Salt-assisted liquid-liquid microextraction of Cr(VI) ion using an ionic liquid for preconcentration prior to its determination by flame atomic absorption spectrometry

We report on the salt-assisted liquid-liquid microextraction of cationic complexes of Cr(VI) ion using the hydrophilic ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoraborate and potassium hydrogen phosphate. This is a novel, simple, non-toxic and effective technique for sample pretreatment technique that displays large extraction efficiency and represents a new platform where Cr(VI) is complexed with 1,5-diphenylcarbazide (DPC) in sulfuric acid medium. It was applied to the extraction of Cr(VI) in the form of the Cr(VI)-DPC complex prior to its determination by flame atomic absorption spectrometry. Cr(III) ion also can be determined by this procedure after oxidation to Cr(VI). Extraction is mainly affected by the amount of water-soluble IL, the kind and quantity of inorganic salts, by pH and the concentration of DPC. Calibration plots are linear in the range from 3 to 150?μg?L^?1 of Cr(VI), and the limit of detection is 1.25?μg?L^?1. The method was successfully applied to the speciation and determination of trace levels of Cr(III) and Cr(VI) in environmental water samples containing high levels of dissolved salts or food grade salts.

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).     

Electroanalytical Detection of Cr(VI) and Cr(III) Ions Using a Novel Microbial Sensor

A microbial sensor, namely carbon paste electrode (CPE) modified with Citrobacter freundii (Cf–CPE) has been developed for the detection of hexavalent (Cr(VI)) and trivalent (Cr(III)) chromium present in aqueous samples using voltammetry, an electroanalytical technique. The biosensor developed, demonstrated about a twofold higher performance as compared to the bare CPE for the chosen ions. Using cyclic voltammetry and by employing the fabricated Cf–CPE, the lowest limit of detection (LLOD) of 1x10⁻⁴ M and 5x10⁻⁴ M for Cr(VI) and Cr(III) ions respectively could be achieved. By adopting the Differential Pulse Cathodic Stripping Voltammetric technique, the LLOD could be further improved to 1x10⁻⁹ M and 1x10⁻⁷ M for Cr(VI) and Cr(III) ions respectively using the biomodified electrodes. The reactions occurring at the electrode surface‐chromium solution interface and the mechanisms of biosorption of chromium species onto the biosensor are discussed. The stability and utility of the developed biosensor for the analysis of Cr(VI) and Cr(III) ions in chromite mine water samples has been evaluated.     

Electrode processes occurring during the electrodeposition of chromium-carbon coatings from solutions of Cr(III) salts with carbamide and formic acid additions

The regularities and possible mechanisms of electrode processes occurring during the electrodeposition of the Cr-C alloy from an electrolyte based on Cr(III) sulfate with carbamide and formic acid as organic components were analyzed. The chromium electrodeposition was found to impose its kinetics on carbon inclusion in the alloy. The mechanism of carbon incorporation in the deposit was assumed to be the chemical interaction of the adsorbed organic particles with highly active chromium adatoms. During the stage discharge of Cr(III) ions to the metal, the rate of the limiting stage Cr(II) + 2e → Cr(0) increased when OH^? or F^? ions were included in the structure of the electrochemically active complex.     

Zur Chemie des Bis(methylsulfonyl)-amins (Dimesylamins). III. Onium-Salze des Dimesylamins

Investigations on Bis(methylsulfonyl)-amine (Dimesylamine). III. Onium Salts of Dimesylamine Thirty-five salts of the strong acid HN(SO₂Me)₂ containing onium cations are reported. These include higher quaternary ammonium salts which show excellent solubility in solvents of low polarity, thus being particularly useful for preparing solutions of “naked” (MeSO₂)₂N^? or corresponding ion-pairs. Pyrolysis of the quaternary ammonium salts at temperatures of 200—250°C produces tertiary amines and N-alkyl-dimesylamines by (N → N′)-alkyl transfer. Alkene formation is not observed.     

Amorphous aluminosilicates containing trivalent chromium in a non-octahedral coordination environment

Amorphous aluminosilicates containing up to 5.8 wt.% chromium have been prepared by cation exchange on an amorphous sodium aluminosilicate using Cr(III) salts. Electronic spectroscopy has shown that the ligand arrangement around the Cr(III) sites does not correspond to the octahedral geometry. No isomorphous substitution of Al³⁺ by Cr³⁺ in the aluminosilicate occurs, and it is not possible to exchange back chromium by Na⁺ ions. The amorphous chromium-substituted aluminosilicates (abbr. ACSAS) are slightly acidic and when heated in air at 800 °C no oxidation of Cr(III) takes place. The chromium species in the ACSAS undergoes ligand replacement reactions.

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Amorphe Alumosilicate mit dreiwertigem chrom in nicht-oktaedrischer Koordinationsumgebung

Zusammenfassung Bis zu 5,8 Gew.% Chrom enthaltende amorphe Alumosilicate wurden durch Kationenaustausch am amorphen Natriumalumosilicat unter Anwendung von Cr(III)-Salzen hergestellt. Elektronenspektroskopische Untersuchungen zeigten, daß die Ligandanordnung um die Cr(III)-Lagen nicht der oktaedrischen Geometrie entspricht. Im Alumosilicat erfolgt keine isomorphe Substitution von Al³⁺ durch Cr³⁺ und es ist nicht möglich, Chrom durch Na⁺ zurückzuersetzen. Die amorphen durch Chrom ersetzten Alumosilicate (abgekürzt ACSAS) sind schwach sauer und wenn sie in der Luft auf 800 °C aufgeheizt werden, erfolgt keine Oxydation von Cr(III). Das in den ACSAS vorliegende Chrom unterliegt Ligandaustauschreaktionen.

     

Investigation of deceleration causes in gold electrowinning from aqueous cyanide solutions on porous carbon electrodes

Deceleration of gold electrowinning from model cyanide solutions on porous cathode of graphitized carbon felt was investigated. It was established that calcium ions present in the solution did not negatively affect the electrolysis rate. The main reason of the termination of gold electrowinning on the graphitized cathode was the corrosion of the stainless steel anode resulting in penetration into the cathode chamber of the electrolyzer of CrO₄²⁻ anions further reduced to Cr(III) and probably Cr(II) compounds. X-ray Photoelectron Spectroscopy investigation of the passivated carbon cathode showed that the film formed thereon consisted of Cr(III) compounds containing cyanide and hydroxy ligands. This film covers the active sites on the surface of the porous carbon cathode preventing the deposition thereon of ad-ions of gold(I). Inert aqua and hydroxo complexes of chromium(III) stronger impede the rate of gold(0) deposition on the porous carbon cathode than chromium(III) cyanide compounds. This fact originates presumably from the requirement of lower cathode potentials for the reduction of the former to labile chromium(II) complexes than those necessary for the thermodynamically stable anions [Cr(CN)₆]³⁻.     

Speciation of chromium in environmental samples by dual electromembrane extraction system followed by high performance liquid chromatography

This study proposes the dual electromembrane extraction followed by high performance liquid chromatography for selective separation-preconcentration of Cr(VI) and Cr(III) in different environmental samples. The method was based on the electrokinetic migration of chromium species toward the electrodes with opposite charge into the two different hollow fibers. The extractant was then complexed with ammonium pyrrolidinedithiocarbamate for HPLC analysis. The effects of analytical parameters including pH, type of organic solvent, sample volume, stirring rate, time of extraction and applied voltage were investigated. The results showed that Cr(III) and Cr(VI) could be simultaneously extracted into the two different hollow fibers. Under optimized conditions, the analytes were quantified by HPLC instrument, with acceptable linearity ranging from 20 to 500 μg L⁻¹ (R² values ≥ 0.9979), and repeatability (RSD) ranging between 9.8% and 13.7% (n = 5). Also, preconcentration factors of 21.8–33 that corresponded to recoveries ranging from 31.1% to 47.2% were achieved for Cr(III) and Cr(VI), respectively. The estimated detection limits (S/N ratio of 3:1) were less than 5.4 μg L⁻¹. Finally, the proposed method was successfully applied to determine Cr(III) and Cr(VI) species in some real water samples.     

Determination of bismuth in pharmaceutical products by resonance light scattering using quaternary ammonium salts as molecular probe

Resonance light scattering (RLS) of bismuth-iodide-quaternary ammonium salts and their analytical application have been studied. Bismuth can react with a large excess of I^? to form [BiI₄]^? in the presence of Tween-20; [BiI₄]^? further reacts with quaternary ammonium salts such as tetramethyl ammonium bromide (TMAB), tetrapropyl ammonium bromide (TPAB) and tetrabutyl ammonium bromide (TBAB) to form an ion-association compound. Moreover, the characteristics of RLS spectra of the complexes, the optimum conditions and the influencing factors have been investigated. The method has been applied to determine Bi(III) in pharmaceutical products with satisfactory results which are in agreement with that of atomic absorbance spectrometry (AAS).     

Solubility and Phase Behavior of Cr(III) Oxides in Alkaline Media at Elevated Temperatures

A platinum-lined, flowing autoclave facility is used to investigate the solubility behavior of Cr₂O₃ and FeCr₂O₄ in alkaline sodium phosphate, sodium hydroxide, and ammonium hydroxide solutions between 21 and 288°C. Baseline Cr(III) ion solubilities were found to be on the order of 0.1 nmolal, which were enhanced by the formation of anionic hydroxo and phosphato complexes. At temperatures below 51°C, the activity of Cr(III) ions in aqueous solution is controlled by a Cr(OH)₃·3H₂O solid phase rather than Cr₂O₃; above 51°C the saturating solid phase is -CrOOH. Measured chromium solubilities were interpreted via a Cr(III) ion hydrolysis/complexing model and thermodynamic functions for the hydrolysis/complexing reaction equilibria were obtained from least-squares analyses of the data. The existence of four new Cr(III) ion complexes is reported: Cr(OH)₃(H₂PO₄)^–, Cr(OH)₃(HPO₄)^2–, Cr(OH)₃(PO₄)^3–, and Cr(OH)₄(HPO₄)-(H₂PO₄)^4–. The last species is the dominant Cr(III) ion complex in concentrated, alkaline phosphate solutions at elevated temperatures.     

Darstellung,Isolierung und Charakterisierung gemischter Thiocyanato(1,3-diaminopropan)chrom(III)-Komplexe

Preparation, Isolation, and Characterization of Mixed Thiocyanato(1.3-diaminopropane) -chromium(III) Complexes The complexes [Cr(NCS)₄dap]^? and trans-[Cr(NCS)₂(dap)₂]⁺ (dap = 1.3-diaminopropane) are prepared for the first time, isolated as salts and characterized by CHN analysis, chromium contents, the electrophoretic behaviour in dimethylformamide, and by UV, VIS, and IR spectra.     

Cation exchange separation of51Cr-labelled species in aqueous Cr(VI)−Cr(III) solution

A convenient and precise separation procedure for aqueous⁵¹Cr(VI),⁵¹Cr(III)-monomer,⁵¹Cr(III)-dimer and⁵¹Cr(III)-polymer species, using a cation exchange resin, is described. Standard deviations of 0.3% for each component may be routinely obtained.     

Molecular structure and hydrolytic stability amidinium salts derived from triazatricyclo[5.2.1.0]decane

Five amidinium salts have been prepared from triazatricyclo[5.2.1.0^4,10]decane (tacnoa) and characterised by mass spectrometry, NMR spectroscopy and X-ray crystallography. The X-ray structures revealed a long distance between the methine carbon and the ammonium nitrogen, viz., C-N distance 1.64-1.70 Å, cf. other C-N distances of 1.40-1.50 Å. An NMR study of 1-ethyl-4,7-diaza-1-azoniatricyclo[5.2.1.0^4,10]decane and 1-benzyl-4,7-diaza-1-azoniatricyclo[5.2.1.0^4,10]decane, confirmed that these amidinium salts hydrolyse in aqueous solution, the latter 60 times faster than the former. Tacnoa, which has C-N distances typical of single bonds, showed no evidence of hydrolysis after several days at 80 °C. Molecular modeling calculations indicate that the preferred gas phase structure of the salts is one where the positive charge is delocalised over the two secondary amines and the methine carbon. The calculated distance between this carbon and the ammonium nitrogen is 0.15-0.4 Å longer than in the crystal structure. The energy difference between the preferred gas phase and solid state conformations is 2 kJ mol⁻¹ and presents little barrier to nucleophilic attack of the methine carbon. Further analysis of the methine carbon geometry (C(7)) reveals that the bond angles in the benzyl salt are closer to those expected for an sp² centre than in the ethyl salt and that this could be the origin of the faster hydrolysis rate.     

Speciation of chromium in Australian fly ash

The concentrations of chromium (III) and (VI) in fly ash from nine Australian coal fired power stations were determined. Cr(VI) was completely leached by extraction with 0.01 M NaOH solution and the concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). This was confirmed by determining Cr(III) and Cr(VI) in the extracts of fly ash that had been spiked with chromium salts. These analytical measurements were done using a combination of ion-exchange chromatography and ICP-AES. The elutant was 0.05 M HNO₃ containing 0.5%-CH₃OH. When the column was operated at a flow rate of 1.2 ml min⁻¹ and samples were injected by use of a sample loop with a volume of 100 μl, Cr(III) and Cr(VI) in sample solution was exclusively separated within approximately 10 min. The detection limits (3σ) were 5 ng for Cr(III) (0.050 mg l⁻¹) and 9 ng for Cr(VI) (0.090 mg l⁻¹), respectively. A relative standard deviation of 1.9% (n = 6) was obtained for the determination by IC-ICP-AES of 0.25 mg l⁻¹ Cr(III) and Cr(VI).     

Use of Different Salt Solutions in Salting-Out TLC of Co(III) Complexes on Silica Gel

Saturated aqueous solutions of 28 different salts have been studied as a potential mobile phases for salting-out thin-layer chromatography, on silica gel, of a series of four mixed bis-aminocarboxylato cobalt(III) complexes. In addition, by linear regression analysis of chromatographic data obtained for fifteen mixed aminocarboxylato Co(III) complexes (four series) with solutions of ammonium chloride, three alkali metal chlorides, and four alkaline earth metal chlorides, four linear dependences previously established on different adsorbents with (NH₄)₂SO₄ solutions were confirmed. The qualities of the separations achieved with the salts were compared and Li⁺, Mg²⁺, and Ca²⁺ chlorides are proposed as the most suitable.     

Optimization of the Conditions for the Cr (III) Adsorption on Activated Carbon

In order to understand the patterns of the adsorption equilibrium of Cr (III) on activated carbon, the adsorption process was studied by two different ways: classical batch experiments on commercial Norit and Merck activated carbons and their oxidized forms in a wide range of pHs; and extended time-based tests at the same pH values on the same adsorbents. This approach allowed us to understand the role of texture, chemical carbon surface functionality and experimental conditions (initial pH of the solution, contact time and adsorbate/adsorbent ratio) on the effectiveness of Cr (III) removal. The adsorption process of Cr (III) at (24 ± 1^∘C) on Merck and Norit activated carbons and their oxidized forms were studied at pH values between 1.5 and 5 (either adjusted or buffered). Chromium concentration was fixed at 200 ppm. The carbon loading ranged from 1.2 to 20 g/l. The carbon/Cr (III) solution contact time was varied from 0.5–1 month to 5 months, to ensure that the saturation of the carbon level was reached. According to the data obtained, the presence of carboxylic groups on carbon surface seems to enhance Cr (III) uptake at initial pH of the solution in the range between 2 and 4. Depending on the nature of the adsorbent surface chemistry, the contact time to reach equilibrium may range from 3 to 5 months. There is an optimum carbon loading which limits the Cr (III) uptake/removal at given pH value. In order to understand the adsorption process, an ion exchange, surface complex and surface precipitation were considered. This paper was presented in the 5th Brazilian Meeting on Adsorption, held at Natal, Brazil, 18-21 July, 2004.     

Effect of Cationic Micelles on the Kinetics of Interaction of [Cr(III)-Gly-Gly] with Ninhydrin

The effect of cationic micelles of cetyltrimethylammonium bromide (CTAB) on the interaction of chromium dipeptide complex ([Cr(III)-Gly-Gly]²⁺) with ninhydrin under varying conditions has been investigated. The rates of the reaction were determined in both water and surfactant micelles in the absence and presence of various organic and inorganic salts at 70 °C and pH 5.0. The reaction followed first- and fractional-order kinetics with respect to [Cr(III)-Gly-Gly²⁺] and [ninhydrin]. Increase in the total concentration of CTAB from 0 to 40×10⁻³ mol·dm⁻³ resulted in an increase in the pseudo-first-order rate constant (kψ) by a factor of ca 3. Quantitative kinetic analysis of kψ−[CTAB] data was performed on the basis of the pseudo-phase model of the micelles. As added salts induce structural changes in micellar systems that may modify the substrate-surfactant interactions, the effect of some inorganic (NaBr, NaCl, Na₂ SO₄) and organic (NaBenz, NaSal, NaTos) salts on the rate was also explored. It was found that the tightly bound counterions (derived from organic salts) were the most effective.     

Tetraalkylammonium cations in electron impact mass spectrometry

Twenty-five tetraalkylammonium halide salts were investigated by means of electron impact mass spectrometry. In all cases, corresponding ammonium cations have been detected at low electron energy. Moreover, [C+M]⁺ have been detected (where C and M denote ammonium cation and molecule, respectively) for both symmetrical salts and iodine salts. Quaternary ammonium salts are considered to be more stable than believed previously.     

Determination of chromium(III) and total chromium in natural waters using a surface ion-imprinted silica gel as selective adsorbent

A sensitive and selective method has been developed to determine Cr(III) and total Cr in natural water samples by ICP-AES with a Cr(III)-imprinted aminopropyl-functionalised silica gel adsorbent. The Cr(III)-imprinted and non-imprinted adsorbent were prepared by an easy one-step reaction with a surface imprinting technique. Their maximum static adsorption capacities for Cr(III) were 11.12 mg g^?1 and 3.81 mg g^?1, respectively. The relative selectivity factors (α _r) for Cr(III)/Co(II), Cr(III)/Au(III), Cr(III)/Ni(II), Cr(III)/Cu(II), Cr(III)/Zn(II), and Cr(III)/Cr(VI), were 377, 21.4, 15.4, 27.7, 26.4, and 31.9, respectively. Under the optimal conditions, Cr(III) can be absorbed quantitatively, but Cr(VI) was not retained. Total chromium was obtained after reducing Cr(VI) to Cr(III) with hydroxyammonium chloride. The detection limit (3σ) for Cr(III) was 0.11 ng mL^?1. The relative standard deviation was 1.2%. The proposed method has been validated by analysing two certified reference materials and successfully applied to the determination and speciation of chromium in natural water samples with satisfactory results.     

On-line preconcentration using dual mini-columns for the speciation of chromium(III) and chromium(VI) and its application to water samples as studied by inductively coupled plasma-atomic emission spectrometry

On-line preconcentration system for the selective, sensitive and simultaneous determination of chromium species was investigated. Dual mini-columns containing chelating resin were utilized for the speciation and preconcentration of Cr(III) and Cr(VI) in water samples. In this system, Cr(III) was collected on first column packed with iminodiacetate resin. Cr(VI) in the effluent from the first column was reduced to Cr(III), which was collected on the second column packed with iminodiacetate resin. Hydroxyammonium chloride was examined as a potential reducing agent for Cr(VI) to Cr(III).The effects of pH, sample flow rate, column length, and interfering ions on the recoveries of Cr(III) were carefully studied. Five millilitres of a sample solution was introduced into the system. The collected species were then sequentially washed by 1 M ammonium acetate, eluted by 2 M nitric acid and measured by ICP-AES. The detection limit for Cr(III) and Cr(VI) was 0.08 and 0.15 μg l⁻¹, respectively. The total analysis time was about 9.4 min.The developed method was successfully applied to the speciation of chromium in river, tap water and wastewater samples with satisfied results.