Hisao Aoyagi

Evaluation of Gibbs free energy for the transfer of a highly hydrophilic ion from an acidic aqueous solution to an organic solution based on ion pair extraction

Abstract A method to determine the standard Gibbs free energy for the transfer, ΔG°tr, of a highly hydrophilic metal ion from an aqueous solution, W, in the presence of high concentration of H+ to an organic solution, O, was proposed based on the theoretical consideration of the distribution process of ions between W and O. The usefulness of the proposed method was verified experimentally by comparing ΔG°tr of Mg2+ determined by the method with that obtained by voltammetry for the ion transfer at the W|O interface. The O examined were nitrobenzene (NB) and 1,2-dichloroethane (DCE). By applying the proposed method, ΔG°tr of NpO2+, UO22+, NpO22+ and PuO22+ from an acidic W to NB were determined.

Photoreduction of uranyl ion in aqueous solution. I. With ethanol in sulphuric acid solutions

Abstract Photoreduction of UO22+ with ethanol (EtOH) in H2SO4 solutions has been studied by an electrochemical quantitative analysis of UIV formed as a final product. The quantum yield f(UIV) of UIV increased rapidly at [EtOH] up to 0.5 mol dm−3 and then reached a maximum in deaerated 3.0 mol dm−3 H2SO4. At [EtOH] = 1.5 mol dm−3, f(UIV) under deaerated conditions increased gradually with [H2SO4], while f(UIV) under oxygenated conditions was almost one-half of that under deaerated conditions. In order to explain these experimental results, f(UIV) was expressed on the basis of both formation of an encounter complex between excited UO22+ (∗UO22+) and EtOH and a successive reaction of UO22+ with α-hydroxyalkyl radical formed after the complex formed. Kinetic parameters for the quenching of ∗UO22+ by EtOH were derived from a plot of f(UIV)−1 vs. [EtOH]−1 and checked by the measurements of the photon emission from ∗UO22+ with a time-resolved laser and steady state light excitation. It was confirmed that the proposed photoreduction mechanism could reproduce the experimental results.

Plutonium(III)-ion selective electrode of liquid membrane type using multidentate phosphine oxide ionophore

Abstract Plutonium(III)-ion selective electrode consisting of phosphine oxides such as bis-(diphenylphosphoryl)-methane as an ionophore was developed. Fundamental data, which were utilized for designing the electrode, on the ion transfer of Pu3+ across aqueous (w)/organic (org) solutions interface facilitated by the phosphine oxide derivatives were obtained by ion-transfer polarography at the w/org interface. The Pu(III)-ISE prepared exhibited Nernstian response to the concentration of Pu3+ in the sample solution. Interference of UO2+2, U4+, NpO+2 and Pu4+ was investigated.

Spent fuel reprocessing based on electrochemical extraction process (SREEP)

Abstract A new concept of the electrochemical process for the reprocessing of nuclear spent fuel, SREEP, has been developed on the basis of fundamental studies of the electrochemical reactions of actinide elements and fission products. SREEP consists of unit processes for (1) spent fuel dissolution by electrolytic oxidation, (2) recovery of platinum group metals by electrodeposition, (3) TBP extraction of U, Pu and Np after the successive electrolytic preparation of U(VI), Pu(IV) and Np(VI), (4) recovery of Cs and Sr by electrolytic ion transfer extraction and (5) separation of transplutonium and lanthanide elements by electrodeposition in the molten salt.

Voltammetric study on the transfer of U, Np, and Pu ions at the aqueous-organic interface facilitated by phosphine oxides

Abstract Transfer of U, Np, and Pu ions across the interface between aqueous and 1,2-dichloroethane phases, which was facilitated by bidentate phosphine oxide, was studied by ion transfer polarography using an aqueous electrolyte dropping electrode. The ionic species participating in the transfer reaction were determined on the basis of wave analyses of the polarograms and results of the dependence of half-wave potential or limiting current on the concentration of metal ion in the aqueous phase or phosphine oxide in the organic phase. Applicability of the ion transfer reaction to the electrolytic separation of ions or the liquid-membrane-type ion selective electrode was investigated.

Determination of concentration and distribution of (CuO)+ in high-Tc superconducting La2−xSrxCuOy pellet by flow-coulometry

Abstract The novel wet-chemical method for the analysis of the concentration and the distribution of constituent(s) at a higher oxidation state, [CuO] + , in superconducting La 2− x Sr x CuO y was developed. The sample pellet was dissolved in a flow of acid solution containing Fe 2+ , and the solution was introduced continuously into a flow-coulometric system composed of two-step column electrodes of glassy carbon fibers as working electrodes. Fe 3+ generated in the solution through the stoichiometric redox between [CuO] + and Fe 2+ was detected at the first column electrode, and Cu 2+ in the dissolving solution was detected at the second column. It was obvious that the [CuO] + concentration in the surface region of ca. 200 μm in depth from the surface was lower than that of the bulk pellet and the distribution of [CuO] + near the surface was dependent on such sample-preparation parameters as sintering time, annealing atmosphere and storage time of the pellets, etc.

Electrode processes of plutonium ions in phosphate media

SummaryThe electrolytic behaviour of plutonium ions in a mixture of phosphate and nitrate solutions was studied by flow-coulometry with column electrodes of glassy carbon(GC)-fibers and voltammetry with a GC disc electrode, and compared with that in phosphate-free media. The redox processes, PuO22+/PuO2+and Pu4+/Pu3+, were demonstrated to be reversible even in the phosphate media and their half wave potentials shifted more negatively due to the formation of PuO2(H2PO4)+ and Pu(HPO4)2. The rate of the irreversible reduction of PuO2+to Pu3+ increased in the presence of phosphoric acid and the quantitative reduction was attained with the column electrode even at +0.35 V vs. saturated KCl-Ag/AgCl. The reduction process of PuO2+was elucidated considering an intermediate Pu(IV)-species, PuO2+, which decomposed into Pu4+ by a post-chemical reaction. Analytical advantages of the use of phosphate media are discussed.

Kinetics of Reduction of Uranium(VI) to Uranium(IV) at Titanium Electrode in Nitric Acid and Hydrazine Media

The kinetic study of the reduction of U(VI) at the titanium electrode in nitric acid and hydrazine media was carried out at 25±0.5°C. Uranium (VI) is reduced through a totally irreversible process and kinetic parameters are affected by both the concentration of nitric acid and the electrolytic potential. An empirical equation to express the rate of the reduction of U(VI) was proposed and was confirmed to be in fairly good agreement with the amount of U(IV) produced by the controlled-potential electrolysis. The optimal condition for the preparation of U(IV) from U(VI) was found to be the electrolysis at -0.5V vs. Ag-AgC1/sat.KC1 in 1–2 M HNO3 and 0.1–0.2M hydrazine.

Determination of traces of chlorine in sodium metal by coulometric titration

SummaryProcedures have been developed for the determination of less than 1 ppm of chlorine in sodium. An amalgamation or a vacuum distillation technique is used for the separation of chloride from 3–5 g of sodium. After the separation, the residue is dissolved in ethanol containing perchloric acid and sodium perchlorate, and chloride in the ethanol solution is determined by coulometric titration with mercury ions. The endpoint is determined potentiometrically.ZusammenfassungVerfahren zur Bestimmung von weniger als 1 ppm Chlor in Natrium wurden ausgearbeitet. Amalgamierung oder Vakuumdestillation dienen zur Trennung des Chlorids von 3–5 g Natrium. Der Rückstand nach der Trennung wird in Äthanol gelöst, das Perchlorsäure und Natriumperchlorat enthält. Das in äthanolischer Lösung vorliegende Chlorid wird coulometrisch mit Quecksilber titriert. Der Endpunkt wird potentiometrisch bestimmt.

New method for the separation of actinide ions by controlled-potential electrolysis at the interface between aqueous and organic solutions

Controlled-potential electrolysis (CPE) method for the transfer of an ion at the interface of two immiscible electrolyte solutions has been developed. The CPE method was applied to the transfer of actinide ions such as UO22+ and Am3+ between aqueous (W) and nitrobenzene (NB) solutions. The transfer of actinide ions from W to NB facilitated by bis(diphenyl-phosphoryl)methane (BDPPM) was studied, and it was found that CPE was successfully applied to the electrolytic separation of UO22+ and Am3+ from W to NB containing BDPPM.