Thermal behaviour of uranium(VI) complexes

The solid-state syntheses of complexes of uranyl acetate dihydrate andN-phenylthiourea have been attempted by heating various stoichiometric mixtures of the reactants directly in a DSC and in a TA apparatus. Both the DSC and the TG results indicate that only the 1∶1 adduct is formed, independently of the molar ratios of the reactants. It appears that the reaction is complete only with a large excess ofN-phenylthiourea, in agreement with IR data.     

Thermal behaviour of chromium (III) perchlorate hexahydrate

Thermal behaviour of intimate mixtures of chromium(III) oxide and lithium¹, potassium², rubidium³, cesium³ and thallium(I)⁴ perchlorates revealed that chromium(III) oxide not only catalyses the decomposition by lowering the decomposition temperatures of the pure metal perchlorates but also chemically interacts resulting in the formation of metal dichromate. The oxidation of chromium(III) into the hexavalent state is attributed to the abstraction of oxygen from the perchlorate moiety during the decomposition. In this context, it was thought interesting to study the thermal behaviour of chromium(III) perchlorate and to identify the decomposition products in order to find out whether chromium(III) is oxidized into chromium(VI) by the perchlorate group. Except for a report⁵ on the preparation of chromium(III) perchlorate with different molecules of water of hydration no work seems to have been carried out on the thermal decomposition of this compound. In the present study, the decomposition characteristics are followed by TG and DTA techniques and the decomposition products have been examined by chemical analysis, X-ray powder diffraction patterns and infrared spectral measurements.     

Thermal decomposition of sodium chlorate and chromium(III) oxide mixtures

A study of the thermal decomposition of intimate mixtures of sodium chlorate and chromium(III) oxide in different molar ratios was made employing thermogravimetry, differential thermal analysis, chemical analysis, infrared spectroscopy and X-ray diffraction analysis. Sodium chlorate in the presence of chromium(III) oxide starts to decompose around 180°, which is much below the decomposition temperature of pure NaClO₃. Each mole of Cr₂O₃ consumes 8/3 moles of NaClO₃, undergoing oxidation to sodium dichromate.     

Thermal analysis of chromium(III) and chromium(VI) systems with silica and sodium silicate

Sodium silicate, or a mixture of silica and sodium carbonate, reacts with chromium(III) oxide in the presence of oxygen to give sodium chromate, within the approximate temperature range 300–900°. Above 900° the reaction is reversed and chromium(III) oxide regenerated.

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Zusammenfassung Natriumsilikat oder eine Mischung von Siliziumdioxid mit Natriumkarbonat ergeben bei der Reaktion mit Chrom(III)oxid in Gegenwart von Luft im Temperaturbereich zwischen 300° und 900° Natriumchromat. Über 900° wird die Reaktion umgekehrt und Chrom(III)oxid zurückgebildet.

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Résumé Le silicate de sodium ou un mélange de silice et de carbonate de sodium réagissent, en présence d'oxygène, avec l'oxyde de chrome trivalent pour former le chromate de sodium, approximativement entre 300 et 900°. Au-dessus de 900°, la réaction s'inverse et l'oxyde de chrome(III) est régénéré.

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(III) , 3002900°. 900° .

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We thank Messrs. Albright and Wilson for a grant (to D. A. F.)     

Sorption of chromium(III) and chromium(VI) on lead sulfide

The sorption of chromium(III) and chromium(VI) on lead sulfide has been investigated in dependence on pH, time of sorption and the concentrations of sorbate and sorbent. The mechanisms of the sorption of Cr³⁺ and CrO ₄ ^2– traces on lead sulfide are discussed; a difference between CrO ₄ ^2– sorption on PbS and -Fe₂O₃ has been found. Sulfates and molybdates affect the removal of chromates from aqueous solutions. Lead sulfide carrier prepared in this work was also used for the preconcentration of chromium(III) and chromium(VI) from tap water.     

Sorption of chromium(VI) and chromium(III) on aluminium hydroxide

Factors that influence the sorption of Cr(VI) and Cr(III) on aluminium hydroxide were investigated. The sorption of chromates decreases as the pH of the suspension increases. The mechanism of CrO ₄ ^2– sorption was interpreted in terms of reactions between chromates and –OH and/or H₂O groups at the hydroxide/liquid interface. It has been shown that chromates are more tightly sorbed on aluminium hydroxide compared to other anions, e.g. chlorides. On the other hand, specifically absorbed anions, such as molybdates, compete strongly with chromates for the sorption sites. The sorption of chromium(III) increases with the pH of the suspension. Also, the sorption of chromium(III) is suppressed in the presence of citrate ions. The best conditions for the fixation of Cr(VI) and Cr(III) by aluminium hydroxide are presented.     

Thermal decomposition of pentacoordinate uranium(VI) complexes

The thermal decomposition of some mixed uranyl complexes with Schiff bases and DMSO, EtOH or (Ph)₃PO as neutral ligand, were investigated and the corresponding activation energy, E*_a, and enthalpy of dissociation, ΔH_d, values were calculated.

The results obtained indicate that for the same neutral ligand, the thermal stability of the uranyl complexes is influenced by the Schiff base used; for the same Schiff base, the presence of (Ph)₃PO results in a greater thermal stability of the mixed complexes than when the other two neutral ligands are present.     

Mononuclear and homobinuclear vanadium(IV), chromium(III), molybdenum(III), and uranium(VI) chelates with ortho-cresolphthalein complexone

Mononuclear and homobinuclear o-cresolphthalein complexone complexes with VO²⁺, Cr³⁺, MoO⁺, and UO₂ ²⁺ have been prepared and their structures investigated. The empirical formulas, the mode of bonding, and the geometry of the complexes were obtained from elemental and thermal analyses, IR, electronic and ESR spectra, magnetic moment determinations, DC and CV polarographic studies.     

Novel chromium(III)/(VI) adducts of XPS-determined mixed valence,from electroreduced chromium(VI)

Summary Conditions of chromium(VI) and acidity have been found at which a novel brown deposit from electroreduction. Its 11 chromium(III):chromium(VI) composition, and the 21 ratio of the alkali-leached product, were established by XPS. The composition bounds for electrodeposition of the 11 solid and the competing insulative chromium(III) film were established electrochemically. Earlier reported chromium(III) and chromium(VI) solid or gels, some deposited from alkali, differ in colour and composition properties.     

Ultrafast concentration and speciation of chromium(III) and (VI)

There is an increasing need to know the concentrations of chromium(III) and (VI) separately rather than only the total chromium content. A method is described for accomplishing this very quickly using only low-cost, portable equipment. Two small, resin-loaded extraction disks are placed one on top of the other in a plastic holder. Then a syringe containing the aqueous sample is attached to the holder and the sample is pushed through the disks. In a matter of seconds, all of the chromium(VI) is retained on the top anion-exchange disk and chromium(III) is extracted by the second cation-exchange disk. The concentrations on each disk are several hundredfold higher than they were in the original sample. The amounts of chromium(III) and (VI) extracted are measured directly on the surface of the respective disks by diffuse reflectance spectroscopy (DRS). Despite the low molar absorptivity of chromium(III) in aqueous solution, the concentration on the upper most layer on the extraction disk is high enough to permit the determination of chromium(III) in samples at the low mg/L range. Chromium(VI) can also be determined at low to sub-mg/L concentrations. A study of the cation-exchange disks was undertaken to compare the performance characteristics of disks containing sulfonated resins and those with iminodiacetate functionality. In addition, data are presented to show the effects of heating the iminodiacetate disks after the initial extraction. The disks were heated in hot water for 15-30 min to complete the slow complexation reaction on the surface.     

Polarographic behaviour of uranium (VI) in organic solutions

Polarographic determination of uranium (VI) has been studied in the organic extraction phase TBP—diluent along with a selected aprotic solvent (i. e. dimethyl sulfoxide or N,N-dimethylformamide). DMF was found more suitable because it dissolves the organic extraction phase better than DMSO. U (VI) extracted in TBP-kerosene, n-hexane, cyclohexane, n-dodecane, benzene, from nitric acid medium can be determined in an organic solution (e. g. 50% DMF-30% TBP-20% kerosene) where it gives a well defined wave. In the organic solution, nitric acid added and/or extracted from the aqueous phase was found as an excellent supporting electrolyte for uranium determination.     

Spectrophotometric determination of chromium(III, VI) by use of chromium(III, VI)-Chrome AzurolS-cetylpyridinium bromide-hydroxylamine hydrochloride-zinc(II) system

This paper shows that the sensitivity of the Cr(III, VI)—Chrome Azurol S (CAS)-cetylpyridinium bromide (CPB)—hydroxylamine hydrochloride system can be increased and the wavelength of maximum absorption slightly shifted by addition of zinc(II) and that the analytical data are practically identical for both Cr(III) and Cr(VI), indicating that under the conditions used both initial oxidation states of chromium yield the same final oxidation state, Cr(III). On the basis of the Cr(III, VI)—CAS—CPB—NH₂OH·HCl—Zn systems a new, highly sensitive and selective method for spectrophotometric determination of microamounts of Cr(III, VI) has been developed, with molar absorptivity of 1.27 × 10⁵ 1. mole⁻¹ . cm⁻¹ for the complex at 620 nm and linear calibration up to 0.4 μg/ml chromium. Various foreign ions do not interfere. The method can be applied to direct determination of chromium in steels.     

Speciation studies by capillary electrophoresis- Simultaneous determination of chromium(III) and chromium(VI)

A method for the simultaneous determination of chromium(III) and chromium(VI) by capillary electrophoresis (CE) has been developed. The chromium(III) has been chelated with 1,2-cyclohexanediaminetetraacetic acid (CDTA) in order to impart a negative charge and similar mobility to both the chromium(III) and the chromium(VI) species. The effects of the amount of the reagent, pH and heating time required to complete the complexation have been studied. Factors affecting the CE behaviour such as the polarity of electrodes and the pH of electrophoretic buffer have been investigated. The separated species have been monitored by direct UV measurements at 214 nm. The detection limits achieved are 10 g/l for Cr(VI) and 5 g/l for Cr(III) and linear detector response is observed up to 100 mg/l. The procedure has been applied to the determination of both chromium species in industrial electroplating samples and its accuracy was checked by comparing the results (as total chromium) with those of atomic absorption spectrometry. No interference occurred from transition metal impurities under optimized separation conditions. The method is also shown to be feasible for determining Cr(III) as well as other metal ions capable to form complexes with CDTA (like iron(III), copper(II), zinc(II) and manganese(II)) in pharmaceutical preparations of essential trace elements.     

Speciation studies by capillary electrophoresis- Simultaneous determination of chromium(III) and chromium(VI)

A method for the simultaneous determination of chromium(III) and chromium(VI) by capillary electrophoresis (CE) has been developed. The chromium(III) has been chelated with 1,2-cyclohexanediaminetetraacetic acid (CDTA) in order to impart a negative charge and similar mobility to both the chromium(III) and the chromium(VI) species. The effects of the amount of the reagent, pH and heating time required to complete the complexation have been studied. Factors affecting the CE behaviour such as the polarity of electrodes and the pH of electrophoretic buffer have been investigated. The separated species have been monitored by direct UV measurements at 214 nm. The detection limits achieved are 10 microg/l for Cr(VI) and 5 microg/l for Cr(III) and linear detector response is observed up to 100 mg/l. The procedure has been applied to the determination of both chromium species in industrial electroplating samples and its accuracy was checked by comparing the results (as total chromium) with those of atomic absorption spectrometry. No interference occurred from transition metal impurities under optimized separation conditions. The method is also shown to be feasible for determining Cr(III) as well as other metal ions capable to form complexes with CDTA (like iron(III), copper(II), zinc(II) and manganese(II)) in pharmaceutical preparations of essential trace elements.     

Simultaneous and sequential determination of chromium(VI) and chromium(III) by unsegmented flow methods

Summary Several different configurations for simultaneous and sequential photometric speciation of Cr(VI) and Cr(III) based on the reversed flow injection analysis and completely continuous modes are proposed in this paper. The determination of these species at theg · ml^–1 level is achieved with sampling frequencies between 30 and 100 h^–1 and an r.s.d. of less than ±1% for simultaneous methods and of less than ±3% for the sequential method. The proposed methods are suitable for chromium speciation in waters. A simulation of the continuous monitoring of Cr(VI) and periodical of Cr(III) in natural and waste waters has been performed. The most frequent interferents in these types of samples have been investigated.

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Simultane und sequentielle Bestimmung von Chrom(VI) und Chrom(III) durch unsegmentierte Durchflußmethoden

Zusammenfassung Verschiedene Arten der simultanen und sequentiellen photometrischen Cr(III)- und Cr(VI)-Bestimmung nach der Technik der reversed-flow Injektionsanalyse werden beschrieben. Die relative Standardabweichung für die Bestimmung dieser Chromspezies im g/ml-Bereich bei einer Probenfrequenz von 30 bis 100 je Stunde beträgt weniger als +1% für die Simultan- und weniger als +3% für die sequentiellen Methoden. Eine simulierte kontinuerliche Überwachung von Chrom(VI) sowie eine periodische von Chrom(III) in natürlichem Wasser und Abwasser wird beschrieben. Die häufigsten Störungen werden diskutiert.

     

Determination of antimony(III) by titration with chromium(VI)

Summary There is no need for a catalyst in the potentiometric or spectrophotometric titration of antimony(III) with chromium(VI). The failure of titrations using indicators is due to slowness of the oxidized indicator-Sb(III) reaction; the indicator reaction can be catalysed by iodine. The spectrophotometric titration yields accurate results, and shows that the Sb(III)-Cr(VI) reaction is rapid. Slowness in the potentiometric titration is due to unfavourable electrode kinetics.

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Titration von Antimon(III) mit Chrom(VI)

Zusammenfassung Bei der potentiometrischen oder spektrophotometrischen Titration von Sb(III) mit Cr(VI) bedarf es keines Katalysators. Der Fehler bei Titrationen unter Verwendung eines Indikators hat seine Ursache in der langsam verlaufenden Reaktion zwischen dem oxydierten Indikator und Sb(III); die Indikator-Reaktion kann mit Jod katalysiert werden. Die spektrophotometrische Titration gibt genaue Resultate und zeigt, daß die Sb(III)-Cr(VI)-Reaktion rasch abläuft. Die Langsamkeit der potentiometrischen Titration ist durch die ungünstige Kinetik der Elektrode verursacht.

     

Retention of chromium(III) and chromium(VI) in rat tissues after different drying procedures

Losses of chromium(III) and chromium(VI) during freeze-drying and ovendrying at different temperatures from various rat tissues and feces containing the radioactive isotopes are assessed. Significant loss of chromium(III) occurs from fur samples (hair with skin) on freeze-drying. Oven-drying at 80°C does not cause losses of either form of chromium but at 105°C there are minor losses from kidney and feces samples. At 120°C, chromium(III) is lost from all samples to varying extents whereas losses of chromium(VI) are less significant.     

Neutron activation analysis for chromium(III) and (VI) in lixiviated liquid through a calcined chromium(VI) adsorbed hydrotalcite

Summary Electrophoresis followed by neutron activation analysis was utilized to determine chromium(III) and (VI) in mixed solutions. These solutions proceeded from Cr(VI) adsorbed hydrotalcites heated at 800 °C to partially immobilize Cr in the Mg-Al oxide solid solution. Immobilization was studied by Cr lixiviation with NaCl solutions through the heated hydrotalcites. The results have shown that Cr lixiviated was in the form of CrO₄^2- ions, mainly because some Cr(VI) was not completely reduced to Cr(III) during heating. Chromium lixiviated from HT-Cr sample, heated at 800 °C and γ-irradiated at 1000 kGys, was found, as well, in the form of CrO₄^2- ions. Although γ-irradiation increases Cr immobilization in the solid, it does not reduce completely all CrO₄^2- ions present in the solid and, therefore, some Cr is lixiviated through the solid in the form of CrO₄^2- ion.