Sequential determination of Se(IV) and Se(VI) by flow injection-hydride generation-atomic absorption spectrometry with HCl/HBr microwave aided pre-reduction of Se(VI) to Se(IV)

In this study a flow injection (FI) system used in conjunction with hydride generation (HG), atomic absorption spectrometry (AAS) and microwave (MW) aided pre-reduction of selenite (Se(IV)) to selenate (Se(IV)) with HCl:HBr has been developed in order to differentiate both inorganic selenium species. As full control of the MW reduction step is possible, the experimental approach allows the use of milder acidic conditions (10% v/v of HCl and HBr) than those conventionally accomplished with hydrochloric acid alone (≥50% v/v). Experimental parameters were optimized by the univariate optimization method. In either case, the linear range was from 1.0 to 30 μg l⁻¹. The detection limits based on 3σ of the blank signal were 0.25 μg l⁻¹ for Se(IV) and 0.30 μg l⁻¹ for Se(VI). The reproducibility, about 3% RSD and recoveries of different amounts of Se(VI) and Se(IV) added to water and orange juice samples (97–103%) were good. The main advantage of the proposed method is that the sequential determination of Se(IV) and Se(VI) is performed at a high sampling frequency (ca. 50 samples per h) in a closed system without Se losses, and with a minimum sample waste, operator attention, and sample manipulation.     

Flow injection for the determination of Se(IV) and Se(VI) by hydride generation atomic absorption spectrometry with microwave oven on-line prereduction of Se(VI) to Se(IV)

An on-line flow injection system has been developed for the selective determination of Se(IV) and Se(VI) in citric fruit juices and geothermal waters by hydride generation atomic absorption spectrometry with microwave-aided heating prereduction of Se(VI) to Se(IV). The samples and the prereductant solutions (4 mol l⁻¹ HCl for Se(IV) and 12 mol l⁻¹ HCl for Se(VI)) which circulated in a closed-flow circuit were injected by means of a time-based injector. This mixture was displaced by a carrier solution of 1% v/v of hydrochloric acid through a PTFE coil located inside the focused microwave oven and mixed downstream with a borohydride solution to generate the hydride. The linear ranges were 0–120 and 0–100 μg l⁻¹ of Se(IV) and Se(VI), respectively. The detection limits were 1.0 μg l⁻¹ for Se(IV) and 1.5 μg l⁻¹ for Se(VI). The precision (about 2.0–2.5% RSD) and recoveries (96–98% for Se(IV) and 94–98% for Se(VI)) were good. Total selenium values were also obtained by electrothermal atomic absorption spectrometry which agreed with the content of both selenium species. The sample throughput was about 50 measurements per hour. The main advantage of the method is that the selective determination of Se(IV) and Se(VI) in citric fruit juices and geothermal waters is performed in a closed system with a minimum sample manipulation, exposure to the environment, minimum sample waste and operator attention.     

Optimization of the reduction of Se(VI) to Se(IV) in a microwave oven

 Parameters for the reduction of Se(VI) to Se(IV) in HCl medium by heating in a microwave oven have been optimized. The reduction resulted to be quantitative applying 100% power, corresponding to 600 W heating for 2 min in 6 mol/L or for 3 min in 4 mol/L HCl. The behavior of selenomethionine and selenocystine under the optimized reduction conditions was studied in order to evaluate a possible interference of these selenium species in the determination of Se(VI). The final determination of Se(IV), and Se(VI) were done by hydride generation-atomic absorption spectrometry. The analytical merits of the method are reported. The method was applied to the selective determination of Se(IV), and Se(VI) in spiked river and lake water. Received: 6 December 1996/Revised: 1 April 1997/Accepted: 3 April 1997     

Thin-layer chromatographic separation of Se(IV), Te(IV), V(V), and Mo(VI) as ternary mixtures and their identification with potassium thiocarbonate (PTC) reagent

Summary Thin-layer chromatography of Se(IV), Te(IV), V(V), and Mo(VI) as ternary mixtures has been described. The separation was effected on a silica gel G layer by employing two different solvent systems: diethyl oxalate-HCl (601v/v) andn-butyl acetate-HCl (400.6v/v). The chromatograms were visualized with 0.1M potassium thiocarbonate (PTC) spray and the limits of identification as determined, lie between 1.27 and 2.04g.

---

Zusammenfassung Die Dünnschichtchromatographie ternärer Gemische von Se(IV), Te(IV), V(V) und Mo(VI) wurde beschrieben. Die Trennung wurde auf Schichten von Kieselgel G mit zwei verschiedenen Lösungsmittelsystemen durchgeführt: Diäthyloxalat—Salzsäure (601) und n-Butylacetat—Salzsäure (400,6). Die Chromatogramme wurden mit 0,1-m Kaliumthiocarbonat gesprüht. Die Nachweisgrenze liegt zwischen 1,27 und 2,04g.

     

Separation and preconcentration of Se(IV)/Se(VI) speciation on algae and determination by graphite furnace atomic absorption spectrometry in sediment and water

A novel method for the separation and preconcentration of Se(IV)/ Se(VI) with algae and determination by graphite furnace atomic absorption spectrometry (GFAAS) has been developed. The Se(VI) is extracted with algae from the solution containing Se(IV)/Se(VI) at pH 5.0, and the remaining Se(IV) is then preconcentrated pH 1.0. The detection limits (3σ, n = 11) of 0.16 μg L^–1 for Se(IV) and 0.14 μg L^–1 for Se(VI) are obtained using 40 mL of solution. At the 2.0 μg L^–1 level the relative standard deviation is 2.6% for Se(IV) and 2.3% for Se(VI). The method has been applied to the determination of Se(IV)/Se(VI) in sediment and water samples. Analytical recoveries of Se(IV) and Se(VI) added to samples are ¶97 ± 5% and 102 ± 6% (95% confidence), respectively.     

Separation and preconcentration of Se(IV)/Se(VI) speciation on algae and determination by graphite furnace atomic absorption spectrometry in sediment and water

A novel method for the separation and preconcentration of Se(IV)/ Se(VI) with algae and determination by graphite furnace atomic absorption spectrometry (GFAAS) has been developed. The Se(VI) is extracted with algae from the solution containing Se(IV)/Se(VI) at pH 5.0, and the remaining Se(IV) is then preconcentrated pH 1.0. The detection limits (3σ, n = 11) of 0.16 μg L^–1 for Se(IV) and 0.14 μg L^–1 for Se(VI) are obtained using 40 mL of solution. At the 2.0 μg L^–1 level the relative standard deviation is 2.6% for Se(IV) and 2.3% for Se(VI). The method has been applied to the determination of Se(IV)/Se(VI) in sediment and water samples. Analytical recoveries of Se(IV) and Se(VI) added to samples are ?97 ± 5% and 102 ± 6% (95% confidence), respectively. Received: 10 February 1999 / Revised: 21 June 1999 / /Accepted: 22 June 1999     

Nucleophilic attack by bromide ions as a first step of conversion of Se(VI) to Se(IV)

Numerous commonly used analytical methods allow only determination of a total amount of selenium in a given sample. Electroanalytical methods as well as those based on hydride generation or on formation of piazselenol allow only determination of Se(IV). To determine Se(VI) by these procedures, present alone or in mixtures with Se(IV), it is first necessary to convert Se(VI) to Se(IV). Such conversion is effective in the presence of excess of halides in acidic media or by photoreduction. In the often used conversion of Se(VI) in the presence of chlorides or less frequently of that of bromides, it has been assumed that the halide ion acts as a reducing agent. Kinetic studies of conversion of Se(VI) in acidic solutions containing an excess of bromide ions indicated that the rate determining first step of the reaction with Se(VI) is a nucleophilic substitution of the OH₂⁺ group in the protonated form of H₂SeO₄ by bromide ions. For the overall reaction with rate −d[Se(VI)]/dt = k₁[H⁺][Br⁻]^1.15[Se(IV)] the rate constant 1 × 10⁻³ L² mol⁻² s⁻¹ was found. The following formation of Se(IV) from the bromo derivative is a fast reaction probably resulting in elimination of HBrO.     

Determination of Se(IV) and Se(VI) in Italian mineral waters

This paper deals with determination of selenium and analysis of its speciation in some Italian mineral waters. Selenium was determined by differential pulse cathodic stripping voltammetry (DPCSV) even if square wave cathodic stripping voltammetry (SWCSV) was also taken into consideration. The selenium determined in the mineral waters here investigated is not over 600 ng L(-1); in three samples, it was found below the detection limit. Analysis of speciation revealed that Se(VI) is the highly prevailing form present: only two of the examined samples revealed a detectable amount (few ng L(-1)) of Se(IV). DPCSV made possible to detect, in two of the samples, the presence of a specie(s) able to interact with Se(IV). The apparent interaction constant for the adduct formation was evaluated and the species concentration determined. However, the nature of such compound(s) remains unknown.     

Separation and preconcentration of Se(IV)/Se(VI) species by selective adsorption onto nanometer-sized titanium dioxide and determination by graphite furnace atomic absorption spectrometry

A simple and sensitive method for the selective determination of Se(IV) and Se(VI) in natural water and sludge samples through an adsorptive process on a nanometer-sized TiO(2) (anatase) was developed. The conditions for quantitative and reproducible preconcentration, elution, and subsequent GFAAS determination were established. The proposed method gave a concentration factor of 50 for a 100 mL sample volume, characterized by high precision, high reproducibility, and direct determination of Se(IV)/Se(VI). The detection limits (3 sigma, n=11) were 4.7ng L(-1) for Se(IV) and 6.3ng L(-1) for Se(VI); the precision (relative standard deviation) was 0.7% for Se(IV) and 0.9% for Se(VI) at the 0.5microg L(-1) level.     

Microwave assisted reduction of Se(VI) to Se(IV) and determination by HG/FI-ICP/MS for inorganic selenium speciation

Speciation of inorganic selenium using hydride generation method is a widespread analytical method nowadays. However, a reduction step of Se(VI) to Se(IV) is necessary as the hydride-forming species is HSeO(3)(-) (oxydation state+IV). This paper describes the development of a batch assisted microwave system allowing a rapid (<5 min) conversion of Se(VI) to Se(IV). Hydride generation is performed by a flow injection system and detection by ICP/MS. Detection limits of 6 and 8 pg for Se(IV) and for Se(VI) (by using a sample loop of 200 mul) respectively have been achieved. This method has been validated by participating in a European certification exercise for inorganic Se speciation in aqueous solutions.     

Preconcentration of inorganic selenium species (Se (IV) and Se (VI)) in an alumina filled microcolumn and on-line determination by hydride generation atomic absorption spectrometry

A flow injection system has been developed consisting of on-line preconcentration of selenium species in a microcolumn filled with activated alumina, reduction of Se(VI) to Se(IV) and determination by HG-AAS. When 0.01 mol/L HNO₃ is used both as carrier and activation reagent for the alumina microcolumn, up to 150 ng of Se(IV) and Se(VI) can be preconcentrated and quantitatively eluted by 500 L of 2 mol/L NH₃. The preconcentration factor is 50 when 25 mL of sample is used. The detection limit is about 6 ng/L, the precision is 5% for low concentrations such as 150 ng/L and 3% at high concentrations such as 120 ng/mL. The proposed method is suitable for natural water samples and inorganic Se speciation can be performed by determining Se(IV) and total selenium [Se(VI) is evaluated from the difference].     

Voltammetric determination of Se(IV) and Se(VI) in saline samples—Studies with seawater, hydrothermal and hemodialysis fluids

Determination of Se(IV) and Se(VI) in high saline media was investigated by cathodic stripping voltammetry (CSV). The voltammetric method was applied to assay selenium in seawater, hydrothermal and hemodialysis fluids. The influence of ionic strength on selenium determination is discussed. The CSV method was based on the co-electrodeposition of Se(IV) with Cu(II) ions and Se(VI) determined by difference after sample UV-irradiation for photolytic selenium reduction. UV-irradiation was also used as sample pre-treatment for organic matter decomposition. Detection limit of 0.030 μg L⁻¹ (240 s deposition time) and relative standard deviation (RSD) of 6.19% (n = 5) for 5.0 μg L⁻¹ of Se(IV) were calculated. Linear calibration range for selenium was observed from 1.0 to 100.0 μg L⁻¹. Concerning the pre-treatment step, best results were obtained by using 60 min UV-irradiation interval in H₂O₂/HCl medium. Se(VI) was reduced to the Se(IV) electroactive species with recoveries between 91.7% and 112.9%. Interferents were also investigated.     

Differential pulse polarography of germanium(IV), tin(IV), arsenic(V), antimony(V), selenium(IV) and tellurium(VI) at the static mercury drop electrode in catechol—perchlorate media

The differential pulse polarography of Ge(IV), Sn(IV), As(V), Sb(V), Se(IV) and Te(VI) has been investigated in perchlorate media containing catechol using a static mercury drop electrode. Under optimum conditions, Ge(IV), Sn(IV), As(V), and Sb(V) undergo reduction to yield well-defined peaks; detection limits of 82 ppb, 28 ppb, 4 ppm, and 25 ppb, respectively, have been calculated. Few electrolytes are known for which these ions exhibit a quantitatively useful polarographic response. While Se(IV) and Te(VI) may be detected at levels of 115 ppb and 17 ppb, respectively, addition of catechol does not enhance the peak current relative to that observed in simple perchlorate solutions, as was the case for the other ions studied. The determination of germanium, arsenic and antimony in samples is described.     

On line speciation of Se(VI), Se(IV), and trimethylselenium by HPLC-microwave oven-hydride generation-atomic absorption spectrometry

An on-line system is proposed consisting of an anion-exchange chromatographic column, microwave-induced thermooxidation of trimethylselenium in the presence of persulphate, and microwave-induced thermoreduction of Se(VI) to Se(IV) in HCl medium, followed by hydride generation and atomic absorption for the determination of trimethylselenium (TMeSe), Se(IV) and Se(VI). Trimethylselenium is eluted in the dead volume of an anion-exchange column (Hamilton PRP-X-100), before elution of Se(IV) and Se(VI). Optimum chromatographic conditions have been obtained using 100 mmol L^–1 phosphate buffer (pH=6.8) H₂PO ₄ ^– /HPO ₄ ^2– as the mobile phase. Recoveries were around 100%, absolute detection limits were 1.1, 1.4 and 2.2 ng for TMeSe, Se(IV) and Se(VI), respectively. Precision was lower than 10% in all cases. The method has been applied to tap water.     

Syntheses and characterization of some mixed Te/Se polychalcogenide anions [Te(m)Se(n)]2-

Several mixed Te/Se polychalcogenide anions [Te(m)Se(n)](2-) were synthesized at 293 K by reactions between Te(n)(2-)and Se(n)(2-) anions in N,N-dimethylformamide (DMF) in the presence of different-size ammonium or phosphonium cations, in some cases in the presence of metal species. The structures of these anions were determined by single-crystal X-ray diffraction methods. The crystal structures of [NEt(4)](2)[Te(3)Se(6)] (1) and [NEt(4)](2)[Te(3)Se(7)] (2) consist, respectively, of one-dimensional infinite 1(infinity)[Te(3)Se(6)(2-)] and 1(infinity)[Te(3)Se(7)(2-)] anionic chains separated by NEt(4)(+) cations. In compound 1, each chain comprises Te(3)Se(5) eight-membered rings bridged by Se atoms. The Te(3)Se(5) ring has an "open book" conformation. The NMR spectrum of a DMF solution of [NEt(4)](2)[Te(3)Se(6)] crystals at 223 K shows (77)Se resonances at delta = 290, 349, and 771 ppm and a single (125)Te resonance at delta = 944.7 ppm. In compound 2, each chain comprises Te(3)Se(6) five- and six-membered rings bridged by Se atoms. The Te(3)Se(6) ring can be regarded as an inorganic analogue of bicyclononane. The anion of [PPh(4)](2)[Te(2)Se(2)] (4) contains a Se-Te-Te-Se chain with the terminal Se atoms trans to one another. The new compounds [PPN](2)[TeSe(10)] (3), [NMe(4)](2)[TeSe(3)].DMF (5), and [NEt(4)](2)[TeSe(3)] (6) contain known anions.     

Electrometric titrations of Bi(III), Cu(II), Pt(IV), Te(VI) and Te(IV) with standard iron(II) solution in alkaline medla containing hexitols

Potentiometric, bipotentiometric and biamperometric titrations with an iron(II) salt in alkaline solutions containing sorbitol are reported. Suitable conditions are discussed for determinations based on the Bi(III) → Bi°, Cu(II) → Cu(I) → Cu°, Pt(IV) → Pt(II) → Pt° and Te(VI) → Te(IV) → Te° → telluride systems.     

Hydride generation atomic absorption spectrometry with insitu graphite tube trapping for the determination of Se (IV) and Se (VI) in baltic sea water samples

This paper reports the results of an optimisation study for a procedure to determine the total selenium and its inorganic species, Se(IV) and Se(VI) using atomic absorption spectrometry combined with hydride generation and in-situ trapping of the analyte on the inner walls of the graphite tube. With the use of the proposed modification, a detection limit (3σ) of 0.018 ng/ml is achieved. This paper presents exemplary results, according to the proposed procedure, for selenium determination in samples of marine water. The concentrations of selenium in the samples ranged from <0.02 ng/ml to 0.16ng/ml of Se(IV) and from <0.02 ng/ml to 0.10 ng/ml of Se(VI).     

Ultramikrobestimmung von Selen(IV) mit Hilfe der Chlorat-Chlorid-Hydrazin-Reaktion

Zusammenfassung Zum katalytischen Nachweis von Selen(IV) wird das Chlorat-Chlorid-Hydrazin-System empfohlen. Die Grenzkonzentration beträgt 0,004g Se/ml, die Grenzverdünnung 1 2,5 · 10^–8.In Ultramikrokonzentration kann Selen(IV) mit der Simultankomparationsmethode oder chronometrisch auf Grund des LandoltEffektes bestimmt werden. V(V), Os(VIII) Cu(II), Mo(VI) und Te(IV) wirken in diesem System ebenfalls katalytisch.

---

Ultramicrodetermination of selenium(IV) by use of the chlorate-chloridehydrazine system

Summary Selenium(IV) may be catalytically detected by use of the chlorate-chloride-hydrazine system. This reagent is recommended. The limiting concentration is 0.004g Se/ml, the limiting dilution is 1 2.5 · 10^–8. Ultramicro amounts of selenium(IV) may be determined with the simultaneous comparison method or chronometrically on the basis of the Landolt effect. V(V), Os(VIII), Cu(II), Mo(VI) and Te(IV) likewise have a catalytic effect in this system.

     

Sorption of selenium(IV) and selenium(VI) onto synthetic pyrite (FeS2): spectroscopic and microscopic analyses

Pyrite was hydrothermally synthesized and used to remove Se(IV) and Se(VI) selectively from solution. Surface analyses of pyrite before and after contact with Se(IV) and Se(VI) were conducted using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and atomic force microscopy (AFM). All solid samples were acquired by allowing 3.1 mmol/L of Se(IV) or Se(VI) to react with 1 g/L of pyrite for 1, 15, or 30 days. The XPS spectra were fitted using the XPSPEAK program that applies a Gaussian Lorentzian function. The fitted spectra indicate that Se(IV) more strongly reacts with the surface-bound S than with the surface-bound Fe of pyrite. However, there is no apparent evidence of surface reaction with Se(VI). Specifically, fitted XPS spectra showed the presence of sulfide and tetrathionate on the surface, indicating that sulfur (S(2)(2-)) at the surface of pyrite can be both oxidized and reduced after contact with Se(IV). This occurs via surface disproportionation, possibly resulting in the formation of surface precipitates. Evidence for the formation of precipitates was seen in SEM and AFM images that showed rod-like particles and a phase image with higher voltage. In contrast, there were no important changes in the pyrite after contact with Se(VI) over a period of 30 days.     

Interaction of uranyl with Se(IV) and Se(VI) in aqueous acid solutions by means of capillary electrophoresis

The uranyl–selenium(IV) and uranyl–selenium(VI) interactions were studied by CE in aqueous acid solutions, containing U(VI) and Se(IV) or Se(VI) at different concentrations, at pH 1.5, 2.0 and 2.5. The method proposed in this paper allows one with the use of CE data on metal ion mobilities at different pHs to establish the ligand species interacting with metal ion and complex species formed. In the case of Se(VI) a selenate, as demonstrated, interacts with uranyl ions, in the case of Se(IV) this is a hydroselenite. It was also shown that the equilibria for the U(VI)–Se(VI) and U(VI)–Se(IV) systems can be established from CE data. The formation of UO₂SeO₄, UO₂(SeO₄), UO₂HSeO and UO₂(HSeO₃)₂ species is demonstrated. The stability constant values were measured at different ionic strengths (from 0.02 to 0.2 mol/L). The logarithms of the stability constant values (β°) extrapolated to ionic strength 0 by the specific ion interaction theory (SIT) are found to be log β°₁=2.93±0.06 for UO₂SeO₄ formation, log β°₂=4.030.18 for UO₂(SeO₄) formation, log β°₁=3.270.15 for UO₂HSeO formation and log β°₂=5.510.11 for UO₂(HSeO₃)₂ at 25°C. The results for the first constant values for each of systems are consistent with the published values. For UO₂(SeO₄) formation, a new constant stability value is given. The existence of UO₂(HSeO₃)₂ complex species is demonstrated and its constant stability value is given for the first time.