The complexation of Cr(III) and Cr(VI) with flavones in micellar media and its use for the spectrophotometric determination of chromium

The complexation of chromium by different flavonoid dyes in micellar media has been studied, in particular the reaction between chromium and quercetin. Micellar effects, the reaction pathway proposed and the application of the method to the determination of Cr(VI) and Cr(VI) + Cr(III) mixtures are discussed.     

Oxidation of alcohol by lipopathic Cr(VI): a mechanistic study

The oxidation kinetics of various aliphatic primary and secondary alcohols having varied hydrocarbon chain length were studied using cetyltrimethylammonium dichromate (CTADC) in dichloromethane (DCM) in the presence of acetic acid and in the presence of a cationic surfactant. The rate of the reaction is highly sensitive to the change in [CTADC], [alcohol], [acid], [surfactant], polarity of the solvents, and reaction temperature. A Michaelis-Menten type kinetics was observed with respect to substrate. The chemical nature of the intermediate and the reaction mechanism were proposed on the basis of (i) observed rate constant dependencies on the reactants, that is, fractional order with respect to alcohol and acid and a negative order with respect to oxidant, (ii) high negative entropy change, (iii) inverse solvent kinetic isotope effect, k(H2O)/k(D2O) = 0.76, (iv) low primary kinetic isotope effect, kH/kD = 2.81, and (v) the k(obs) dependencies on solvent polarity parameters. The observed experimental data suggested the self-aggregation of CTADC giving rise to a reverse micellar system akin to an enzymatic environment, and the proposed mechanism involves the following: (i) formation of a complex between alcohol and the protonated dichromate in a rapid equilibrium, equilibrium constant K = 5.13 (+/-0.07) dm(3) mol(-1), and (ii) rate determining decomposition (k(2) = (7.6 +/- 0.7) x 10(-3) s(-1)) of the ester intermediate to the corresponding carbonyl compound. The effect of [surfactant] on the rate constant and the correlation of solvent parameters with the rate constants support the contribution of hydrophobic environment to the reaction mechanism.     

Hydrogen peroxide oxidation of hydrocarbons catalyzed by Cr(VI) oxo compounds

>CrO₃, (NBu₄)₂CrO₄, (NBu₄)₂Cr₂O₇, and (NBu₄)Cr₄O₁₃ catalyze the hydrogen peroxide oxidation of cyclohexane, ethylbenzene, and styrene in acetonitrile. The active species is apparently a Cr(VI) peroxo complex.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 210–211, January, 1990.     

A novel lipopathic Cr(VI) oxidant for organic substrates: Kinetic study of oxidation of benzyl alcohol

A novel lipopathic oxidizing agent, cetyltrimethylammonium dichromate, was used for oxidation of benzyl alcohol in various organic solvents and in surfactant systems. The reaction kinetics was investigated with change in [acid], [substrate], [oxidant], [surfactant], and temperature. The rate constant values led to propose that the reaction occurs in a reversed micellar system produced by the oxidant, akin to an enzymatic environment. The rate variation with variation in [surfactant] and solvent isotope effect suggest that the path of reaction to be through the formation of an ester complex, the decomposition of which is the rate‐determining step. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 651–656, 2006     

Flavin mononucleotide chemiluminescence in cationic micellar media for determination of chromium(III + VI) by flow injection

The flavin mononucleotide chemiluminescence system, originally developed for the determination of copper(II), is modified with cationic surfactant micelles for the determination of chromium(III + VI). In order for chromium to be detected predominantly, the copper-induced luminescent reaction is significantly suppressed by virtue of the cationic micellar effect. The limit of detection (signal-to-noise ratio = 2) is 5 × 10^?8 M chromium (50-μl sample injection).     

Kinetics of edta catalyzed oxidation of I? by Cr(VI)

The activity of various iron-containing ore catalysts during hydrogenation of Kansk-Achinsk lignite into liquid products in H-donor solvent (tetraline) has been studied. Ore samples contained pyrite, hematite and magnetite minerals. The most active appear to be pyrite samples. The catalytic effect of ore systems is, apparently, associated with the fact that during hydrogenation more active than tetraline H-donors are formed due to the hydrogenation of polycyclic aromatic molecules produced by thermal destruction of lignite.

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Kinetics of Cr(VI) and V(V) oxidation of trans-stilbene and indene

Title reactions are acid catalyzed and strongly dependent on the permittivity of the medium. With Cr(VI) indene reacts faster than trans-stilbene, but with V(V), trans-stilbene reacts faster than indene. The mechanism is discussed.

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. Cr(VI) , -, V(V) - , . , .

     

Efficient silver nanocluster photocatalyst for simultaneous methyl orange/4-chlorophenol oxidation and Cr(VI) reduction

Metal nanoclusters have shown great potential in photocatalysis, while simultaneous removal of both inorganic and organic contaminants by metal nanoclusters under visible light is less explored. Here, we synthesized Ag_m(SR)_n (SR represents 3-mercaptopropyltriethoxysilane ligand) nanoclusters (∼1 nm) via a reduction of silver triphenylphosphine under ambient conditions in the presence of 3-mercaptopropyltriethoxysilane. The nanocluster was characterized by UV–vis spectroscopy, high resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectrum (FTIR), and X-ray photoelectron spectroscopy (XPS). Under 5 W blue LED, the Ag_m(SR)_n/P25 exhibits enhanced catalytic activity for simultaneous methyl orange (MO) oxidation and Cr(VI) reduction, and also for synchronous 4-chlorophenol oxidation and Cr(VI) reduction. Mechanism studies by electrochemical impedance spectroscopy (EIS), photoluminescence (PL), electron spin resonance (ESR) etc. and control experiments reveal that the unique structure of silver nanoclusters with thiolate ligands is vital to the high catalytic performance, and both the photo-generated holes and superoxide radicals are responsible for the decomposition of MO.     

Amperometric Determination of Cr(III) and Cr(VI) by Flow Injection Analysis

Speciation of Cr(III) and Cr(VI) can be attained by flow injection analysis with amperometric detection. Cr(VI) is reduced in an acidic medium to Cr(III) with a glassy carbon electrode at —0.1 V vs. Ag/AgCl and the current is recorded. Cr(III) is oxidised on-line to Cr(VI) with alkaline hydrogen peroxide solution. From the difference of the total chromium and Cr(VI), the amount of Cr(III) was obtained. A linear calibration curve for Cr(VI) was obtained for the concentration ranges 0.01-5.0ppm of Cr(VI) and we have calculated the limit of determination to be about 0.5ppb. We have studied the degree of reproducibility obtained using the solid electrodes under various conditions. The influence of flow rate, coil length, interfenences and the extent of reaction were studied.     

Kinetics and mechanism of the oxidation of diaquabis(1,10-phenanthroline) iron(II) by periodate in aqueous and micellar media

The kinetics of oxidation of [Fe^II(phen)₂(H₂O)₂]²⁺ (phen = 1,10-phenanthroline) by periodate were investigated in aqueous acidic medium at different [H⁺] over a temperature range of 20–40 °C. The reaction was studied under pseudo-first-order conditions by taking [IO ₄ ^? ] > tenfold over [Fe^II(phen)₂(H₂O) ₂ ²⁺ ]. The reaction rate increases with increasing [H⁺], and the kinetics of oxidation obeyed the following rate law:

$$ {\text{Rate}} = \left[ {{\text{Fe}}^{\text{II}} ({\text{phen}})_2({\text{H}}_{2} {\text{O}})_{2}^{2 + } } \right]\left[ {{\text{IO}}_{4}^{ - } } \right]\left\{ {k_{4} K_{2} + k_{5} K_{1} K_{3} [{\text{H}}^{ + } ]} \right\} $$

The surfactant sodium dodecyl sulfate was found to enhance the rate, whereas cetyltrimethylammonium bromide had little effect. Activation parameters associated with k ₂ and k ₃ were calculated. An electron transfer from Fe(II) to I(VII) is identified as the rate-determining step. The I(VI) species thus generated reacts in a fast step with another Fe(II) complex.

     

Cr(VI) oxidation using cetylpicolinium dichromate: Kinetics of oxidation of benzaldehydes with a green protocol

Oxidation kinetics of benzaldehyde and some of its ortho- and para-monosubstituted derivatives have been studied using cetylpicolinium dichromates, a class of novel phase transfer oxidants, in dichloromethane medium. The rate of reaction is first order with respect to oxidant and fractional order with respect to the substrates. The Michaelis–Menten type oxidation was observed with respect to the substrates. Benzaldehydes are found to be oxidized to their corresponding acids. The mechanism of oxidation reaction has been suggested based on the solvent isotope effect, Hammett plot, and thermodynamic study. The solvent isotope effect (k_CHCl3/k_CDCl3 = 1.57) indicates the involvement of hydrogen exchange with the medium during oxidation reactions. A strong influence of specific solute–solvent interactions on the rate of the reaction is observed. Both the electron-withdrawing and electron-releasing substituents on the substrates accelerate the rate of reaction.     

Speciated isotope dilution analysis of Cr(III) and Cr(VI) in water by ICP-DRC-MS

An isotope dilution method has been developed for the speciation analysis of chromium in natural waters which accounts for species interconversions without the requirement of a separation instrument connected to the mass spectrometer. The method involves (i) in-situ spiking of the sample with isotopically enriched chromium species; (ii) separation of chromium species by precipitation with iron hydroxide; (iii) careful measurement of isotope ratios using an inductively coupled plasma mass spectrometer (ICP-MS) with a dynamic reaction cell (DRC) to remove isobaric polyatomic interferences. The method detection limits are 0.4 μg L⁻¹ for Cr(III) and 0.04 μg L⁻¹ for Cr(VI). The method is demonstrated for the speciation of Cr(III) and Cr(VI) in local nullah and synthetically spiked water samples. The percentage of conversion from Cr(III) to Cr(VI) increased from 5.9% to 9.3% with increase of the concentration of Cr(VI) and Cr(III) from 1 to 100 μg L⁻¹, while the reverse conversion from Cr(VI) to Cr(III) was observed within a range between 0.9% and 1.9%. The equilibrium constant for the conversion was found to be independent of the initial concentrations of Cr(III) and Cr(VI) and in the range of 1.0 (at pH 3) to 1.8 (at pH 10). The precision of the method is better than that of the DPC method for Cr(VI) analysis, with the added bonuses of freedom from interferences and simultaneous Cr(III) determination.     

Selective Oxidation of Betulin by Cr(VI) Reagents

Oxidation of betulin by pyridinium dichromate, pyridinium chlorochromate, and K₂Cr₂O₇ -H₂SO₄ in the presence of tetrabutylammonium bromide was studied. Products of regioselective C-3, C-28-, and exhaustive oxidation, 28-hydroxylup-20(29)-3-one, 3-hydroxy- and 3-oxolup-20(29)-en-28-al were obtained.     

Biotechnology of Cr(VI) transformation into Cr(III) complexes

The dose-dependent formation of Cr(III) complexes and uptake of chromium by Arthrobacter oxydans — a Gram-positive bacterium from contaminated Columbian basalt rocks (USA) — were studied along with the testing under aerobic conditions of two bacterial strains of Arthrobacter genera isolated from the polluted basalts from the Republic of Georgia. Instrumental neutron activation analysis (INAA) was used to track the accumulation of chromium in the bacterial cells. To monitor and identify Cr(III) complexes in these bacteria, electron spin resonance (ESR) spectrometry was employed.     

Determination of Cr(VI) and Cr(III) by Using Activated Carbon-Atomic Absorption Spectrometry

A method for the separation and preconcentration of Cr(III) and Cr(VI) on activated carbon in presence of diethyldithiocarbamate as a complexing reagent was optimized. The method makes it possible to achieve 200- to 500-fold Cr(VI) concentrating depending on the initial volume of the solution to be analysed and the final volume eluted. The Cr(VI) concentration in the background solution determined with RSD 30% was equal to 1.5 g L. The limit of Cr(VI) determination was equal to 0.9 g L.     

Speciation of Cr(III) and Cr(VI) by Means of Melamine-Urea-Formaldehyde Resin and FAAS

 A method is described for the quantitative preconcentration and separation of trace chromium in water by adsorption on melamine-urea-formaldehyde resin. Cr(VI) is enriched from aqueous solutions on the resin. After elution the Cr(VI) is determined by FAAS. The capacity of the resin is maximal at ∼ pH 2. Total chromium can be determined by the method after oxidation of Cr(III) to Cr(VI) by hydrogen peroxide. The relative standard deviations (10 replicate analyses) for 10 mg/L levels of Cr(VI), Cr(III) and total chromium were 1.5, 3.5 and 2.8% respectively. The procedure has been applied to the determination and speciation of chromium in lake water, tap water and chromium-plating baths.     

Simultaneous Determination of Cr(III) and Cr(VI) in Ground and Drinking Waters by IC-ICP-MS

Abstract

The coupling of ion chromatography (IC) and inductively-coupled plasma mass spectrometry (ICP-MS) provides a powerful tool for the simultaneous determination of Cr(III) and Cr(VI) in water samples. The experimental set-up of the technique is described in detail and performance data are given, whereby detection limits of 0.1 μg/l could be achieved for both chromium species without pre-concentration. With this method, a complete analysis can be done in less than 3 minutes. In order to improve the repeatability of the method, Rh(III) as internal standard is used. Stability of the two chromium species in water turned out to be a major problem and optimal conditions for storage were determined. For real water samples storage at 4 °C in the dark at neutral pH and analysis as fast as possible is recommended. The application of the IC-ICP-MS technique to real water samples and the comparison to another, more time-demanding and less sensitive method proves the capacity of the method.