Determination of the O/M ratios of polynary uranium oxides by Ce(IV)-Fe(II) back titration after dissolution in mixed sulphuric and phosphoric acids

Uranium (IV) in polynary uranium oxides is determined after the solid has been dissolved in a warm mixed solution of sulphuric and phosphoric acids containing excess Ce(IV). The latter is titrated with a Fe(II) standard solution using ferroin as indicator. This method is especially effective for (mixed) uranium oxides which are difficult to dissolve in hot Ce(IV) sulphuric acid. The standard deviation of the determined x value in polynary oxides is estimated to be below ± 0.004 for samples of 10–30 mg.     

A new volumetric method for the determination of uranium(VI)

Summary A new volumetric method for the estimation of uranium(VI) salt based on its photoreduction in the presence of diethyl ether has been developed. The recommended procedure consists of exposing uranium(VI) solution in about 1 N sulphuric acid solution with an excess of saturated aqueous ether solution in a glass vessel to the light from a Phillips repro lamp or sun light for 1 hour. The uranium(IV) salt formed is estimated by titration with a standard solution of sodium vanadate.The reduction does not proceed to uranium(III) stage under any conditions of exposure. Fluoride, phosphate, arsenate and perchlorate are not found to interfere either with the photochemical reduction or with the subsequent oxidimetric titration. But chloride and bromide ions markedly inhibit the photochemical reaction.     

N-oxides of 4-(5-nonyl)pyridine and trioctylamine as extractants for cerium(IV) and its separation from thorium(IV)

Trace level cerium has been oxidized to the quadrivalent state with potassium dichromate and shown to be preferentially extracted from very dilute mineral acid solutions and also from moderate nitric acid media by 0.1M solutions of 4-(5-nonyl)pyridine oxide and trioctylamine oxide dissolved in xylene. The dependence of extraction on the type of N-oxide, acid concentration and the N-oxide concentration has been investigated. The influence of the concentration of salting-out agents is described. Separation factors for a number of metal ions relative to cerium(IV) are reported for 0.1 M 4-(5-nonyl)pyridine oxide/xylene-0.1M sulphuric acid system. The ratio of the D for Ce(IV) to that of Ce(III) is greater than 10⁵, and the D for Ce(IV) is much greater than that for thorium(IV). Separation of cerium(IV) from thorium has been achieved from 0.1M sulphuric acid solutions using 0.1M 4-(5-nonyl)pyridine oxide/xylene as an extractant.     

Controlled potential coulometry: the application of a secondary reaction to the determination of plutonium and uranium at a solid electrode

A method is described for the simultaneous determination of plutonium and uranium in mixed oxides by controlled potential coulometry at a gold working electrode in two stages: first a coulometric oxidation, at 0.73 V vs. a silver/silver chloride electrode, of Pu(III) and U(IV) to Pu(IV) and U(VI) by a combination of a direct electrode reaction and a secondary chemical reaction proceeding concurrently, and secondly, a coulometric reduction at 0.33 V of Pu(IV) to Pu(III), leaving uranium as U(VI). The determination is carried out in a mixture of sulphuric and nitric acids, and Ti(III) is used to reduce plutonium and uranium to Pu(III) and U(IV) before electrolysis. The precision (3sigma) of Pu:U ratio results obtained from mixtures containing about 30% and 2% plutonium was 0.5% and 1-5% respectively. The effect of experimental variables on the time taken to complete the coulometric determination is discussed.     

Catalytic amperometric and catalytic constant-current potentiometric titrations of silver(I), palladium(II) and mercury(II)

Amperometry and constant-current potentiometry were used to follow the course of catalytic titrations of silver(I), palladium(II), and mercury(II) with potassium iodide. The Ce(IV)As(III) and Ce(IV)Sb(III) systems in the presence of sulphuric acid were used as indicator reactions. The possibilities of application of platinum, palladium, gold, graphite, and glassy-carbon indicator electrodes were investigated. Graphite appeared to be somewhat more advantageous than the other electrode materials. The effect of concentration of the components of the indicator reactions, the presence of organic solvents and acids on the shape of the catalytic titration curves was studied. Amounts of 30-3000 mug of silver(I) nitrate, 90-900 mug of palladium(II) chloride, 130-1300 mug of mercury(II) chloride, and 150-1500 mug of mercury(II) nitrate were determined with a relative standard deviation less than 1.0%. The results obtained were in good agreement with those of comparable methods. The catalytic titration method developed was applied to determination of mercury in Unguentum Hydrargyri.     

Spectrophotometric determination of the oxygen to uranium ratio in uranium oxides based on dissolution in sulphuric acid

The oxygen to uranium ratio in uranium oxides such as U(3)O(8), UO(2+x) powders and UO(2) fuel pellets has been determined by a new spectrophotometric method. The method can be used for determination of O/U ratio in UO(2) pellets and powders on a routine basis. In the described method, uranium oxides in the powder form are dissolved in 2M sulphuric acid containing a few drops of HF. The concentrations of U(IV) and U(VI) are directly determined by means of the absorbances of these species at different wavelengths. For determination of the O/U ratio in U(3)O(8) powder samples, 630 and 310 nm are the wavelengths chosen for U(IV) and U(VI), respectively. For UO(2+x) powder, where the O/U ratio lies between 2.04 to 2.15, U(IV) and U(VI) are determined at 630 and 300 nm respectively, whereas for UO(2) fuel pellets, where the O/U ratio is less than 2.01, 535 and 285 nm are used. The molar absorptivity of U(IV) at 630 and 535 nm is 21.4 and 6.8 l.mole(-1).cm(-1) and that of U(VI) at 310, 300 and 285 nm is 178.1, 278.6 and 585 l.mole(-1).cm(-1), respectively. Standard deviations of +/-0.002 O/U ratio units for pellets and +/-0.004 O/U ratio units for powders have been achieved.     

Pyridine-2-aldoxime and 6-methylpyridine-2-aldoxime as gravimetric reagents for estimation of palladium(II) and uranium(VI)

Pyridine-2-aldoiumc (I) has been found to be a sensitive reagent for the gravimetric determination of palladium(II). From chloride medium, precipitation is complete at pH 3.0-11.0, and in solution containing 1NHNO(3) to pH6.0. The compositions of the precipitates (dried at 130 degrees ) correspond to PdL(2), and PdL(2). HNO(3) (HL representing the reagent) respectively. Pd(II) can be estimated gravimetncally in presence of acetate, oxalate, tartrate, phosphate, fluoride borate, perchlorate, Cu(II), Cd, Co(II), Fe(II), Ni, Zn, Pb, Bi, Sb(III), Pt(IV), Ir(IV), Ru(III), Rh(III); Os(IV) in quantities more than twice that of Pd(II), and Ag(I), Au(III) and Fe(II) even m traces cause serious interference. The yellow uranium(VI) complex with (I) is precipitated quantitatively over the pH range 3.5-10.5 and, after washing and drying corresponds to the composition (c(6)h(5)n(2)o)(2)uo(2), The uranium(VI) complex with 6-methylpyridine-2-aldoxime (II) is precipitated quantitatively over the pH range 3.0-10.5, and after washing and drying at 120-130 degrees corresponds to UO(2),(C(7),H(7),N(2)O)(2). Both (I) and (II) are suitable for the estimation of 1-50 mg of uranium(VI) in the presence of up to 10-fold quantities ofTh(IV), La(III) and Ce(III) even when present together. Ce(IV) in quantities more than three times that of U must be reduced to Ce(III). Tartrate, citrate, phosphate, Ti(IV) and Zr interfere, but acetate, oxalate, and borate do not.     

Kinetics and Mechanism of Oxidation of Adipic Acid by Ce(IV) Ion in Acidic Medium

Kinetic study of oxidation of adipic acid by Ce(IV) ion in aqueous solution of sulphuric acid shows that the reaction follows first order kinetics in both Ce(IV) and adipic acid and the over all reaction order ascertained is two. The specific rate constant increases with an increase in the concentration of adipic acid. Effects of hydrogen ion concentration, bisulphate ion and temperature have been studied in detail. Various kinetic parameters have been computed. The experimental findings are consistent with the mechanism involving rapid resersible formation of an activated complex between Ce(IV) and adipic acid followed by a rate determining step involving C-C bond fission.     

Sequential determination of uranium (IV), free acidity and hydrazine in a single aliquot

A simple analytical procedure for the sequential determination of uranium (IV), free acidity and hydrazine in presence of hydrolysable ions is developed and described. In this method, first, uranium (IV) is determined using fiber optic aided spectrophotometry then same solution is used for determination of free acidity and hydrazine. Free acid is titrated with standard sodium carbonate solution after uranium (IV) is masked with EDTA. Once the end point for the free acid is determined at pH 3.0, an aliquot of formaldehyde is added to liberate the acid equivalent to hydrazine which is then titrated with the same standard sodium carbonate solution using an automatic titration system. The described method is simple, accurate and reproducible. The overall recovery of uranium (IV), nitric acid and hydrazine is 98% with 3% relative standard deviation respectively. The major advantage of the method is that it uses sodium carbonate a primary standard as titrant and generation of corrosive analytical wastes containing oxalate or sulphate is avoided. Valuable metals like uranium and plutonium can easily be recovered from analytical waste before final disposal.     

Solvent extraction of uranium(VI) into toluene by dicyclohexano-18-crown-6 from mixed aqueous-organic solutions

Extraction behaviour of uranium(VI) from mixed organo-aqueous solutions containing water-miscible protic aliphatic alcohols and several aprotic solvents was investigated by using dicyclohexano-18-crown-6(DC18C6) as an extractant. The organic phase was a binary solution of DC18C6 and toluene while the polar phase was a three component solution of uranyl nitrate, polar additive and aqueous nitric acid. Methanol, ethanol, isobutanol, dioxane, acetone, propylene carbonate and acetonitrile were used as the organic components of the mixed (polar) phase. Propylene carbonate, acetone, acetonitrile and dioxane increased the extractability of U(VI), whereas alcoholic additives showed only an antagonistic effect. The relative increase in extraction was found to be more at lower nitric acid concentrations. Possible reasons for such behaviour are briefly discussed. Recovery of U(VI) from loaded organic phase was easily accomplished using dilute perchloric acid and sulphuric acid. A sample method was standardized for the separation of plutonium(IV) from uranium(VI) based on its reductive stripping.     

A scheme for the analysis of monazite and monazite concentrates

A scheme using ion-exchange methods is described for the analysis of monazites and monazite concentrates. The sample is opened up with concentrated sulphuric acid, and the resultant solution is applied to a column of Zeocarb 225 resin. After phosphate has been washed out, lead, aluminium, titanium, iron, uranium, calcium and magnesium are eluted with N hydrochloric acid and determined by specific, mainly spectrophotometric, methods. Rare earth elements are eluted with 3 N hydrochloric acid. Cerium is separated from the other rare earths by solvent extraction of its nitrate with methyl iso-butyl ketone; both groups are determined gravimetrically. Thorium is eluted from the ion-exchange resin with 3.6 N sulphuric acid and determined spectrophotometrically with thorin.The sulphuric acid-insoluble minerals are brought into solution by a double fusion method, and the determinations are carried out by a combination of ion-exchange and photometric procedures. Silica, phosphorus pentoxide, tin and chromium are determined by photometric methods, using separate portions of the sample.Lanthanum, yttrium and ytterbium are determined in a 1 M perchloric acid solution of the mixed rare earth oxides (less cerium) using flame photometry. Samarium, praseodymium and neodymium are determined by spectrophotometry.     

Coulometric determination of uranium with a platinum working electrode

Experimental conditions have been established which enable uranium to be determined coulometrically by the reduction of uranium(VI) to uranium(IV) at a platinum working electrode, by controlled-potential or controlled-potential-limit techniques. The procedure has been used successfully as a subsidiary method in the routine determination of uranium in pure uranyl nitrate solutions. The platinum electrode has several important practical advantages over the well established mercury-pool electrode for the coulometric determination of uranium. The consecutive determination of iron(III) and uranium(VI), or plutonium(IV) and uranium(VI) can be carried out with the same working electrode in the same solution and the coulometric oxidation of uranium(IV) to uranium(VT) is practicable. The rate of stirring of the cell liquor is much less critical in the case of the platinum electrode. Two main problems had to be overcome before a practical procedure could be achieved; hydrogen evolution during the uranium(VI)-(IV) reduction had to be eliminated so that 100% current efficiency could be obtained for the desired reaction and electrode-surface poisoning phenomena had to be controlled so that reaction times could be kept reasonably short. It was found that selection of a hydrochloric acid base solution containing a small amount of bismuth(III) enabled hydrogen evolution to be avoided: also electrode-surface poisoning with this base solution was not particularly serious and could be maintained at a satisfactorily low level by occasionally anodizing the electrode in dilute sulphuric acid. Bismuth(III) forms a complex with chloride ions and its presence increases the hydrogen overvoltage at the working electrode: no visible deposit of bismuth metal forms on the electrode during the uranium reduction. Samples containing nitrate can be analysed provided sulphamic acid is added to this hydrochoric acid base solution.     

Titrimetric determination of U(iv) alone and in mixtures with V(IV), Mn(II), Ce(III) and Fe(II)

Summary A rapid and accurate method is described for the potentiometric determination of uranium(IV) with permanganate at room temperature using trace amounts of ortho-phosphoric acid as a catalyst. The procedure has been extended for the differential potentiometric determination of mixtures with vanadium, manganese or cerium. The methods are easy, non-time consuming and free from interference by a large number of foreign ions. Conditions are also developed for the differential photometric determination of uranium and iron in mixtures.Based on these procedures, a differential titrimetric procedure has been developed for determination of iron(III), vanadium(V), chromium(VI) and manganese(VII) [or cerium(IV)] in a single solution at room temperature. This procedure has also been tested on Bureau of Standard samples.

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Titrimetrische Bestimmung von U(IV) allein und in Mischungen mit V(IV), Mn(II), Ce(III) und Fe(II)Anwendung auf die Analyse von Stählen und Legierungen

Zusammenfassung Eine schnelle und genaue Methode wird beschrieben zur potentiometrischen Bestimmung von Uran(IV) mit Permanganat bei Raumtemperatur unter Verwendung von Spuren Orthophosphorsäure als Katalysator. Das Verfahren wurde auf die differentialpotentiometrische Bestimmung im Gemisch mit V, Mn und Ce ausgedehnt. Die rasch und einfach ausführbare Methode wird durch zahlreiche Fremdionen nicht gestört. Eine differentialphotometrische Bestimmung von U(IV) und Fe(II) im Gemisch wird ebenfalls angegeben, außerdem eine differentialtitrimetrische Bestimmung von Fe(III), V(V), Cr(VI), Mn(VII) [oder Ce(IV)] in einer Lösung. Anwendungsbeispiele für Stähle und Legierungen werden beschrieben.

     

Visible-light induced oxo-bridged Zr(IV)-O-Ce(III) redox centre in tetragonal ZrO2-CeO2 solid solution for degradation of organic pollutants

Tetragonal ZrO(2)-CeO(2) solid solutions with composition Zr(1-x)Ce(x)O(2) (x = 0.1, 0.2, and 0.3) were synthesized in a citrate complexation route and characterized by XRD, XPS, UV-vis diffuse reflectance and ESR measurements. The formation of the homogeneous solid solution Zr(1-x)Ce(x)O(2) constructed the oxo-bridged bimetallic Zr(IV)-O-Ce(III) linkage between two neighboring flattened tetrahedrons of the structural framework. As compared to their parent oxides, the ZrO(2)-CeO(2) solid solutions exhibited optical absorption extending to longer wavelengths in the visible region. The red shift in the absorption spectrum was demonstrated to be partially due to a metal-to-metal charge transfer (MMCT) transition of the oxo-bridged Zr(IV)-O-Ce(III) linkage. The visible-light induced MMCT transition of Zr(IV)-O-Ce(III)→ Zr(III)-O-Ce(IV) resulted in the generation of the additional Ce(IV) and superoxide anion radical formed by the interaction of Zr(III) with adsorbed O(2). Catalytic activity evaluation revealed that the photoexcitation of the MMCT over the solid solution can initiate the degradation of RhB and 2,4-DCP upon visible-light irradiation, whereby Zr(III) and Ce(IV) act as a site-specific reducing and oxidizing center, respectively. The structure of the solid solution Zr(1-x)Ce(x)O(2) and the oxidation states of Zr and Ce species are also discussed in detail.     

The oxidative decarboxylation of polyaminocarboxylic acidm-VI Reaction of diethylenetriaminepenta-acetic acid with cerium(IV) in sulphuric acid media

The oxidation of diethylenetrianunepenta-acetic acid (DTPA) by Ce(IV) in sulphuric add was investigated spectrophotometrically by the stopped-flow technique. The rate of reaction is influenced by the acidity, but can be expressed by a simplified rate law At [H(2)SO(4)] below 0.75M the reaction proceeds stepwise as shown by formation of a 1:1 Ce(IV)-DTPA complex with measurable rate of formation and decay. At higher acid strengths, the formation of an intermediate is not evident. The rate is maximal in ~ 0.75M sulphuric add. The overall stoichiometry varies with time.     

Determination of plutonium and uranium in mixed oxide fuels by sequential redox titration

The use of a sequential determination of uranium and plutonium in a single sample solution results in a saving in analysis time and apparatus requirements. The method starts with U(IV) and Pu(in) in a mixture of sulphuric and nitric adds. Titration with dichromate, using amperometry at a pair of polarizable electrodes, produces two well-defined end-points corresponding to the sequential oxidation of U(IV) to U(VI) and Pu(III) to Pu(IV). The quantitative oxidation of U(IV) to U(VI) is achieved via the action of Pu(IV) as intermediate, and is dependent upon establishing conditions which favour rapid reaction between U(IV) and Pu(IV). The method is precise and accurate. With Pu-U mixtures containing between 15 and 30% plutonium the precision (3sigma) of the Pu: U ratio results is +/-0.6% on samples containing 100-120 mg of plutonium plus uranium. Iron and vanadium interfere quantitatively with plutonium, copper interferes non-quantitatively with uranium, and gross amounts of molybdenum mask the uranium end-point.     

Fluorimetric determination of uranium(VI) in waters by flow-injection analysis after preconcentration on a silica gel microcolumn

A method was developed for the selective determination of trace concentrations of uranium(VI) by flow-injection analysis (FIA) with fluorimetric detection. Uranium(VI) is selectively separated and/or pre-concentrated from a volume up to 20 ml on an activated silica gel microcolumn (2 x 40 mm) from a medium of 0.03M EDTA, 0.06M tartrate, and/or 0.05M NaF at pH = 9.3. After washing the column the uranium is eluted with a mixture of 1.33M sulphuric and phosphoric acids and determined with a relative standard deviation not exceeding 6% for concentrations in the range 10-250 mug/l. The detection limit was estimated to be 0.1-0.2 mug of uranium. The method has been verified on artificial water samples with high content of the interfering elements and applied to analysis of waste and natural waters.     

Determination of oxygen to uranium ratio in irradiated uranium dioxide by controlled potential coulometry

This work reports the determination of oxygen to uranium (O/U) ratio in irradiated UO_2+x fuel pellet of burnup of ca. 34 GWd/t by controlled potential coulometry. The method is based on the dissolution of the nuclear fuel in strong phosphoric acid (SPA) at 180–190 °C under an inert atmosphere. After dissolution, 8% sulphuric acid is added in order to obtain a 20% SPA in 8% sulphuric acid. A controlled potential coulometric determination of uranium(VI) is carried out at ?0.60V vs. ferri-ferrocyanide. The uranium(IV) contained in an aliquot of the fuel solution is oxidised to uranium(VI) with cerium(IV) sulphate, and the total uranium content is then determined by coulometry. Optimum experimental conditions have been established using simulated irradiated fuel solution containing various fission products which include cerium, tellurium, palladium, ruthenium, molybdenum and zirconium. Interference of the fission products and the possible removal of their interferences by preelectrolysis at +0.5 V vs. saturated calomel electrode (SCE) have been investigated. The accuracy of the coulometric method is confimed by polarographic measurement using several unirradiated UO_2+x fuel of known stoichiometry.     

Some triphenylmethane dyes as redox indicators in the titration of antimony(III) with cerium(IV) sulphate

Erioglaucine A, Eriogreen B, Patent Blue and Xylene Cyanol FF work satisfactorily for the titration of Sb(III) with Ce(IV) in 1-2N sulphuric acid medium with iodine as catalyst. In hydrochloric acid medium the colour of the oxidized dye is very evanescent but is very much improved in intensity and stability by the addition of manganese(II) sulphate; no catalyst is needed.     

Successive complexometric determination of thorium and uranium in sulphuric acid media

A selective analytical extraction method for rapid successive complexometric determination of thorium(IV) and uranium(VI) in sulphuric acid media is described. The method is based on the extraction of thorium and uranium from sulphuric acid media with N-butylaniline or N-benzylaniline in chloroform. Both thorium and uranium are selectively and quantitatively extracted in the presence of ascorbic acid and EDTA. Most cations and anions do not interfere. The reduction of uranium(VI) with sodium dithionite at room temperature is rapid and quantitative and superior to that with ascorbic acid, which reduces uranium(VI) in boiling solution. The method is simple, rapid and accurate, and the experimental conditions are not highly critical.