The catalytic adsorptive stripping voltammetric determination of chromium with TTHA and nitrate

A sensitive voltammetric method is presented for the determination of trace amounts of total chromium (Cr(III) and Cr(VI)) in natural waters, employing the square wave mode. The method is based on the preconcentration of the Cr(III)-TTHA complex by adsorption at the HMDE at the potential of –1.0 V vs. Ag/AgCl. The adsorbed complex is then reduced producing a response with a peak potential of –1.29 V and the peak height of the Cr(III) reduction is measured. The catalytic action of the nitrate ions on the Cr(III)-TTHA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a.c. voltammetry and the potential range of adsorption was determined.Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 155–2000 ng 1^–1 have been established. The determination limit is 15 ng 1^–1 and the RSD is 3.5% for chromium concentrations 200 ng 1^–1.The usefulness and wide scope of this method for reliable and highly sensitive chromium analysis down to the ultra trace levels existing in various types of natural waters is demonstrated by determinations of the total chromium content in lake, sea and rain water.     

Separation of Cr(III) and Cr(VI) using melamine-formaldehyde resin and determination of both species in water by FAAS

A method is described for the determination of Cr(VI) and total chromium by FAAS. Cr(VI) is separated from Cr(III) by adsorption on melamine-formaldehyde resin. After elution of Cr(VI) with 0.1 mol/l NaAc solution, it is analysed by FAAS. Total chromium is determined by FAAS after conversion of Cr(III) to Cr(VI) by oxidation with hydrogen peroxide, total Cr(VI) is concentrated as above. If the total concentration of chromium is sufficient, the determination can be directly made by FAAS. Cr(III) can then be calculated by subtracting Cr(VI) from the total Cr. This method was successfully applied to the determination of chromium in lake water.     

Speciation of Chromium in Natural Waters,Tea, and Soil with Membrane Filtration Flame Atomic Absorption Spectrometry

A sensitive and selective method was developed for the speciation of chromium(III) and chromium(VI) in environmental samples based on membrane filtration and determination by flame atomic absorption spectrometry. Chromium(III) reacts with cochineal red A, yielding a complex that is adsorbed on a cellulose acetate membrane filter, whereas chromium(VI) remains in aqueous solution, permitting separation. After reduction of chromium(VI) to chromium(III) with hydroxylamine hydrochloride, the total concentration of chromium was determined, and the concentration of chromium(VI) was calculated by subtraction. The pH, amount of cochineal red A, and sample volume were optimized on the basis of the recovery of Cr(III). The influence of matrix ions was also investigated. The preconcentration factor was 94. The detection limit (3 sigma) for Cr(III) was 1.4 micrograms per liter. The method was validated using environmental certified reference materials. The method was successfully employed for the speciation of chromium in wastewater and lake water.     

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.     

Tea-industry waste activated carbon, as a novel adsorbent, for separation, preconcentration and speciation of chromium

Activated carbon was produced from tea-industry wastes (TIWAC) and employed as a low cost and effective solid phase material for the separation, preconcentration and speciation of chromium species without using a complexing agent, prior to determination by flame atomic absorption spectrometry (FAAS). The characterization of TIWAC was performed by utilizing several techniques such as Fourier Transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM), and elemental analysis. The adsorption experiments were conducted in a batch adsorption technique. Under the experimental conditions, Cr(VI) adsorption amount was nearly equal to zero, however the adsorption percentage of Cr(III) was in the range of 95–100%. Therefore total chromium was determined after the reduction of Cr(VI) to Cr(III) and Cr(VI) was calculated by subtracting Cr(III) concentration from total chromium concentration. The suitable conditions for adsorption and speciation processes were evaluated in terms of pH, eluent type and volume, TIWAC concentration, adsorption and desorption contact time, etc. Adsorption capacity of TIWAC was found to be 61.0 mg g⁻¹. The detection limit for Cr(III) was found to be 0.27 μg L⁻¹ and the preconcentration factor was 50 for 200 mL of sample volume. The procedure was applied to the determination and speciation of chromium in stream, tap and sea water. Also, the proposed method was applied to total chromium preconcentration in microwave digested tobacco and dried eggplant samples with satisfactory results. The method was validated by analyzing certified reference materials (CRM-TMDW-500 Drinking Water and CRM-SA-C Sandy Soil C) and the results were in good agreement with the certified values.     

Trace determination of chromium in various water types by adsorption differential pulse voltammetry

Summary A new sensitive voltammetric method is presented for the determination of trace amounts of total chromium [Cr(III) and Cr(VI)] in natural waters. The method is based on the preconcentration of the Cr(III)-DTPA complex by adsorption at the HMDE at the potential of –1.0 V. The adsorbed complex is then reduced producing a response with a peak potential of –1.22 V and the peak height of the Cr(III) reduction is measured. The catalytic action of nitrate and bromate ions on the Cr(III)-DTPA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a. c. voltammetry and the potential range of adsorption was determined. Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 20–2,000 ng/l have been established. The determination limit is 20 ng/l and the RSD is 5% for chromium concentrations 200 ng/l.The usefulness and wide scope of this new voltammetric method for reliable and highly sensitive chromium analysis down to the natural ultra trace levels existing in various types of natural waters is demonstrated by determinations of the total dissolved chromium content in river, lake, sea and rain water.

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Spurenbestimmung von Chrom in verschiedenen Wassertypen durch Adsorptions-Differentialpuls-Voltammetrie

Zusammenfassung Eine neue voltammetrische Methode zur Spurenbestimmung von Chrom [als Summe von Cr(III) und Cr(VI)] in natürlichen Gewässern wurde entwickelt. Die Methode beruht auf einer Anreicherung des Cr(III)-DTPA-Komplexes durch Adsorption an der hängenden Queck-silbertropfenelektrode beim Potential –1.0 V. Der adsorbierte Komplex wird anschließend im differentiellen Pulsmodus reduziert und die Peakhöhe beim Peakpotential –1.22 V gemessen. Die katalytische Wirkung von Nitrat- und Bromationen auf die Cr(III)-DTPA-Reduktion wurde mit der cyclischen Voltammetrie geklärt. Die Adsorption der Cr-Komplexe wurde zusätzlich mit der a.c.-Voltammetrie (kapazitive Komponente) untersucht und der Potentialbereich der Adsorption ermittelt. Aufgrund der Untersuchungen wurden die optimalen Bedingungen zur Chrombestimmung im Konzentrationsbereich 20–2000 ng/l festgelegt. Die Bestimmungsgrenze liegt bei 20 ng/l und die relative Standardabweichung beträgt 5% für Konzentrationen 200 ng/l. Die weite Anwendbarkeit der Methode für die zuverlässige und hochempfindliche Analyse von Chromspuren bis zu den natürlichen Ultraspurengehalten in verschiedenen Typen natürlicher Wässer wird an Beispielen der Analyse des gelösten Gesamtgehaltes von Chrom in Flußwasser, Seewasser, Meerwasser und Regenwasser aufgezeigt.

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Dedicated to Prof. Dr. H. Monien on the occasion of his 60th birthday

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Attached from Department of Chemistry of Warsaw University, Poland, within the scope of the joint research project on Eco-toxic Metals in the Environment     

Chromium(III)-imprinted silica gel for speciation analysis of chromium in environmental water samples with ICP-MS detection

A new chromium(III)-imprinted 3-(2-aminoethylamino) propyltrimethoxysilane (AAPTS)-functionalized silica gel sorbent was synthesized by a surface imprinting technique and was employed as a selective solid-phase extraction material for speciation analysis of chromium in environmental water samples prior to its determination by inductively coupled plasma mass spectrometry (ICP-MS). The prepared Cr(III)-imprinted silica gel shows the selectivity coefficient of more than 700 for Cr(III) in the presence of Mn(II). The static adsorption capacity of the ion-imprinted and non-imprinted sorbent for Cr(III) were 30.5 mg g(-1) and 13.4 mg g(-1). It was also found that Cr(VI) could be adsorbed at low pH by the prepared imprinted silica gel, and this finding makes it feasible to enrich and determine Cr(VI) at low pH without adding reducing reagents. The imprinted silica gel sorbent offered a fast kinetics for the adsorption and desorption of both chromium species. Under the optimized conditions, the detection limits of 4.43 pg mL(-1) and 8.30 pg mL(-1) with the relative standard deviations (R.S.D.s) of 4.44% and 4.41% (C=0.5 ng mL(-1), n=7) for Cr(III) and Cr(VI) were obtained, respectively. The proposed method was successfully applied to the speciation of trace chromium in environmental water samples. To validate the proposed method, two certified reference materials were analyzed and the determined values were in a good agreement with the certified values. The developed method is rapid, selective, sensitive and applicable for the speciation of trace chromium in environmental water samples.     

Trace Analysis of Iron in Environmental Water and Snow Samples from Poland

Abstract

A voltammetric method for the determination of iron at detection limit of 4 μg/l is described, using the catalytic current of the reduction of the Fe(III)-triethanolamine (TEA) complex in the presence of bromate ions. the determination was performed at a mercury hanging drop electrode without preconcentration, using the TEA alkaline solution as a supporting electrolyte and the differential pulse technique. A peak current for the Fe(III)-TEA catalytic reduction was observed at a potential of-1.0 V (Ag/AgCl saturated electrode). the influence of TEA, BrO³ and NaOH concentrations on the peak height was studied. It was found that a 100-fold excess of Mn, a 50-fold excess of Cr(VI) and Zn did not interfere in the determination. This method was applied to the determination of iron in water, snow and waste water samples.     

Catalytic Adsorptive Stripping Voltammetric Procedure for Determination of Total Chromium in Environmental Materials

A sensitive catalytic adsorptive stripping voltammetric procedure for determination of traces of total chromium in environmental samples is reported. The method is based on the preconcentration of a Cr(III)? H₂DTPA complex by adsorption at the HMDE from an acetate buffer solution at the potential ?1.0 V vs. Ag/AgCl. Total chromium was determined as Cr(III) after reduction of Cr(VI) to Cr(III) by NaHSO₃. In order to stabilize the signal of Cr(III) the measurements were performed at 5 °C. The calibration graph for chromium for an accumulation time of 60 s was linear in the range from 5×10^?10 to 5×10^?8 mol L^?1. The relative standard deviation for a chromium concentration of 1×10^?8 mol L^?1 was 3.9% (n=5). The detection limit for accumulation time of 60 s was about 8×10^?11 mol L^?1. The validation of the procedure was performed by the analysis of the certified reference materials.     

Speciation of Cr(III) and Cr(VI) in environmental samples by using coprecipitation with praseodymium(III) hydroxide and determination by flame atomic absorption spectrometry

A speciation procedure has been established for the flame atomic absorption spectrometric determination of Cr(III) and Cr(VI) based on coprecipitation of Cr(III) by using praseodymium(III) hydroxide (Pr(OH)₃) precipitate. In the presented system, Cr(III) was quantitatively (>95%) recovered at the pH range of 10.0?C12.0 on Pr(III) hydroxide, while the recoveries of Cr(VI) were below 10%. The method was applied to the determination of the total chromium after reduction of Cr(VI) to Cr(III) by using hydroxylamine hydrochloride. The concentration of Cr(VI) is calculated by difference of total chromium and Cr(III) levels. The analytical parameters including pH of the aqueous medium, amount of Pr(III), centrifugation speed, sample volume were optimized. The influences of matrix ions were also investigated. The method was validated by the analysis of TMDA 70 fortified lake water certified reference material. The method was applied to the speciation of chromium in water samples.     

Simultaneous determination of chromium(III) and cadmium(II) by differential pulse anodic stripping voltammetry on a stannum film electrode

A stannum film electrode has been developed for the simultaneous determination of trace levels of chromium(III) and cadmium(II) by differential pulse anodic stripping voltammetry (DPASV). The stannum film electrode was generated in situ by depositing simultaneously the stannum film and the metals obtained by reduction of Cd(II) and Cr(III) at −1.4 V on a glassy carbon electrode. Then, the reduced products were oxidized by scanning the potential of the electrode from −1.4 to −0.4 V using DPASV. The electrode exhibited well-defined and separated stripping signals for both metals accompanied with a low background contribution. The possible mechanism of this design was proposed. Under the optimized working conditions, the detection limit was 2.0 and 1.1 μg l⁻¹ for Cr(III) and Cd(II) at a deposition time of 3 min. Finally, the stannum film electrode was successfully applied to the determination of Cd(II) in tap water with satisfactory results.     

Preconcentration and Determination of Chromium Species Using Octadecyl Silica Membrane Disks and Flame Atomic Absorption Spectrometry

A novel and selective method for the fast determination of trace amounts of chromium species in water samples has been developed. The procedure is based on the selective formation of chromium diethyldithiocarbamate complexes at different pH in the presence of Mn(II) as an enhancement agent of chromium signals followed by elution with organic eluents and determination by atomic flame absorption spectrometry. The maximum capacity of the employed disks was found to be (396±3) µg and (376±2) µg for Cr(III) and Cr(VI), respectively. The detection limit of the proposed method is 49 and 43 ng·L^?1 for Cr(III) and Cr(VI), respectively. The proposed method was successfully applied for determination of chromium species Cr(III) and Cr(VI) in different water samples.     

Polarographic studies of iron complexes as potential contrast as agents in magnetic resonance imaging

Iron(III) complexes of polyaminocarboxylic acids of potential use as contrast agents in magnetic resonance imaging wre investigated by differential pulse polarography. The complexes with diethylenetrinitrilopentaacetic acid, trans-1,2-cyclohexylenedinitrilotetraacetic acid and triethylenetetranitrilohexaacetic acid (TTHA) wre found to decompose slowly at pH 7.2. With TTHA, a mixture of 1:1 and 2:1 complexes was obtained, and the transformation between the two complexes was slow. Ethylenediimonobis[(2-hydroxyphenyl)acetic acid] (EHPG) was found to be the most suitable ligand. The complex formation is kinetically slow, and a special procedure for the preparation of the complex is required; the complex is then stable for one week at pH 7.2. The polarographic measuremeents are preferably made at pH 9.2, where a well-defined reduction peak is obtained. The complex is stable for 2 days at pH 9.2. At pH values below 9, a double peak is obtained, except for low concentrations of the complex. Both Fe(III)EHPG and Fe(II)EHPG adsorb at the mercury electrode. The polarographic determination can be done in the presence of 10% of urine or serum without interference. The detection limit is in the low μM range.     

ICP-AES detection of ultratrace aluminum(III) and chromium(III) ions with a microcolumn preconcentration system using dynamically immobilized 8-hydroxyquinoline on TiO2 nanoparticles.

Titanium dioxide nanoparticle dynamically loaded with 8-hydroxyquinoline (nanometer TiO2-Oxine) was used as a solid-phase extractant for the preconcentration of trace amounts of aluminum(III) and chromium(III) prior to their determination by inductively coupled plasma atomic emission spectrometry (ICP-AES). The optimal conditions for preparing nanometer TiO2-Oxine were obtained. Also, the separation/preconcentration conditions of analytes, including the effects of the pH, the sample flow rate and the volume, the elution solution and the interfering ions on the recovery of the analytes were investigated. At pH 6.0, the adsorption capacity of nanometer TiO2-Oxine was found to be 5.23 mg g(-1) and 9.58 mg g(-1) for Al(III) and Cr(III), respectively. An enrichment factor of 50 was achieved by this method, and the detection limits (3sigma) for Al(III) and Cr(III) were 1.96 and 0.32 microg L(-1) respectively. The proposed method was applied for the determination of trace Al(III) and Cr(III) in biological samples and lake water with satisfactory results.     

Trace Determination of Chromium by Square‐Wave Adsorptive Stripping Voltammetry on Bismuth Film Electrodes

This works reports the use of adsorptive stripping voltammetry (AdSV) for the trace determination of chromium on a rotating‐disk bismuth‐film electrode (BFE). During the reductive accumulation step, all the chromium species in the sample were reduced to Cr(III) which was complexed with cupferron and the complex was accumulated by adsorption on the surface of a preplated BFE. The stripping step was carried out by using a square‐wave (SW) potential‐time voltammetric signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements in the presence of dissolved oxygen. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for chromium was 100 ng L^?1 (for 120 s of preconcentration) and the relative standard deviation was 3.6% at the 2 μg L^?1 level (n=8). Finally, the method was applied to the determination of chromium in real samples with satisfactory results.     

Preconcentration method on modified silica fiber for chromium speciation

A new method involving pre-concentration on modified silica fiber is described for the speciation of chromium(III) [Cr(III)] and chromium(VI) [Cr(VI)] in aqueous media. This method is based on the different chelating behavior of Cr(III) and Cr(VI) with morpholine-4-carbodithioate (MDTC). Both complexes are extracted on silica fiber modified by sol-gel technology by using 3-aminopropyltriethoxysilane (APS) as a precursor. All extracted samples are directly injected into an high-performance liquid chromatography injector for the simultaneous determination of Cr(III) and Cr(VI). Cr(VI) forms two different complexes, and Cr(III) forms a single complex with MDTC. Therefore, the concentration of Cr(VI) is determined directly from the peak area obtained at 5.4 min; whereas, the assay of Cr(III) is based on subtracting the peak area of Cr(VI) from the total peak area obtained at 4.3 min. Under the optimized conditions, the limits of detection for Cr(III) and Cr(VI) are found to be 0.7 ng/mL and 0.2 ng/mL, respectively.     

Gas Chromatographic Determination of Trace Amount of Chromium

Trace amount of chromium was first converted quantitatively to volatile chromium chelates such as chromium(III) acetylacetonate, Cr(acac)₁, chromium(III) trifluoracetylacetonate, Cr(tfa)₃, and chromium(III) hexafluoroacetylacetonate, Cr(hfa)₃, and the chelate solution was then analyzed by gas chromatography. It was found that Cr(tfa)₃ is the most suitable chemical form for this purpose. The method was profitably applied to the determination of chromium(III) incorporated in the six kinds of commerical products of potassium Chromate.     

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.     

Study of the Complexation,Adsorption and Electrode Reaction Mechanisms of Chromium(VI) and (III) with DTPA Under Adsorptive Stripping Voltammetric Conditions

The complexation of Cr(III) and Cr(VI) with diethylenetriaminepentaacetic acid (DTPA), the redox behavior of these complexes and their adsorption on the mercury electrode surface were investigated by a combination of electrochemical techniques and UV/vis spectroscopy. A homogenous two‐step reaction was observed when mixing Cr(III), present as hexaquo complex, with DTPA. The first reaction product, the electroactive 1 : 1 complex, turns into an electroinactive form in the second step. The results indicate that the second reaction product is presumably a 1 : 2 Cr(III)/DTPA complex. The electroreduction of the DTPA‐Cr(III) complex to Cr(II) was found to be diffusion rather than adsorption controlled.The Cr(III) ion, generated in‐situ from Cr(VI) at the mercury electrode at about ?50 mV (vs. Ag|AgCl) (3 mol L^?1 KCl), was found to form instantly an electroactive and adsorbable complex with DTPA. By means of electrocapillary measurements its surface activity was shown to be 30 times higher than that of the complex built by homogenous reaction of DTPA with the hydrated Cr(III). Both components, DTPA and the in‐situ built complex Cr(III) ion were found to adsorb on the mercury electrode.The effect of nitrate, used as catalytic oxidant in the voltammetric determination method, on the complexation reaction and on the adsorption processes was found to be negligible.The proposed complex structures and an overall reaction scheme are shown.     

Simultaneous determination of hexavalent and total chromium in water and plating baths by spectrophotometry

A new spectrophotometric determination method of hexavalent chromium in waste water and plating baths is described based on the oxidation of beryllon III by chromium(VI) in 0.02M sulphuric acid medium. The decrease in the absorbance of beryllon III was measured at 482 nm with an apparent molar absorptivity of 5.15 x 10(4)1.mole(-1).cm(-1). Beer's law was obeyed for chromium(VI) over the range 0-25 mug/25 ml. After the oxidation of Cr(III) to Cr(VI) by ammonium persulphate, total chromium can be determined. Therefore, chromium(III) can be calculated by subtracting chromium(VI) from total chromium. The detection limit is 0.015 and 0.020 mug/25 ml for chromium(VI) and total chromium, respectively. A sensitive spectrophotometric method for trace Cr(III) and Cr(VI) in waste water and plating baths was developed with good precision and accuracy. The reaction is also discussed.