Infrared absorption of chromium trioxide and its suboxides

The chromium oxides, obtained by the thermal decomposition of chromium trioxide, presented a bewildering array of possible mixed valency compounds. The IR absorption measurements in the range 400–5000 cm^?1 revealed the presence of the valencies +6, +5, +4, +3 in the samples between CrO₃ and Cr₂O₃.     

Physico-chemical properties of chromium trioxide and its suboxides

Seven samples of chromium suboxides of composition CrO_3-x are prepared from the thermal decomposition of CrO₃. The O/Cr ratio of each is determined by chemical analysis and ignition loss. X-Ray analysis reveals that they are more or less of the correct structure contaminated with the nearest suboxide. The specific gravity is determined pycnometrically and varies linearly with the O/Cr ratio.TGA indicates weight losses which agree with the calculated O/Cr ratio. DTA and DTG give endothermic peaks corresponding to the temperature of preparation of the successive samples.     

The electrical conductivity of chromium trioxide and its suboxides

The electrical conductivity of CrO₃ and seven samples between CrO₃ and Cr₂O₃ is measured between room temperature and the temperature of preparation of the corresponding sample using an AC circuit. CrO₃ is found to be a semiconductor having a resistance of the order of 10⁶ Ω when completely dried by heating to 100°C under a vacuum of 10^?3mm Hg. Cr₂O₃ gives a resistance of the order of l0⁵–l0³ Ω in the temperature range 25–370°C. The samples in between possess values ranging between 10⁵ and 10⁴ Ω. The activation energies are interpreted as being due to the extrinsic range.     

Preparation of chromium catalysts bearing bispyridylamine and its performance in ethylene oligomerization

Transition Metal Chemistry - A series of bispyridylamine ligands and chromium complexes designed for ethylene oligomerization have been synthesized, which was made up of both neutral organic ligand...     

Preconcentrative separation of chromium(VI) species from chromium(III) by coprecipitation of its ethyl xanthate complex onto naphthalene

A rapid, sensitive and selective method is described for the determination of chromium(VI) in presence of 100-fold amounts of chromium(III) by flame atomic absorption spectrometry (FAAS) in conjunction with coprecipitative preconcentration of its ethyl xanthate complex onto naphthalene. The solid mixture consisting of the chromium(VI) complex together with naphthalene is dissolved in 8.0 ml of dimethyl formamide (DMF) and chromium(VI) content was established by FAAS. Calibration graphs were rectilinear over the chromium(VI) concentration in the range 0-200 μg l⁻¹. Five replicate determinations of 20 μg of chromium(VI) present in 1.0 l of sample solution gave a relative standard deviation of 3.1%. The detection limit corresponding to three times the standard deviation of the blank was found to be 0.5 μg l⁻¹. The developed procedure has been successfully utilized for the estimation of chromium(VI), chromium(total) (after oxidation with bromate) and chromium(III) (by subtracting chromium(VI) content from chromium(total) value contents of several tannery industries.     

The interconversion of orthorhombic chromium oxy-hydroxide and chromium dioxide

The thermal behaviour of CrOOH and CrO₂ in different atmospheres has been studied by means of DTA, TG and X-ray diffraction. The easy interconversion between the two substances at one atmosphere pressure is explained on the basis of the results obtained.

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Zusammenfassung Es wurde das thermische Verhalten von CrOOH und CrO₂ in verschiedenen Atmosphären mit Hilfe der Differentialthermoanalyse, Thermogravimetrie und Röntgendiffraktion untersucht und auf Grund der Ergebnisse die leichte Umwandlung wischen den zwei Verbindungen unter 1 Atm. Druck erklärt.

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Résumé Étude du comportement thermique de CrOOH et de CrO₂ dans différentes atmosphères, par ATD, TG et diffraction de rayons X. Interprétation de l'interconversion légère entre les deux substances sous une pression d'une atmosphère.

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lO₂ , -. , .

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Thanks are due to Prof. Dr. S. C. Bevan of Brunei University and to Dr. E. Hockings of RCA (USA) for valuable discussions, and also to the D.G.E.S.I. (Spain) for a grant (to M.A.A.F.).     

Separation of chromium (VI) using complexation and its determination with GFAAS

In acidic medium and in the presence of chloride ions 2-[2-(4-methoxy-phenylamino)-vinyl]-1,3,3-trimethyl-3H-indolium chloride forms complex with Cr(VI). The optimum conditions (pH, concentration of Cl^- and the complex forming reagent) of the separation and extraction of Cr(VI) into toluene using this basic dye as a complexing reagent have been determined and the possible interferences of Ca, Mg, Na, K, Cr(III), Ni, Pb, Hg, Mn, Al, Cu have been studied. An electrothermal atomic absorption spectrometer (GFAAS) was used for the determination of Cr(VI). The detection limit of the method for Cr(VI) found to be 0.15 μg dm^− 3 and RSD for spiked drinking water was better than 3%.     

Microscopic detection of chromium as chromium(III)

Summary Trisodium phosphate is recommended as a reagent for the microscopic detection of chromium (III). Sensitivity of the test is 6 of chromium (III) in a volume of 0.01 ml. Of particular importance is the fact that aluminum and iron (III) do not react with the reagent to form crystalline precipitates.

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Zusammenfassung Tertiäres Natriumphosphat gibt mit Chrom(III)-Lösungen eine charakteristische Fällung, in der unter dem Mikroskop rechteckige Kristalle zu erkennen sind. Weder Aluminium noch Eisen(III) gibt mit diesem Reagens einen kristallinen Niederschlag. Die Reaktion gestattet den Nachweis von 6 Chrom in 0,01 ml Lösung. Auch Quecksilber(II) gibt charakteristische Kristalle.

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Résumé On recommande le phosphate trisodique comme réactif pour la recherche microscopique du chrome-III. La sensibilité de l'essai est 6 de chrome-III dans un volume de 0,01 ml. Le fait que l'alumine et le fer-III ne se combinent pas avec le réactif pour former des précipités cristallins présente une importance particulière.

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With 2 figures.     

The interconversion of orthorhombic chromium oxy-hydroxide and chromium dioxide

The thermal behaviour of CrOOH and CrO₂ in different atmospheres has been studied by means of DTA, TG and X-ray diffraction. The easy interconversion between the two substances at one atmosphere pressure is explained on the basis of the results obtained.     

A Rhodamine-based fluorescent sensor for chromium ions and its application in bioimaging

A rhodamine-based sensor (1) has been developed for the detection of chromium ions. Cr³⁺-induced opening of the rhodamine spirocycle in sensor (1) led to the distinct colorimetric and fluorescence responses. Among all the tested ions, only Cr³⁺ generated a significant fluorescence enhancement of up to 13-fold, which indicated the high selectivity of 1. Sensor (1) was successfully applied in the in vivo fluorescence imaging of Cr³⁺ in C. elegans. The results provided solid evidences for the future estimation of Cr³⁺ in environmental applications and tobacco samples.     

Determination of chromium by atomic absorption spectrophotometry of chromium acetylacetonate : Determination of chromium in sea water

A sensitive atomic absorption method is described for the determination of chromium at sub-microgram levels. Chromium(III) is converted to its acetylacetone complex and extracted into MIBK. The atomic absorption sensitivity is thus en hanced two-fold compared to chromium (III) in aqueous medium. The detection limit of chromium is 0.015 p.p.m. with an acetylene-air flame; the sensitivity obtained with other chromium, methods under the same instrumental conditions is compared. The method can be applied to the determination of chromium in sea water; at a level of ca. 1.6 μg Cr/1, the precision is ±0.06 μg/1.     

Elemental speciation for chromium in chromium picolinate products

Chromium picolinate products have been examined for different forms of chromium, using chromatographic separation and inductively coupled plasma mass spectrometric detection. The brands we evaluated contained no detectable amount of elemental chromium(VI), the toxic form. Since chromium picolinate might have other chromium forms as impurities, different products may contain different forms of chromium species. Compared with ion-exchange, reversed-phase chromatography showed excellent chromium recovery based on the amount stated on the product label.     

Synthesis of chromium N-heterocyclic carbene complexes using chromium Fischer carbenes as a source of chromium carbonyls

Chromium Fischer carbene complexes, [Cr{OMe(R)}(CO)₅] have been utilized as a source of chromium carbonyls in the synthesis of chromium NHC complexes. Using the synthetic method, chromium complexes of various NHC ligands were isolated in reasonable yields. Moreover, the method can be employed for the synthesis of molybdenum and tungsten NHC compounds.     

Removal and recovery of chromium and chromium speciation with MINTEQA2

Chromium(III) is commonly found in large quantities in tannery wastewaters. For this reason, the recovery of the chromium content of these wastewaters is necessary for environmental protection and economic reasons. Removal and recovery of chromium were carried out by using ion exchange resins. To this purpose, two weakly acidic exchange resins Amberlite IRC 76 and Amberlite IRC 718 and a strongly acidic exchange resin Amberlite IR 120 were used. Basic chromium sulphate [Cr(4)(SO(4))(5)(OH)(2)] solutions in different concentrations and pH were used in all experiments as tanning baths. The resins were prepared in two different ionic forms as Na(+) and H(+). The effects of concentration, pH, stirring time and resin amount were investigated. The concentration range varied between 5 and 100 mg l(-1), pH range was between 1 and 8, stirring time between 5 and 60, and resin amount was between 50 and 1000 mg. Stirring speed was 2000 rpm during all these experiments. Exchange capacities, moisture contents and optimum conditions of these resins were determined in batch system. The results obtained showed that Amberlite IRC 76 and 718 weakly acidic resins had shown better performance than Amberlite IR 120 strongly acidic resin for removal and recovery of chromium(III) in Na(+) form. Optimum conditions were found as concentration 10 mg l(-1), pH 5, stirring time 20 min, and resin amount 250 mg. Furthermore, chromium(III) speciation was investigated for optimum concentration and pH with MINTEQA2 computer programme. The studied pH range was between 1 and 8 and concentration range was between 5 and 100 mg l(-1). Cr(OH)(2+) species were found to be dominant at pH 5 and 10 mg l(-1) concentration in batch studies. There was a correlation between experimental and computerised results.     

Mechanism of photochemical reduction of chromium(VI) by alcohols and its environmental aspects

The idea of photochemical abatement of the Cr(VI) pollution has been verified by investigating the photoreduction mediated by aliphatic alcohols under conditions mimicking the environmental ones. Effects of the alcohol nature, pH and presence of oxygen are analysed. The time-resolved spectra are used to follow the generation of the transient chromium Cr(V), Cr(IV) and Cr(II) species and the R₁R₂CHOH√⁺ radicals. A direct interaction between chromate(VI) and an electron donor is a precondition of the photoreduction via the photoinduced electron transfer (PET). Two pathways of the PET are identified: one-electron transfer for intermolecular and two-electron transfer for the intramolecular systems. In the case of the alcohol mediators the option is pH-controlled.     

Rapid chelatometric estimation of chromium, iron and aluminium in chromium + iron and chromium + aluminium mixtures

Summary A study of the kinetics of the reaction of chromium(III) with EDTA [1,2] had shown that the rate of reaction, usually slow, can be accelerated in the presence of hydrogen carbonate, sulphite, nitrite and organic solvents like alcohols, ketone, dioxane, etc., or suppressed in the presence of oxalate, acetate, etc. Based on this principle, a rapid titrimetric method using EDTA was developed for the estimation of chromium(III), iron and aluminium in chromium(III) + iron and chromium(III) + aluminium mixtures. The method has been extended for the estimation of iron and chromium(III) in stainless steel. Results are satisfactory.

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Zusammenfassung Die Reaktion von Chrom(III) mit äDTA kann durch Hydrogencarbonat Sulfit, Nitrit sowie organische Lösungsmittel beschleunigt oder durch Oxalat, Acetat u. a. unterdrückt werden. Aufgrund dieser Erscheinung wurde ein chelatometrisches Verfahren zur Bestimmung von Chrom(III), Eisen und Aluminium in Gemischen von Cr + Fe oder Cr + Al ausgearbeitet, das auch auf die Bestimmung von Eisen und Chrom in rostfreiem Stahl angewendet wurde. Es wurden befriedigende Ergebnisse erhalten.

     

Thermal characterisation of alumina supported chromium and platinum–chromium catalysts

This study presented results on reduction of alumina supported chromium and platinum–chromium catalysts using temperature programmed reduction method (TPR). It has been shown that catalysts after earlier oxidation step but without calcinations one undergo reduction in lower temperature in comparison to calcined only catalysts. Moreover, addition platinum to Cr/Al₂O₃ catalysts also caused decrease of reduction temperature. It has been observed that over the examined catalysts oxidation CO to CO₂ and reduction CO to CH₄ occurs. However, on Pt–Cr catalysts both reactions proceed at lower temperature compare to Cr catalysts.     

Substoichiometric extraction of chromium

Substoichiometric extraction of chromium with tetraphenylarsonium chloride (TPACl), tri-n-octylamine (TNOA), diethylammonium diethyldithiocarbamate (DDDC) and ammonium pyrrolidinedithiocarbamate (APDC) was examined in detail. Chromium can be extracted substoichiometrically in a pH range, which is 1.1–2.6 for the TPACl compound, 0.6–2.3 for the TNOA compound, 5.1–6.4 for the DDCC chelate and 3.9–4.9 for the APDC chelate. Chromium in high-purity calcium carbonate, Orchard Leaves (NBS SRM-1571) and Brewers Yeast (NBS SRM-1569) was determined by neutron activation analysis combined with substoichiometric extraction by DDDC and APDC. The values of 2.0±0.02 ppm and 2.6±0.2 ppm were obtained for Brewers Yeast and Orchard Leaves, respectively. These values were in good agreement with the values reported by NBS. The reaction mechanism and the reaction ratio between hexavalent chromium and dithiocarbamate were also discussed.