The addition of concentrated K2SO4 solution was used to precipitate BaSO4 and KClO4. The analysis of H2O2 was conducted by a permanganatometric method. The LiOH and NaOH used were high-purity grade (the content of iron in the 17 mol dm–3 NaOH solutions supplied was less than 3×10–5%).The solutions were prepared with twice distilled water. The study on the reaction of NpVI with hydrogen peroxide was carried out by recording the change in the intensity of its wide charge-transfer band of optical absorption in the near UV region using spectrophotometers SF-46 (Russia) and ‘Shimadzu UV-3100’ (Japan).To investigate this reaction, the solution of alkali was placed in a quartz cell (optical path lengths 1 or 5 cm), the spectrum was recorded, an aliquot of the stock NpVI solution was added upon vigorous stirring, and the spectrum was recorded again. The solution of hydrogen peroxide was then inserted, and the spectrum or absorbance at the chosen wavelength (usually at 320 nm) were periodically measured. Note that hydrogen peroxide in alkaline solutions exists in the form of HO2 – or O2 2–.For simplicity, the designation HO2 – is used in this paper. It was found that alkaline NpVI solutions became yellowbrown as a result of the addition of hydrogen peroxide.Optical absorption spectra of ~ 8 mol dm–3 NaOH solutions containing various amounts of NpVI and hydrogen peroxide which were recorded 20–40 s after mixing the solutions are analogous to the spectrum obtained by us upon the addition of hydrogen peroxide to alkaline NpV solutions and coincide with those described in the literature5–7 for NpV peroxo complex. The molar absorption coefficient of the complex obtained from the measurements of absorbance of ~ 8 mol dm–3 NaOH solution at different ratios of NpV and HO2 – concentrations is equal to 3.8×102 m2 mol–1 at 320 nm.To determine the stoichiometry of reaction between HO2 – and NpVI, HO2 – solution was added to a 1 mol dm–3 solution of LiOH, containing 1×10–3 mol dm–3 NpVI, up to a concentration 4×10–4 mol dm–3.The solution became turbid over several minutes, and a precipitate was formed. The latter was separated by centrifugation, then it was dissolved in a 0.1 mol dm–3 solution of HClO4. HClO4 was added to the supernatant to adjust the pH to approximately 1. Absorption spectra were recorded in both solutions. They showed the presence of NpV. The total content of NpV in both solutions allowed us to conclude that ratio D[NpVI] / [HO2 –]0 ~ 1.8, where D[NpVI] designates the difference between initial and final NpVI concentrations, i.e., the total reaction that took place in the solution can be described as: The stoichiometry of reaction (1) was also studied for NaOH solutions.In this case, excess NpVI was also used, and alkaline NpVI solutions, stored for 1–2 days in the dark to finish the partial reduction of NpVI by organic impurities present in stock NaOH solution (it was supplied from the manufacturer in a polyethylene vessel), were utilized.The data obtained are shown in Table 1, where n = D[NpVI] / [HO2 –]0. It is evident that hydrogen peroxide is consumed in NpVI reduction and also in side reactions; their fraction increases with increasing NaOH concentration.The dependence of absorbance of 8.4 mol dm–3 NaOH solutions, containing 1×10–4 mol dm–3 NpVI and different HO2 – amounts, at 320 nm (A320) on time was investigated. The data obtained are shown in Figure 1. The analysis of the data allows us to draw the following conclusions. At [HO2 –]0/[NpVI]0 > 1, absorbance for 23–25 s reaches 93–95% of the maximal value, i.e., 4t1/2 (t1/2 is the half-life of one of the reagents, for example, NpVI) elapsed to this moment, and t1/2 = 6 s.The constancy of t1/2 within the range of HO2 – concentrations from 1×10–4 to 1×10–3 mol dm–3 requires us to accept the following reduction mechanism at these concentration ratios. Initially, the complex of NpVI with HO2 – of 1:m composition where m > 1 is formed. The increase in A320 values for the next 30–40 s at [HO2 –] = 10–3 mol dm–3 obeys the rate law for a first-order reaction but t1/2 = 12 s.At present, we can not 2NpVI + HO2 – + OH– 2NpV + O2 + H2O (1) 2.0 1.6 1.2 0.8 0.4 0 10 20 30 40 50 A320 t/min 1 2 3 4 5 Figure 1 Dependence of A320 on time for an 8.4 mol dm–3 NaOH solution containing 1×10–4 mol dm–3 NpVI and various HO2 – concentrations (mol dm–3): 1, 4.2×10–5; 2, 1×10–4; 3, 2.04×10–4; 4, 3.97×10–4; 5, 1.03×10–3 (temperature 25 °C, optical path length 5 cm).Mendeleev Communications Electronic Version, Issue 6, 1998 (pp. 207–248) explain the increase in t1/2 value. After reaching the maximum, A320 decreases. At [HO2 –]0/[NpVI]0 = 1–10, the absorbance tends to the value corresponding to the equilibrium value for a NpV and HO2 – solution of a concentration that is equal approximately to [HO2 –]0 – 0.5[NpVI]0.Hence, the ligand HO2 – is already present in the coordination sphere, and a complex NpV(HO2 –) appears via reduction of NpVI(HO2 –) complex by the species attacking from outside. At [HO2 –]0/[NpVI]0 > 0.5, A320 decreases to a value corresponding to NpVI concentration minus the amount consumed in reaction with hydrogen peroxide.At HO2 – excess, the NpV(HO2 –) concentration calculated via molar absorption coefficient is equal to 90–95% of the initial neptunium concentration. If reactions: proceed, the NpV(HO2 –) concentration after reduction should be equal to about 0.5[NpVI]0, and then an increase in absorbance should occur because of reaction between NpV and HO2 –. Special experiments showed that the NpV peroxo complex in the reaction of NpV with hydrogen peroxide is formed slowly.For instance, at [HO2 –]0/[NpV]0 = 10:1, the duration of this reaction is 70 min. Since an increase is not observed, it is most probable that reactions (2), (4), (6), (7) and (5) take place: At NpVI excess, reaction (8) should be added to the reactions considered: At [HO2 –]0/[NpVI]0 < 0.5, it is necessary to propose as an explanation of the obtained kinetic data (the initial rate linearly decreases with increasing NaOH concentration but slightly depends on NpVI concentration and increases with increasing [HO2 –]0) that the formed complex NpV(HO2 –) participates in reaction (9): In addition, dissociation of NpV(HO2 –) to NpV and HO2 – occurs, and released hydrogen peroxide reacts with NpVI.As mentioned above, HO2 – is consumed not only in NpVI reduction but also in side reactions. Reactions (10) and (11) can belong to such processes: Radical ion O– formed oxidizes NpV to NpVI. In reaction (11), radical ion O– can appear in the NpV coordination sphere and in the same place can perform its oxidation.References 1 V. P. Shilov, A. V. Gogolev and A. K. Pikaev, Khim. Vys. Energ., 1998, 32, 395 (in Russian). 2 Kompleksnye soedineniya urana (Uranium Complex Compounds), ed. I. I. Chernyaev, Nauka, Moscow, 1964 (in Russian). 3 A. V. Gogolev, V. P. Shilov and A. K. Pikaev, Khim. Vys. Energ., 1996, 30, 255 [High-Energy Chem. (Engl. Transl.), 1996, 30, 229]. 4 A. P. Smirnov-Averin, G. S. Kovalenko, N. P. Ermolaev and N. N. Krot, Zh. Anal. Khim., 1966, 21, 76 [J. Anal. Chem. USSR (Engl. Transl.), 1966, 21, 62]. 5 C.Musikas, Radiochem. Radioanal. Lett., 1970, 4, 347. 6 C.Musikas, J. Chim. Phys. Phys.-Chim. Biol., 1974, 71, 197. 7 A. V. Gogolev, V. P. Shilov and A. K. Pikaev, Mendeleev Commun., 1996, 127. NpVI + HO2 – NpVI(HO2 –) NpVI(HO2 –) NpV+ HO2 HO2 + OH– O2 – + H2O NpVI(HO2 –) +O2 – NpV(HO2 –) +O2 (2) (3) (4) (5) NpVI(HO2 –) +HO2 – NpVI(HO2 –)2 NpVI(HO2 –)2 NpV(HO2 –) +HO2 (6) (7) NpVI + O2 – NpV + O2 (8) NpV(HO2 –) +NpVI 2NpV + HO2 (9) Table 1 Influence of NaOH concentration and initial NpVI and HO2 – concentrations on the stoichiometry of reaction NpVI + HO2 –. [NaOH]/ mol dm–3 [NpVI]0 / 10–4 mol dm–3 [HO2 –]0 / 10–4 mol dm–3 D[NpVI]0 / 10–4 mol dm–3 n 1.0 1.98 0.74 1.29 1.74 1.0 2.48 1.08 1.97 1.82 2.1 2.20 1.01 1.58 1.56 4.1 3.85 1.80 2.73 1.52 4.2 3.50 1.67 2.38 1.43 8.2 8.12 3.28 4.66 1.42 8.2 3.84 1.82 2.36 1.30 HO2 – + O2 – O– + OH– + O2 NpV(HO2 –) + O2 – NpV + O– + OH– + O2 (10) (11) Received: Moscow, 23rd April 1998 Cambridge, 17th July 1998; Com. 8/03092H