Simultaneous, sensitive and selective on-line chemiluminescence determination of Cr(III) and Cr(VI) by capillary electrophoresis

A novel in-capillary reduction and capillary electrophoretic (CE)-chemiluminescence (CL) method was developed for the sensitive and selective determination of chromium(III) and chromium(VI). The proposed method was based on the in-capillary reduction of Cr(VI) with acidic H₂O₂ to form Cr(III) using the zone-passing technique and chemiluminescence detection of Cr(III). The sample [Cr³⁺ and CrO₄²⁻], hydrochloric acid, and H₂O₂ (reductant) solution segments were injected for specified periods of time in this order from the anodic end of a capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr³⁺ migrates to the cathode while CrO₄²⁻ ion, moving oppositely to the anode, reacts with acidic H₂O_2, resulted in formation of Cr³⁺. Based on the migration time difference of both Cr³⁺ ions, they were separated by zone electrophoresis. Running buffer was composed of 0.02 mol l⁻¹ HAc-NaAc (pH 4.7) with 1×10⁻³ mol l⁻¹ EDTA. Parameters affecting CE-CL separation and detection, such as reductant concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, stability of luminol-hydrogen peroxide mixed solution were optimized. The limits of detection for chromium(III) and chromium(VI) (3σ) were 6×10⁻¹³ mol l⁻¹ (mass concentration 12 zmol) and 8×10⁻¹² mol l⁻¹ (160 zmol), respectively. This method offered potential advantages of simplicity, sensitivity, selectivity and applicability to the determination of Cr(III) and Cr(VI) in environmental water.     

Influences of temperature and atmosphere on thermal stability of BaCrO4

In this article, the influences of temperature and atmosphere on thermal stability of BaCrO₄ were investigated. BaCrO₄ powders with an orthorhombic structure were synthesized by a facile aqueous solution route. The synthesized BaCrO₄ products were then heat treated at different atmospheres to evaluate their thermal stability by differential thermal analysis–thermogravimetry (DTA–TG), X-ray photoelectron spectroscopy and X-ray diffraction. BaCrO₄ has a good thermal stability and does not decompose in air up to 1,400 °C. However, the decomposition of BaCrO₄ in vacuum depends mainly upon a two-stage chemical reaction. BaCrO₄ is finally decomposed into BaCr₂O₄ with trivalent Cr³⁺ cations and Ba₃(Cr⁶⁺ Cr⁵⁺)₂O_9?x with both pentavalent Cr⁵⁺ and hexavalent Cr⁶⁺ cations after heat treatments in vacuum.     

Aqueous Solutions of Cr(III) Sulfate: Modeling of Equilibrium Composition and Physicochemical Properties

Chemical equilibria in aqueous solutions of chromium sulfate are studied and the mathematical model of chromium complexation is developed in a wide range of concentrations (0.001–1.2 mol/l). The results of calculations satisfactorily agree with the experimental data, which confirms the adequacy of the suggested mathematical model of the equilibrium composition of an aqueous chromium sulfate solution. The nature of green modification of Cr₂(SO₄)₃ is established. This modification occurs in its diluted solutions (c < 0.1 mol/l) due to predominant polynuclear chromium hydroxo complexes, whereas in the concentrated solutions (c > 0.1 mol/l), it forms due to prevailing chromium sulfate complexes. The diagram was plotted that shows the regions with different prevailing forms of the complexes in the equilibrium system H₂O–Cr₂(SO₄)₃ at different concentrations and pH and explains the experimental data on the basis of some physicochemical properties (electric conductivity, viscosity, light absorption) of chromium sulfate solutions within a wide range of concentrations.     

Comparative study of Cr(III) adsorption by carbon nanotubes and active carbons

The adsorption of trivalent chromium ions from aqueous solutions on the surface of carbon materials, namely, multiwall carbon nanotubes (NTs) and two samples of active carbon, is studied depending on pH and adsorbate concentration in the system. Isotherms of Cr(III) adsorption by the aforementioned materials are obtained. It is shown that chromium ions are predominantly bound by surface carboxyl groups. The adsorption of chromium ions reduces the electrokinetic potential of NTs and, at chromium concentrations C _Cr(III) > 10^–5 M, leads to the reversal of the surface charge. The adsorption value decreases in the series NT > Merck carbon > Norit carbon, in contrast to an increase in the adsorbate affinity to the adsorbent in this series, as determined from the slope of the initial section of the Langmuir isotherms. Small amounts of chromium ions sorbed at low concentrations in solution (C _Cr(III) ≤ 10^–5 M) are comparable with the concentration of hydrogen ions displaced from the surface, thus making it possible to suppose the existence of an ionexchange adsorption mechanism. As the concentration of Cr(III) increases, the equivalent displacement of H⁺ is violated, thereby indicating the development of other adsorption mechanisms (complexation).     

Preparation of BaCrO4 particles in the presence of EDTA from aqueous solutions

BaCrO₄ particles with well-defined morphologies have been synthesized in the presence of EDTA as a crystal growth modifier from aqueous solutions. The influence of pH on the morphology of BaCrO₄ crystals with and without EDTA additive was investigated to better understand the formation mechanism. EDTA has a strong interaction with the crystal faces of BaCrO₄ and influences the final morphology of BaCrO₄ crystals. In the presence of EDTA, BaCrO₄ particles at pH?=?6 always exhibit a spindle-like morphology due to the large inhibition effect of EDTA on the BaCrO₄ crystallization. With decreasing pH, the particle morphology changes into a peanut-type at both pH?=?5 and pH?=?4 due to the relatively low inhibition and the following secondary heterogeneous nucleation and growth. The possible morphological evolution of BaCrO₄ particles is also proposed.     

Speciation of Cr(III) and Cr(VI) in Environmental Samples after Solid Phase Extraction on Amberlite XAD–2000

A method for speciation of Cr(III) and Cr(VI) in real samples has been developed. Cr(VI) has been separated from Cr(III) and preconcentrated as its pyrrolidinedithiocarbamate (APDC) complex by using a column containing Amberlite XAD–2000 resin and determined by FAAS. Total chromium has also been determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with KMnO₄. Cr(III) has been calculated by subtracting Cr(VI) from the total. The effect of pH, flow‐rate, adsorption and batch capacity and effect of various metal cations and salt anions on the sorption onto the resin were investigated. The adsorption is quantitative in the pH range of 1.5–2.5, and Cr(VI) ion was desorbed by using H₂SO₄ in acetone. The recovery of Cr(VI) was 97 ± 4 at a 95% confidence level. The highest preconcentration factor was 80 for a 200 mL sample volume. The adsorption and batch capacity of sorbent were 7.4 and 8.0 mg g^?1 Cr(VI), respectively, and loading half time was 5.0 min. The detection limit of Cr(VI) is 0.6 μg/L. The procedure has been applied to the determination and speciation of chromium in stream water, tap water, mineral spring water and spring water. Also, the proposed method was applied to total chromium preconcentration in microwave digested moss and rock samples with satisfactory results. The developed method was validated with CRM‐TMDW‐500 (Certified Reference Material Trace Metals in Drinking Water) and BCR‐CRM 144R s (Certified Reference Material Sewage Sludge, Domestic Origin) and the results obtained were in good agreement with the certified values. The relative standard deviations were below 6%.     

Characterization of Cr(III)-Zr(IV) mixed and Sn(II) doped hydrous oxides

Surface and structural properties of chromium-zirconium mixed and Sn(II)doped hydrous oxide gels have been compared with chromium oxide hydrate gel by the use of thermal analysis, IR spectroscopy, X-ray diffraction, electron microscopy and magnetic measurements. The mixed and doped oxide gels were found to have a hexagonal close packed stacking of O, OH and H₂O ligands with chromium ions distributed in octahedral sites with little degree of order among them. The microstructure of the gels are characterized by the presence of large aggregates of chromium hydroxides, fine granular sheets due to HCrO₂ phase and Cr(OH)₃ microcrystallites. Magnetic susceptibility measurements indicate anti-ferromagnetic behaviour of these gels.     

Speciation Analysis of Cr(III) and Cr(VI) after Solid Phase Extraction Using Modified Magnetite Nanoparticles

A new solid phase extraction (SPE) method has been developed for the speciation of Cr(III) and Cr(VI). This method is based on the adsorption of Cr(VI) on modified alumina‐coated magnetite nanoparticles (ACMNPs). Total chromium in different samples was determined as Cr(VI) after oxidation of Cr(III) to Cr(VI) using H₂O₂. The chromium concentration has been determined by flame atomic absorption spectrometric (FAAS) technique and amount of Cr(III) was calculated by substracting the concentration of Cr(VI) from total chromium concentration. The effect of parameters such as pH, amount of adsorbent, contact time, sample volume, eluent type, H₂O₂ concentration and cetyltrimethylammonium bromide (CTAB) concentration as modifier on the quantitative recovery of Cr(VI) were investigated. Under the optimal experimental conditions, the preconcentration factor, detection limit, linear range and relative standard deviation (RSD) of Cr(VI) were 140 (for 350 mL of sample solution), 0.083 ng mL^?1, 0.1‐10.0 ng mL^?1 and 4.6% (for 5.0 ng mL^?1, n = 7), respectively. This method avoided the time‐consuming column‐passing process of loading large volume samples in traditional SPE through the rapid isolation of CTAB@ACMNPs with an adscititious magnet. The proposed method was successfully applied to the determination and speciation of chromium in different water and wastewater samples and suitable recoveries were obtained.     

Radiochemical determination of chromium at ultratrace levels in biological materials

Determination of chromium at naturally occuring levels in biological materials requires very low detection limits and dust-free conditions during sampling. These levels are of interest in order to distinguish from enhanced levels which may be carcinogenic. To this purpose a radiochemical separation technique which includes extraction of Cr(VI) into tribenzylamine/chloroform and reextraction into alkaline solution has been improved. Without time-consuming precipitation of BaCrO₄ the detection limit has been decreased by a factor of 1.7 compared to earlier, work (to values like 1 ppb) in samples of typically 35 mg by increase of decontamination of⁶⁵Zn by a factor of up to 1000. Formation of⁶⁵Zn(OH)₂ is shown as the reason for this result. Yield determination was carried out by ICP-AAS-measurements and UV/VIS-spectroscopy. The accuracy of the radiochemical procedure has been proved by analysis of NBS-SRM 1577 Bovine Liver.     

Design of BiOBr0.25I0.75for synergy photoreduction Cr（Ⅵ） and capture Cr（Ⅲ） over wide pH range

Conversion of hexavalent chromium (Cr（Ⅵ）) to trivalent chromium (Cr（Ⅲ）) is an effective way to reduce its environmental risk,especially via photoreduction process.However,over a wide range of p H values,it is still a great challenge to achieve a high removal rate,and the disposal of produced Cr（Ⅲ） should be concerned.In this work,we implemented a high removal rate at 98%for Cr（Ⅵ） and total chromium(Cr（T）) over a wide p H range （4–10） through the synergistic effect of adsorption,photoreduction an...     

Continuous flow analysis: Photometric determination of Cr(III) with chrome Azurol S using microwave treatment

The effect of microwave and ultraviolet radiation and ultrasonic treatment on the reaction of chromium(III) with Chrome Azurol S, Arsenazo I, Alizarin, and Thoron was studied in the batch and flow modes. It was found that the reaction of chromium with the above photometric reagents is most efficiently activated by microwave radiation of the power 500-200 Wt. The best analytical properties were found for Chrome Azurol S. A flow system was proposed for the photometric determination of chromium(III) with Chrome Azurol S using microwave treatment. The throughput of the system is 68 samples per hour, the analytical range for chromium(III) is 0.03-60 mg/L. Na, K, Mg, Ca, Zn, Cd, Pb, C1^-, SO ₄ ^2- , NO_3/-, CH₃COO^- in 1000-fold amounts; Cu(II) and F^- in 500-fold amounts; Fe(III) in a 10-fold amount; and Cr(VI) in a sevenfold amount do not interfere with the determination of Cr(III). Procedures for the photometric determination of chromium under batch conditions were developed. The accuracy of the developed procedures was verified in the analysis of tanning agents and dyes for leather. Deceased.     

Polymerization of aniline derivatives by K2Cr2O7 and production of Cr2O3 nanoparticles

Oxidative polymerization of aniline, anthranilic acid, and aniline‐co‐anthranilic acid by potassium dichromate Cr(VI) as an oxidant in acidic medium was investigated. In this study, the polymerization process of aniline, o‐anthranilic acid as well as aniline/o‐anthranlic acid using K₂Cr₂O₇ produced, coordinated Cr(III)/polyaniline (PANI), Cr(III)/polyanthranilic acid (PAA) and Cr(III)/poly aniline‐co‐anthranilic acid (PANAA). The mechanism of polymerization reaction in the presence of dichromate was hypothesized. The precursor chromium doped polymers were characterized by TGA, FT‐IR, UV‐visible, XRD analyses. Cr₂O₃ nanoparticles size were determined using TEM analysis. The calcinations process of synthesized chromium doped PANI, PAA and PANAA yields Cr₂O₃ nanoparticles 26%, 31%, and 34% wt. respectively. Rhombohedral phase of Cr₂O₃ particles in the range from 33 to 61 nm was produced from chromium/polyanthranilic acid (PAA) and chromium/poly(aniline‐co‐anthranilic acid) PANAA. UV‐ visible analysis showed that optical band gaps (E_g) of doped poly aniline and its derivatives are in the range from1.55 to 1.80 using Tacu's law. The band gap values reveal that the doped chromium emeraldine base can be used as semiconductor materials. Copyright © 2016 John Wiley & Sons, Ltd.     

Electrode processes and their kinetic parameters in the reduction and oxidation of Cr(III) and Cr(II) in molten CsCl

The reduction of chromium ions in the CsCl-CrCl₃ melt in wide concentration and temperature ranges is reported. It is assumed that the melt contains polynuclear chromium complexes. The reduction of Cr(III) to Cr(II) is a reversible one-electron process. The electroreduction of divalent chromium to chromium metal is an irreversible process involving dinuclear and mononuclear chromium complex ions. The electron transfer rate constant has been calculated for various temperatures and chromium chloride concentrations in the salt melt. The activation energy of electron transfer and its dependence on the CrCl₃ concentration in the electrolyte have been determined for the Cr(III) ↔ Cr(II) process.     

Photoredox reactions of Cr(III) mixed-ligand complexes

Irradiation of chromium(III) complexes with oxalate and pyridinedicarboxylate ligands (pda = 2,3-, 2,4-, or 2,5-dicarboxylate) leads to diverse behaviors, dictated by light energy, presence of oxygen and the ligand nature. Irradiation within the MC bands is unaffected by O₂ and results in ligand substitution. The LMCT excitation is effective only when oxalate is coordinated to Cr(III); then electron transfer from oxalate to central ion generates an intermediate, consisted of a Cr(II)species and the C₂O₄^? radicals. The species undergo fast redox reactions dependent on the presence of O₂ and the pda ligand.(1) In anoxic medium the fast outersphere electron transfer from Cr(II) to solvent, generates hydrated electrons and re-oxidizes the chromium centre to Cr^III. Then geminate recombination regenerates substrate, whereas competitive release of the C₂O₄^? radical leads to substitution of one oxalate ligand by two water molecules (aquation induced by the LMCT excitation). In the presence of the pda ligand the outersphere electron transfer is accompanied by the innersphere CT, generating Cr(III) coordinated to two radical ligands: C₂O₄^? and pda^3?; the intermediate releases also e_aq^?, but this reaction is slower than that of the homoleptic oxalate complex. Hydrated electrons are scavenged also by the released radicals. All these processes are completed within microseconds and in consequence, the Cr(III) complexes irradiated in deoxygenated solutions are insensitive to subsequent oxygenation.(2) When UV-irradiation is carried out in oxygenated medium reaction of Cr(II) species with molecular oxygen competes with the outer- or inner electron transfer observed in anoxic medium. Both these pathways result in generation of chromate(VI). Quantum yield of the Cr(VI) production is sensitive to the presence and structure of pda ligand, decreasing within the series 2,3-pda > 2,4-pda > 2,5-pda.     

Synthesis and Kinetic Study on the Chromium(III) Complex [Cr(ASA)(en)2]Cl·2H2O

In order to explore the transfer mechanism of chromium(III) in mammals, a novel complex [Cr(ASA)(en)₂]Cl· 2H₂O, bis(ethylenediamine‐ κ ² N,N′)(4‐aminosalicylic acid‐ κ ² O,O′) chromium(III) monochloride dihydrate was synthesized (4‐aminosalicylic acid=H₂ASA, ethylenediamine=en). The crystal structure belongs to orthorhombic system with the space group P2₁2₁2₁ by means of X‐ray diffraction. The characteristic for transfer of Cr³⁺ from the compound to the low‐molecular‐mass chelator EDTA and the iron‐binding protein apoovotransferrin (apoOTf) was followed by UV‐visible (UV‐Vis) and fluorescence spectra in 0.01 mol·L^?1 Hepes at pH 7.4. The second order rate constants were calculated. Those spectra in conjunction were used to obtain more accurate information about the interaction of chromium complex with apoOTf. The experimental results indicate that Cr³⁺ can be transferred from the complex to apoOTf with the retention of the 4‐aminosalicylic acid acting as a synergistic anion.     

Logistic Model of Ligand Selection for Chromium-Plating Electrolytes Based on Cr(III) Complexes

The effect of the ligand nature in chromium(III) complexes on chromium electrodeposition was studied. The predicting logistic model of ligand selection for chromium-plating electrolytes was constructed from experimental data. It was found that high-quality chromium coatings can be deposited from electrolytes of chromium(III) complexes with a ligand having the following acid–basic characteristics in the protonated form: pK ₁ < 4.0 and 4.0 < pK _n < 18.6. The proposed model was verified experimentally and used to prepare a chromium-plating electrolyte based on a chromium(III) malonate complex, which gives high-quality chromium coatings over a wide current density range.     

Protocol for Extraction and Determination of Cr(VI) in Solid Materials with a High Cr(III)/Cr(VI) Ratio Using EDDS as a Leaching Agent for Cr(VI) and a Masking Agent for Cr(III)

A sensitive and selective protocol for the extraction of all forms of Cr(VI) from solid materials followed by determination by catalytic adsorptive stripping voltammetry has been elaborated. Cr(VI) was leached to a solution with 0.2 mol L^?1 (NH₄)₂SO₄/NH₄OH+0.1 mol L^?1 EDDS (pH 9.5) and simultaneously Cr(III) was transferred to a nonactive electrochemical complex with EDDS. The method allows for Cr(VI) determination in solid samples containing even a 1000–2000 fold excess of extractable Cr(III) without its noticeable influence. The effects of several experimental variables such as the composition and pH of the extractant, the time and temperature of the solid sample mixing with the extractant were studied. At the optimized conditions more than 95% of total Cr(VI) recoveries from solid samples were achieved. The validation of the proposed procedure was carried out by Cr(VI) determination in certified reference material CRM 019 Ash, spiked and unspiked with Cr(III), and by comparing the obtained results with those obtained using other common extraction procedures.     

Electrode processes in the course of Cr(III) and Cr(II) reduction/oxidation in KCl melt

A hypothesis about formation of a polynuclear chromium complex is made on the basis of a voltammetric study of the processes of trivalent and divalent chromium reduction within complex chloride ions in a potassium chloride melt. The effect of the reaction of Cr₂Cl ₇ ^3? formation on deviation of theoretical dependences from the experimentally obtained dependences under the terms of linear voltammetry is shown. Models are suggested for the complex chromium ions, quantum-chemical calculations of the bond lengths in them are performed and steric energies of ions are determined. Diffusion coefficients of Cr(III) and Cr(II) are calculated, diffusion activation energies are determined.     

Optimized structures of Cr2(CO)6+ with three different symmetries by density functional calculation

Density functional calculations have been made on a binuclear metal carbonyl ion Cr₂(CO)₆⁺ found in our laser ablation–molecular beam (LAMB) experiment. Optimized structures are calculated for three different conformations: T33 of D_3d symmetry with three terminal carbonyl groups on each chromium atom, B2T22 of D_2h symmetry with two bridging carbonyl groups and two terminal carbonyl groups on each chromium atom, and B4T11 of D_4h symmetry with four bridging carbonyl groups and one terminal carbonyl group on each chromium atom. The most stable conformation is T33 which is 36.76 and 286.44 kJ mol⁻¹ lower in energy than B2T22 and B4T11, respectively. The difference of conformation exerts a significant influence on the internuclear distance between chromium and the carbon of terminal CO, but hardly on the Cr–Cr bond length. For B2T22 and B4T11, longer C–O distances for bridging carbonyls compared with those for terminal ones indicate effective π*-back donation from the chromium atom to the bridging carbonyl groups. Furthermore, the relative abundance of Cr₂(CO)_n⁺ (n = 0–6) observed in our previous experimental study can be explained qualitatively by comparison of the excess energy produced in the formation of a Cr⁺–Cr bond with the CO dissociation energy of Cr₂(CO)₆⁺. © 1998 John Wiley & Sons, Ltd.     

Synthese und Kristallstruktur von [Cr(NH3)6][Cr(NH3)2F4][BF4]2

Synthesis and Crystal Structure of [Cr(NH₃)₆][Cr(NH₃)₂F₄][BF₄]₂ The action of ammonium fluoride on a mixture of boron and chromium in a sealed Monel ampoule at 300 °C yields single crystals of [Cr(NH₃)₆][Cr(NH₃)₂F₄][BF₄]₂. The crystal structure (tetragonal, P4/mbm, Z = 2, a = 1056.0(1), c = 781.7(1) pm; R₁ = 0.0414; wR₂ = 0.1087 for 411 reflections with I₀ > 2σ(I)) contains [Cr(NH₃)₆]³⁺ and [Cr(NH₃)₂F₄]^– octahedra and twice as many [BF₄]^– tetrahedra that are arranged in a quadrupled super‐structure of the CsCl‐type of structure.