The determination of selenium in water samples from the environment

A method for the determination of selenium by preconcentration and thermal neutron activation is presented. Selenium is adsorbed on active charcoal after reduction to the elemental state. By varying the conditions of the reduction, it is possible to measure the total selenium content and tetravalent selenium separately. The contribution of hexavalent selenium is obtained by substraction. The lower limit of determination is 5 ng°1⁻¹. About 40 samples per day can be handled by one analyst. The preconcentration can be performed at the sampling site resulting in samples, which can be stored easily for later analyses by neutron activation.     

Effect of labile hydrogens on measurement of tritium preserved in hydrogens of cellulose

Various f-elements are separated as anionic complexes from both acidic and alkaline solutions by precipitation with alkylpyridinium salts. The precipitates are also cationic surfactants where the simple counter-ion (e.g. nitrate or chloride) is replaced by the negatively charged complex anion of an actinide or lanthanide. The low solubility of these precipitates is explained by a strong affinity of divalent complex counter-ions of f-elements to the quaternary nitrogen. Precipitations in solutions of nitric acid allow to separate tetravalent f-elements from other metals, in alkaline carbonate solutions tetravalent and hexavalent actinides are precipitated simultaneously. The last procedure yields precipitates, which are very intimate mixtures of hexavalent and tetravalent actinides. This allows to prepare mixed oxides in a simple way.     

Determination of hexavalent and tetravalent uranium in phosphate ores through hydrochloric acid selective leaching

The tetravalent and hexavalent uranium content of three Egyptian phosphate type ore samples namely; Sebayia, Abu Tartur and Qatrani have been studied through selective leaching by hydrochloric acid at normal, oxidized and reduced conditions at an amount of hydrochloric acid less than the stoichiometric value i.e. before phosphoric acid production. Oxidizing condition is attained by incorporating 2% of manganese dioxide in the leaching cycle, whereas reducing condition is attained by adding 2% iron powder. The achieved results show that the amount of tetravalent uranium varies between 5 and 95%. As soon as the achieved stoichiometric value of hydrochloric acid is sufficient to produce phosphoric acid both tetravalent and hexavalent uranium dissolve by virtue of phosphoric acid complexing power for uranium. The chemical form of uranium in the ore determines the type of solvent needed to recover it.     

Extraction of actinides in two-phase water-poly(ethylene glycol)—Salt systems in the presence of potassium phosphotungstate

Extraction of actinides has been examined in two-phase aqueous systems based on poly(ethylene glycol) (PEG) from sulfate solutions in the presence of potassium phosphotungstate, which forms strong complexes with ions of tri- and tetravalent transuranium elements. Extraction of these complexes by aqueous PEG solution is complete in contrast to that of penta- and hexavalent actinides. Conditions have been chosen for the separation of actinides in different oxidation states.     

Thermodynamic parameters of Cs+ sorption on natural clays

Electrochemical properties of hexavalent and tetravalent uranium were investigated in various molten organic ionic mixtures. Kinetic parameters and reaction mechanisms were defined by voltammetry, chronoamperometry and sampled polarography. Long-term electrolyses in the mixture phenanthrene/TBABF4 allowed the preparation of both metallic uranium and crystallized uranium dioxide.     

Simultaneous determination of trace arsenic, antimony, bismuth and selenium in biological samples by hydride generation-four-channel atomic fluorescence spectrometry

A new and sample technique for the simultaneous determination of trace arsenic, antimony, bismuth and selenium in biologic samples by hydride generation-four-channel nondispersive atomic fluorescence spectrometry was development. The conditions of instrumentation and hydride generation of arsenic, antimony, bismuth and selenium were optimized. For reducing hexavalent Se to the tetravalent state was to heat the sample with 6 mol l⁻¹ HCl, and then pre-reducing pentavalent As and Sb to the trivalent state was achieved by the addition of 0.05 mol l⁻¹ thiourea. The interferences of coexisting ions were evaluated. Under optimal conditions, the detection limits for As, Sb, Bi and Se were determined to be 0.03, 0.04, 0.04 and 0.03 ng ml⁻¹, respectively. The precision for seven replicate determinations at the 5 ng ml⁻¹ of As, Sb, Bi and Se were 0.9, 1.2, 1.3 and 1.5% (R.S.D.), respectively. The proposed method was successfully applied to the simultaneous determination of As, Sb, Bi and Se in a series of Chinese certified biological reference materials using simple aqueous standard calibration technique, the results obtained are in good agreement with the certified values.     

Two predominance-region diagrams for plutonium

Two predominance-region diagrams for plutonium are illustrated. One diagram plots the pH vs. the equilibrium fraction of hexavalent plutonium. The other diagram plots the equilibrium fraction of tetravalent plutonium vs. the plutonium oxidation number. Both diagrams define the boundaries of the regions where tri-, tetra-, penta-, and hexavalent plutonium are the predominant species. In each diagram, the two principal triple points are located at the intersections of three predominance-region boundary lines.Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy under Contract No. W-7405-ENG-36.     

Two predominance-region diagrams for plutonium

Two predominance-region diagrams for plutonium are illustrated. One diagram plots the pH vs. the equilibrium fraction of hexavalent plutonium. The other diagram plots the equilibrium fraction of tetravalent plutonium vs. the plutonium oxidation number. Both diagrams define the boundaries of the regions where tri-, tetra-, penta-, and hexavalent plutonium are the predominant species. In each diagram, the two principal triple points are located at the intersections of three predominance-region boundary lines.Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy under Contract No. W-7405-ENG-36.     

Sorption of selenium(IV) onto magnetite in the presence of silicic acid

Sorption of selenium(IV) and silicic acid onto magnetite (Fe(3)O(4)) was investigated in binary systems, with concentrations of silicic acid under the solubility limit of amorphous silica. Using the double diffuse layer model (DDLM), surface complexation constants of selenium(IV) and H(4)SiO(4) onto magnetite were extracted using Fiteql 4.0. Then, prediction curves of the sorption of selenium(IV) in the presence of silicic acid onto magnetite were obtained, using the calculated surface complexation constants. Finally, laboratory experiments were performed and showed a competition between selenium(IV) and silicic acid for the surface sites of magnetite. Experimental results matched the model predictions, confirming its ability to model qualitatively and quantitatively the ternary system.     

Electrochemistry of uranium in ionic organic media: Ethylammonium nitrate and acetamide-KSCN eutectic

The electrochemical reduction of hexavalent and tetravalent uranium is investigated in ethylammonium nitrate at 298 K and in the acetamide-KSCN eutectic at 400 K. The transient techniques describe a two-step reduction of UO ₂ ²⁺ in acetamide-KSCN while only one step is observed in ethylammonium nitrate. The reduction of tetravalent uranium proceeds in one step giving trivalent uranium in the two media. In any cases, additional kinetic phenomena are the source of difficulties for the understanding of the reactions. In controlled potential electrolysis, side-reactions between the reduced species and the solvent impede completely the mechanism foreseen by transient methods.     

Effect of oxidation-reduction on the extraction of uranium from wet phosphoric acid by DEHPA/TOPO

Uranium is recovered from wet phosphoric acid by DEHPA/TOPO in kerosene. Uranium is present in wet phosphoric acid in the tetravalent and hexavalent states but DEHPA/TOPO extracts uranium in the hexavalent state only. The ratio of U⁴⁺/U⁶⁺ depends on several factors such as the origin of the phosphate rock, the method of preparation of phosphoric acid and the presence of other impurities. Therefore it is important to oxidize the wet acid to convert all uranium to U⁶⁺ before extraction. Uranium is stripped from the solvent by a reverse process where a concentrated phosphoric acid is used under reducing conditions. This paper studies the oxidation of wet phosphoric acid from Homs plant/Syria by H₂O₂ oxidant and the effect of oxidation on extraction coefficientK. It also studies the reduction by iron and its effect on back extraction of uranium from the solvent to phosphoric acid.     

Thiol-disulfide exchange yields multivalent dendrimers of melamine

[reaction: see text] Thiol-disulfide exchange can be used to prepare multivalent conjugates of a small molecule or octapeptide displayed on dendrimers based on melamine. Exchange of four or eight thiopyridyl groups by captopril occurs at room temperature in methanol almost quantitatively. Exchange using the peptide requires higher temperatures and guanidinium chloride in DMF. While exchange on the tetravalent scaffold with four peptides is almost quantitative, sterics retard formation of the octavalent conjugate: the hexavalent conjugate forms readily.     

化学蒸气发生-四通道原子荧光光谱法同时测定水样中的痕量砷、锑、硒和汞

应用自行设计的化学蒸气发生-四通道无色散原子荧光光谱仪,建立了同时测定水样中As、Sb、Se、Hg的新方法.在实验中优化了四元素同时化学蒸气发生条件和测定的最佳工作参数.在样品预处理阶段用HCl将Se6+还原为Se4+,然后用质量浓度5 g/L硫脲将As5+和Sb5+还原为As3+和Sb3+.在最佳条件下,方法对As、Sb、Se、Hg的检出限分别为0.05、0.03、0.05、0.01 ng/mL(3d);RSD分别为0.42%、0.74%、0.97%、1.0%(对5 ng/mL As、Sb、Se和0.5ng/mL Hg混合标准,n=7).用所建立的方法对不同类型水样中的As、Sb、Se、Hg进行了同时测定,测定结果与用标准方法测定所得结果之间无明显差异,各元素的加标回收率在93%～105%.     

Problems in plutonium chemistry: Generalized proportional equations and polymer formation

Summary This paper illustrates a generalized form of the method of proportional equations. It permits equilibrium calculations for Pu disproportionation reactions at any oxidation number and any pH by means of combining arbitrary proportions of tri- and hexavalent plutonium. Two methods for estimating the work of Pu polymer formation are illustrated. One method uses the work of disproportionation of tetravalent plutonium. The other method suggests the transition between hydrous Pu(IV) oxide and polymer is accompanied by a small change in free energy.     

The gas chromatographic determination of selenium(IV) and total selenium in natural waters with 1,2-diamino-3,5-dibromobenzene

The total selenium and selenium(IV) contents in sea water and river water can be determined directly by a gas chromatographic method with l,2-diamino-3,5-dibromobenzene without preconcentration. The reagent reacts only with selenium(IV) to form a 4,6-dibromopiazselenol; other oxidation states of selenium must therefore be converted to the tetravalent state for total selenium determinations. The piazselenol formed can be extracted quantitatively into 1 ml of toluene from 500 ml of sample water. A method is proposed for the determination of selenium(IV) and total selenium in natural waters at levels as low as 2 ng l^-1. Coastal sea water and river water in Japan contain 8–30 ng of Se(IV) and 20–50 ng of total Se per liter.     

Plutonium disproportionation: maxima in the fractions of two oxidation states

The equilibrium fractions of soluble tri- and hexavalent Pu depend on the pH and the Pu oxidation number. Both fractions can exhibit maxima as the pH changes at constant N. A fraction at maximum can be used to estimate the equilibrium constant of the first hydrolysis reaction of tetravalent Pu. A new formula that renders least-squares estimates of this constant is illustrated.     

Spectrometric,Thermal and Conductometric Studies of Some Lanthanide,Thorium and Uranyl Complexes with Some Hydroxy Nitrosocoumarins

IR, NMR, TG, DTA, DTG and molar conductance studies have been carried out on complexes of La(III) with some hydroxy nitrosocoumarins and those of trivalent Y, Ce, Pr, Nd, Sm, Gd, Dy, Er, Yb; tetravalent Th and hexavalent uranium complexes with 7-hydroxy-4-methyl-8-nitrosocoumarins. The solid complexes were prepared, separated, analysed and their structures elucidated by elemental analysis, IR, NMR, TG, DTA, DTG and molar conductance.     

Alpha coefficients for tetravalent plutonium

The complexation of tetravalent plutonium in aqueous solutions derives from several sources including counterions, hydrolysis, additives, and impurities. A quantitative tool accounting for all such effects, known and unknown, is the alpha coefficient. It can be expressed in six ways by means of the equilibrium fractions of two Pu oxidation states.     

Determination of U(IV) and U(VI) by ion chromatography-inductively coupled plasma mass spectrometry and its application to kinetic studies

The ions normally formed by actinides in aqueous solutions by the oxidation states III-VI are M3-, M4+, MO2+ and MO2(+2), respectively. Oxidation state representatives such as Am3+, Th4+, NpO+ and UO+, which resist oxidation state changes, were used to investigate a method to separate the oxidised species (MO2 and MO2(2+)) from the reduced species (M3+ and M4+). With this method the hexavalent state of uranium could be separated from the tetravalent state of uranium in aqueous media in less than 8 min. Uranium concentrations down to 10(-9) M could be analysed without changing the redox composition during the separation. The oxidation kinetics of the tetravalent uranium for different hydrochloric acid concentrations was investigated. The measurements showed good agreement with values found in the literature, although the uranium concentrations were one million times lower.     

An experimental investigation of uranium redox reactions in an electrolytic cell with a cation exchange membrane

This paper investigates the simultaneous reduction and oxidation reactions of uranium in hydrochloric acid media. The redox reactions were carried out in a cell fitted with a cation exchange membrane and hexavalent uranium was reduced to the tetravalent state in the cathode chamber. The reverse reaction was carried out in the anode side of the cell. The reaction rate was found to be independent of the uranium concentration and the extent of reaction was linear with time. There is evidence of uranium flux across the membrane.