Chromium(III, VI) speciation analysis with preconcentration on a maleic acid-functionalized XAD sorbent

Chromium may exist in environmental waters as Cr(III) and Cr(IV), the latter being the toxic and carcinogenic form. Since atomic absorption spectrometry (AAS) and inductively coupled plasma atomic emission spectrometry can only yield information on total Cr concentration, a polymer resin bearing O,O-donor chelating groups such as the maleic acid-functionalized XAD(CO)CHCHCOOH resin was synthesized to selectively retain Cr(III) at pH 4.0-5.5. The dynamic breakthrough capacity of the resin for Cr(III) at pH 5.0 was 7.52 mg g⁻¹, and the preconcentration factor extended to 250-300. Chromium(III) in the presence of 250-fold Cr(VI)—which was not retained—could be effectively preconcentrated on the NH₄⁺-form of the resin and determined by AAS or diphenylcarbazide (DPC) spectrophotometry. When Cr(VI) was reduced to Cr(III) with Na₂SO₃ solution brought to pH 1 by the addition of 1 M H₂SO₄, and preconcentrated on the resin, total Cr could be determined. The developed method was validated with a blended coal sample CRM-1632. Since the adsorption behavior as a function of pH of possible interferent metal ions, e.g. Ni(II), Co(II), Cu(II), Cd(II), Zn(II), Pb(II) and Fe(III), was similar to that of Cr(III), selective elution of Cr(III) from the resin was realized using a mixture of 1 wt.% H₂O₂+1 M NH₃. The eluate containing Cr as chromate could be directly analyzed by diphenyl carbazide spectrophotometry without any adverse effect from the common interferents of this method, i.e. Fe(III), Cu(II) Hg(II), VO₃⁻, MoO₄²⁻ and WO₄²⁻. Various synthetic waste solutions typical of electroplating bath effluents containing Cr, Cu, Ni, Zn, Na, Ca, cyanide (and chemical oxidation demand (COD), achieved by glucose addition) were subjected to pretreatment procedures such as hypochlorite oxidation (of cyanide) and catalytic oxidation (of COD) with peroxodisulfate. Chromium determination gave satisfactory results. The combined column preconcentration—selective elution—diphenylcarbazide spectrophotometric determination was also successfully applied to the determination of Cr in artificial and real seawater.     

A novel flow injection chemiluminescence determination of Cr(VI) with Dichlorotris(1,10-phenanthroline)ruthenium(II)

A selective novel reverse flow injection system with chemiluminescence detection (rFI-CL) for the determination of Cr(VI) in presence of Cr(III) with Dichlorotris (1,10-phenanthroline)ruthenium(II), (Ru(phen)₃Cl₂), is described in this work. This new method is based on the oxidation capacity of Cr(VI) in H₂SO₄ media. First, the Ruthenium(II) complex is oxidized to Ruthenium(III) complex by Cr(VI) and afterwards it is reduced to the excited state of the Ruthenium(II) complex by a sodium oxalate solution, emitting light inside the detector. The intensity of chemiluminescence (CL) is proportional to the concentration of Cr(VI) and, under optimum conditions, it can be determined over the range of 3-300 μg L⁻¹ with a detection limit of 0.9 μg L⁻¹. The RSD was 8.4% and 1.5% at 5 and 50 μg L⁻¹, respectively. For the rFI-CL method various analytical parameters were optimized: flow rate (1 mL min⁻¹), H₂SO₄ carrier concentration (20% w/V), Ru(phen)₃Cl₂ concentration (5 mM) and sodium oxalate concentration (0.1 M). The effect of Cr(III), Fe(III), Al(III), Cd(II), Zn(II), Hg(II), Pb(II), Ca(II) and Mg(II), was studied. The method is highly sensitive and selective, allowing a fast, on-line determination of Cr(VI) in the presence of Cr(III). Finally, the method was tested in four different water samples (tap, reservoir, well and mineral), with good recovery percentage.     

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.     

A Novel Catalytic Procedure for the Determination of Ultratrace Zirconium

 A novel catalytic procedure for zirconium was proposed based on Zr(IV) catalyzed oxidation of gallocyanine by hydrogen peroxide in hexamethylene tetramine-hydrochloric acid buffer medium. The calibration graph is linear for 0–110 ngċml⁻¹, and the detection limit is 0.4 ngċml⁻¹ Zr(IV). Most foreign ions do not interfere with the determination, except for Cu²⁺, Fe³⁺ and Cr(VI). The interferences of Cu²⁺ and Fe³⁺ could be eliminated by masking with EDTA and mannitol, and that of Cr(VI) by reducing to Cr(III) with ascorbic acid. The typical features of this procedure are that it is sensitive for zirconium, and the determination could be carried out at room temperature. It had been used to the determination of zirconium in zirconium bronze, simulated samples and a certified reference material. The recoveries were 98.6 ∼ 102%, and relative standard deviations (R.S.D.) were 0.9 ∼ 1.5%, respectively. Received September 12, 1999. Revision April 10, 2000.     

Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λ_max = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO₄. The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000 μg l⁻¹ and 30-1200 μg l⁻¹ for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s_r = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 μg l⁻¹ level, respectively). The method developed proved to be adequately selective and sensitive (c_L = 1 and 10 μg l⁻¹ for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.     

A novel fiber-packed column for on-line preconcentration and speciation analysis of chromium in drinking water with flame atomic absorption spectrometry

A novel on-line preconcentration and determination system based on a fiber-packed column was developed for speciation analysis of Cr in drinking water samples prior to its determination by flame atomic absorption spectrometry (FAAS). All variables involved in the development of the preconcentration method including, pH, eluent type, sample and eluent flow rates, interfering effects, etc., were studied in order to achieve the best analytical performance. A preconcentration factor of 32 was obtained for Cr(VI) and Cr(III). The levels of Cr(III) species were calculated by difference of total Cr and Cr(VI) levels. Total Cr was determined after oxidation of Cr(III) to Cr(VI) with hydrogen peroxide. The calibration graph was linear with a correlation coefficient of 0.999 at levels near the detection limit and up to at least 50 μg L⁻¹. The relative standard deviation (R.S.D.) was 4.3% (C = 5 μg L⁻¹ Cr(VI), n = 10, sample volume = 25 mL). The limit of detection (LOD) for both Cr(III) and Cr(VI) species was 0.3 μg L⁻¹. Verification of the accuracy was carried out by the analysis of a standard reference material (NIST SRM 1643e “Trace elements in natural water”). The method was successfully applied to the determination of Cr(III) and Cr(VI) species in drinking water samples.     

Facile preparation of glutathione-stabilized gold nanoclusters for selective determination of chromium (III) and chromium (VI) in environmental water samples

A novel method for selective determination of Cr(III) and Cr(VI) in environmental water samples was developed based on target-induced fluorescence quenching of glutathione-stabilized gold nanoclusters (GSH-Au NCs). Fluorescent GSH-Au NCs were synthesized by a one-step approach employing GSH as reducing/protecting reagent. It was found that Cr(III) and Cr(VI) showed pH-dependent fluorescence quenching capabilities for GSH-Au NCs, and thus selective determination of Cr(III) and Cr(VI) could be achieved at different pHs. Addition of EDTA was able to effectively eliminate the interferences from other metal ions, leading to a good selectivity for this method. Under optimized conditions, Cr(III) showed a linear range of 25–3800 μg L⁻¹ and a limit of detection (LOD) of 2.5 μg L⁻¹. The Cr(VI) ion demonstrated a linear range of 5–500 μg L⁻¹ and LOD of 0.5 μg L⁻¹. The run-to-run relative standard deviations (n = 5) for Cr(III) and Cr(VI) were 3.9% and 2.8%, respectively. The recoveries of Cr(III) and Cr(VI) in environmental water samples were also satisfactory (76.3–116%). This method, with its simplicity, low cost, high selectivity and sensitivity, could be used as a promising tool for chromium analysis in environmental water samples.     

On-line preconcentration and speciation analysis of Cr(III) and Cr(VI) using baker's yeast cells immobilised on controlled pore glass

A study was undertaken to evaluate Saccharomyces cerevisiae as a substrate for the biosorption of Cr(III) and Cr(VI) aiming to the selective determination of these species in aqueous solutions. The yeast cells were covalently immobilised on controlled pore glass (CPG), packed in a minicolumn and incorporated in an on-line flow injection system. The effect of chemical and physical variables affecting the biosorption process was tested in order to select the optimal analytical conditions for the Cr retention by S. cerevisiae. Cr(III) was retained by the immobilised cells and Cr(VI) were retained by CPG. The speciation was possible by selective and sequential elution of Cr(III) with 0.05 mol L⁻¹ HCl and 2.0 mol L⁻¹ HNO₃ for Cr(VI). The influence of some concomitant ions up to 20 mg L⁻¹ was also tested. Quantitative determinations of Cr were carried out by means of inductively coupled plasma optical emission spectrometry (ICP OES). Preconcentration factors of 12 were achieved for Cr(III) and 5 for Cr(VI) when 1.7 mL of sample were processed reaching detection limits of 0.45 for Cr(III) and 1.5 μg L⁻¹ for Cr(VI). The speciation of inorganic Cr in different kinds of natural waters was performed following the proposed method. Spiked water samples were also analysed and the recoveries were in all cases between 81 and 103%.     

Speciation Analysis of Chromium by Adsorptive Stripping Voltammetry in Tap and River Water Samples

An adsorptive stripping voltammetric method for speciation analysis of chromium in natural water samples has been developed. Ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA) were used as complexing agents for Cr(III) present in the sample and formed as products of Cr(VI) reduction, respectively. Under optimum experimental conditions linear relations in the range from 1×10^?6 to 3×10^?5 mol L^?1 without accumulation and from 1×10^?9 to 1×10^?7 at 30 s accumulation time were obtained for Cr(III) and Cr(VI), respectively. For samples in which Cr(III) concentration is higher than 1×10^?6 mol L^?1 the Cr(III) and Cr(VI) were determined simultaneously in one voltammetric cell. For samples in which Cr(III) concentration is below 1×10^?6 mol L^?1 only Cr(VI) was selectively determined in the presence of Cr(III), which did not influence the Cr(VI) signal. The determination of Cr(III) and Cr(VI) was successful with the application of the proposed procedure in the presence of common foreign ions. The presented method was applied for the speciation of chromium in spiked tap and river water samples with satisfactory results.     

Stability of fluoroquinolone antibiotics in river water samples and in octadecyl silica solid-phase extraction cartridges

Airborne particulate matter has been sampled at a location close to a metallurgical plant in North-Rhine-Westphalia, Germany, and first results on the chromium(VI) content in the collected dust are presented. A special procedure using a scrubber as sampling device was used to preserve Cr(VI) during the sampling procedure. The scrubber solution which consisted of 0.1 mol L^–1 TRIS-buffer solution was adjusted to a slightly alkaline pH of 8.6 to reduce the oxidation potential of Cr(VI) and to avoid possible oxidation of Cr(III) to Cr(VI), for example by oxygen (or ozone at ambient concentrations). After sampling Cr(VI) was pre-concentrated on an anion-exchange material and eluted with aqueous 0.6 mol L^–1 sodium perchlorate solution. After elution, a species-selective complex of Cr(VI) with diphenylcarbazide (DPC) was prepared; this was extracted into n-hexanol and quantified by UV–visible spectrophotometry. A detection limit of 0.9 ng m^–3 for Cr(VI) in ambient aerosols can be achieved with this method.     

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.     

Speciation of Cr(III) and Cr(VI) in waters using immobilized moss and determination by ICP-MS and FAAS

The possibility of using moss (Funaria hygrometrica), immobilized in a polysilicate matrix as substrate for speciation of Cr(III) and Cr(VI) in various water samples has been investigated. Experiments were performed to optimize conditions such as pH, amount of sorbent and flow rate, to achieve the quantitative separation of Cr(III) and Cr(VI). During all the steps of the separation process, Cr(III) was selectively sorbed on the column of immobilized moss in the pH range of 4-8 while, Cr(VI) was found to remain in solution. The retained Cr(III) was subsequently eluted with 10 ml of 2 mol l⁻¹ HNO₃. A pre-concentration factor of about 20 was achieved for Cr(III) when, 200 ml of water was passed. The immobilized moss was packed in a home made mini-column and incorporated in flow injection system for obtaining calibration plots for both Cr(III) and Cr(VI) at low ppb levels that were compared with the plots obtained without column. After separation, the chromium (Cr) species were determined by inductively coupled plasma mass spectrometry (ICP-MS) and flame atomic absorption spectrometry (FAAS). The sorption capacity of the immobilized moss was found to be ∼11.5 mg g⁻¹ for Cr(III). The effect of various interfering ions has also been studied. The proposed method was applied successfully for the determination of Cr(III) and Cr(VI) in spiked and real wastewater samples and recoveries were found to be >95%.     

Simultaneous determination of Cr(III) and Cr(VI) with prechelation of Cr(III) using phthalate by ion interaction chromatography with a C-18 column

Ion interaction chromatography has been successfully used for the simultaneous determination of Cr(III) and Cr(VI) in waste water. A C-18 column which had been dynamically coated with octylamine was used for the separation of Cr(III) and Cr(VI) based on anionic interaction. Cr(III) was chelated with potassium hydrogen phthalate (KHP) before injecting into the column since the Cr(III) did not exist in an anionic form like the Cr(VI) (Cr₂O₇²⁻) presented at the optimum condition. The analytes were detected at 200 nm and linear relationship between absorption with the concentration of Cr(III) or Cr(VI) was 0.1-50 mg/L. Most of the interested interferences including alkali metals, heavy metals and organic materials have no significant effect on Cr(III)-KHP complexation and Cr(VI) stability, only NH₄⁺ and ascorbic acid yielded the serious effect on the Cr(VI) stability. The relative standard deviations calculated from both of peak area and retention time were 0.75-2.20%. The sensitivity of the method at the level concentration of sub mg/L enabled the simultaneous determination of Cr(III) and Cr(VI) contents in waste water samples without any special sample preparation step.     

In-situ separation of chromium(III) and chromium(VI) and sequential ETV–ICP–AES determination using acetylacetone and PTFE as chemical modifiers

Electrothermal vaporization–inductively coupled plasma–atomic emission spectrometry (ETV–ICP– ES) has been used for the sequential determination of Cr(III) and Cr(VI). The method is based on the difference between the chelate reactions of the two Cr species and acetylacetone. Cr(III) chelate was separated from Cr(VI) and determined with use of acetylacetone as chemical modifier. The retained Cr(VI) in graphite tube was analyzed subsequently, after addition of polytetrafluoroethylene (PTFE) as chemical modifier. The different factors affecting the vaporization behavior of Cr(III) acetylacetonate were investigated in detail. The detection limits for Cr (III) and Cr(VI) were 0.56 and 1.4 ng mL^–1, respectively, and relative standard deviations for 0.1 μg mL^–1 Cr(III) and 0.1 μg mL^–1 Cr(VI) were 2.5% (n = 6) and 4.8% (n = 6), respectively. The linear ranges of the calibration curve for both Cr(III) and Cr(VI) covered three orders of magnitude. The proposed method was used to analyze water samples with satisfactory results.     

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.     

Reduction of hexavalent chromium at solid electrodes in acidic media: reaction mechanism and analytical applications

The electrochemical detection of hexavalent chromium species was investigated. It was found that Cr(VI) can undergo chemically irreversible reduction in acidic solutions at gold, glassy carbon and boron-doped diamond electrodes. The process was found to be diffusionally controlled at all three electrodes studied. The response obtained at a gold electrode towards the reduction of chromium(VI) produced an electrochemically reversible wave in contrast to those recorded at glassy carbon and boron-doped diamond electrodes. The analytical response of the hexavalent species was studied at gold electrodes in the presence of common environmental interferences: Ni²⁺, Cu²⁺, Fe³⁺, Cr³⁺ and Triton X-100 (surfactant), with an LoD of 4.3 μM obtained in the presence of 5 mM Cr(III).     

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).     

Application of double-spike isotope dilution for the accurate determination of Cr(III), Cr(VI) and total Cr in yeast

A method is presented for the simultaneous determination of Cr(III) and Cr(VI) in yeast using species-specific double-spike isotope dilution (SSDSID) with anion-exchange liquid chromatography (LC) separation and sector field inductively coupled plasma mass spectrometric (SF-ICP-MS) detection. Total Cr is quantitated using ID SF-ICP-MS. Samples were digested on a hot plate at 95±2 °C for 6 h in an alkaline solution of 0.5 M NaOH and 0.28 M Na₂CO₃ for the determination of Cr(III) and Cr(VI), whereas microwave-assisted decomposition with HNO₃ and H₂O₂ was used for the determination of total Cr. Concentrations of 2,014±16, 1,952±103 and 76±48 mg kg⁻¹ (one standard deviation, n=4, 3, 3), respectively were obtained for total Cr, Cr(III) and Cr(VI) in the yeast sample. Significant oxidation of Cr(III) to Cr(VI) (24.2±7.6% Cr(III) oxidized, n=3) and reduction of Cr(VI) to Cr(III) (37.6±6.5% Cr(VI) reduced, n=3 ) occurred during alkaline extraction and subsequent chromatographic separation at pH 7. Despite this significant bidirectional redox transformation, quantitative recoveries for both Cr(III) and Cr(VI) were achieved using the SSDSID method. In addition, mass balance between total Cr and the sum of Cr(III) and Cr(VI) concentrations was achieved. Method detection limits of 0.3, 2 and 30 mg kg⁻¹ were obtained for total Cr, Cr(VI) and Cr(III), respectively, based on a 0.2-g sub-sample.     

Preconcentration and Spectrophotometric Determination of Chromium (VI) and Total Chromium in Drinking Water by the Sorption of Chromium Diphenylcarbazone with Surfactant Coated Alumina

Abstract

A simple and inexpensive method for determining chromium (VI) in drinking water by spectrophotometry after preconcentration with sodium dodecyl sulphate (SDS) coated alumina column is described. Chromium(VI) is reacted with diphenylcarbazide (DPC) and the Cr-DPC complex is quantitatively adsorbed onto a SDS coated alumina column from 800 ml of sample solution. The complex is then eluted with a 8 ml mixture of methanol, acetone and hydrochloric acid and determined by spectrophotometry. Total chromium can be determined after oxidation of chromium (III) to chromium (VI) by KMnO₄. The relative standard deviation (10 replicate analyses) at the 10 μg l^?1 of chromium (VI) and 10 μg l^?1 of total chromium were 3.5% and 3.4% and corresponding limits of detection (based on 3 σ) were 0.040 μg l^?1 and 0.033 μg l^?1, respectively.     

Speciation of chromium in natural waters by micropumping multicommutated light emitting diode photometry

A simple and sensitive multicommutated flow procedure, implemented by employing a homemade light emitting diode (LED) based photometer, has been developed for the determination of chromium (VI) and total chromium in water. The flow system comprised a set of four solenoid micro-pumps, which were assembled to work as fluid propelling and as commutating devices. The core of the detection unit comprised a green LED source, a photodiode and a homemade flow cell of 100 mm length and 2 mm inner diameter. The photometric procedure for the speciation of chromium in natural waters was based on the reaction of Cr (VI) with 1,5-diphenylcarbazide. Cr (III) was previously oxidized to Cr (VI) and determined as the difference between total Cr and Cr (VI). After carrying out the assays to select the best operational conditions the features of the method included: a linear response ranging from 10 to 200 μg l⁻¹ Cr (III) and Cr (VI) (r = 0.999, n = 7); limits of detection of 2.05 and 1.0 μg l⁻¹ for Cr (III) and Cr (VI), respectively; a relative standard deviation lower than 2.0% (n = 20) for a typical solution containing 50 μg l⁻¹ Cr; a sampling throughput of 67 and 105 determinations per hour for total Cr and Cr (VI), respectively, and recovery values within the range of 93-108% for spiked concentrations of the order of 50 μg l⁻¹.