Electrochemical determination of nitrite and nitrate by pneumatoamperometry

Nitrite in aqueous solution is reduced to nitric oxide with hydroquinone in pH 2 phosphate buffer. The nitric oxide is swept with nitrogen to an anodically polarized membrane-covered platinum electrode, where it is oxidized to nitrate. The resulting current is linearly related to nitrite concentration in the original solution from the detection limit of 18 nmol to 5 μmol of nitrite. Nitrate is subsequently determined similarly after reduction to nitric oxide with hydroquinone in 50% sulfuric acid containing ammonium molybdate catalyst. The linear range is from the detection limit of 40 nmol to 10 μmol of nitrate.     

Simultaneous kinetic determination of sulfite and sulfide using artificial neural networks

Artificial neural networks (ANNs) are proposed for the determination of sulfite and sulfide simultaneously. The method is based on the reaction between Brilliant Green (BG) as a colored reagent and sulfite and/or sulfide in buffered solution (pH 7.0) and monitoring the changes of absorbance at maximum wavelength of 628 nm. Experimental conditions such as pH, reagents concentrations, and temperature were optimized and training the network was performed using principal components (PCs) of the original data. The network architecture (number of input, hidden and output nodes), and some parameters such as learning rate (η) and momentum (α) were also optimized for getting satisfactory results with minimum errors. The measuring range was 0.05-3.6 μg ml⁻¹ for both analytes. The proposed method has been successfully applied to the quantification of the sulfite and sulfide in different water samples.     

Simultaneous kinetic-spectrophotometric determination of sulfide and sulfite by partial least squares and genetic algorithm variable selection

Simultaneous multicomponent analysis is usually carried out by multivariate calibration models such as partial least squares (PLS) that utilize the full spectrum. It has been demonstrated by both experimental and theoretical considerations that better results can be obtained by a proper selection of the spectral range to be included in calculations. A genetic algorithm is one of the most popular methods for selecting variables for PLS calibration of mixtures with almost identical spectra without loss of prediction capacity. In this work, a simple and precise method for rapid and accurate simultaneous determination of sulfide and sulfite ions based on the addition reaction of these ions with new fuchsin at pH 8 and 25°C by PLS regression and using a genetic algorithm (GA) for variable selection is proposed. The concentrations of sulfide and sulfite ions varied between 0.05–2.50 and 0.15–2.00 μg/mL, respectively. A series of synthetic solutions containing different concentrations of sulfide and sulfite were used to check the prediction ability of GA-PLS models. The root mean square error of prediction with PLS on the whole data set was 0.19 μg/mL for sulfide and 0.09 μg/mL for sulfite. After the application of GA, these values were reduced to 0.04 and 0.03 μg/mL, respectively. The text was submitted by the authors in English.     

Stabilization of sulfide and sulfite and ion-pair chromatography of mixtures of sulfide, sulfite, sulfate and thiosulfate

Ion chromatography of sulfide, sulfite, sulfate and thiosulfate in a mixture is often difficult because of instability of sulfide and sulfite, poor separation of sulfide from common anions such as bromide or nitrate and similar elution-times for sulfite and sulfate. An ion-pair chromatographic method for the determination of these sulfur anions has been established by stoichiometric conversion of sulfide and sulfite into stable thiocyanate and sulfate, respectively, prior to the chromatographic run. Sulfate, thiosulfate and thiocyanate were resolved on an octadecylsilica column with an acetonitrile-water mobile phase containing tetrapropylammonium salt (TPA) as an ion-paring reagent, and thiosulfate and thiocyanate in the effluent could be measured with a photometric detector (220 nm) and sulfate with a suppressed conductivity detector. When an acetonitrile-water (6:94, v/v) mobile phase (pH 5.0) containing 15 mM TPA and small amounts of acetic acid was used at a flow-rate of 0.6 ml min(-1), the three anions could be eluted within 32 min. Calibration plots of peak height versus concentration for sulfide (detected as thiocyanate) and thiosulfate gave straight lines up to 35 and 60 microM, respectively. The calibration plot for sulfide coincided with that obtained by using thiocyanate. A calibration plot for sulfite, measured as sulfate, was also linear up to 135 microM and was in accord with that of sulfate. Each calibration plot gave a correlation coefficient greater than 0.999. For six replicates obtained for a mixture of 30.0 microM sulfide, 50.0 microM sulfite, 50.0 microM sulfate and 20.0 microM thiosulfate, the proposed method gave a mean value of 30.1 microM with a standard deviation (SD) of 0.77 microM and a relative standard deviation (RSD) of 2.6% for sulfide, 101 microM (SD = 3.5 microM, RSD = 3.5%) for the total of sulfite and sulfate and 20.1 microM (SD = 0.44 microM, RSD = 2.2%) for thiosulfate. Recoveries for sulfide, sulfite plus sulfate, and thiosulfate in hot-spring water samples using the proposed method were found to be quantitative.     

Capillary electrophoretic determination of thiosulfate,sulfide and sulfite using in-capillary derivatization with iodine

A new capillary electrophoresis (CE) method was developed for the rapid, simple and selective determination of thiosulfate, sulfide and sulfite species. The proposed method is based on the in-capillary derivatization of separated sulfur anions by mixing their zones with the iodine zone during the electrophoretic migration and direct UV detection of iodide formed. The optimal conditions for the separation and derivatization reaction were established by varying electrolyte pH, electrolyte counter-ion, concentration of iodine, and applied voltage. The optimized separations were carried out in 20 mmol/L Tris-chloride electrolyte (pH 8.5) using direct UV detection at 214 nm. All three sulfur species were well resolved in less than 4 min. The method gives repeatability comparable or even better than this obtained for sulfur anions using standard CE technique. The proposed CE system was applied to the monitoring of sulfur anions in spent fixing solutions during the electrolytic oxidation.     

Negative ions of ethylene sulfite

The formation of negative ions in molecular beams of ethylene sulfite (ES, alternately called glycol sulfite or ethylene glycol, C(2)H(4)SO(3)) molecules has been studied using both Rydberg electron transfer (RET) and free electron attachment methods. RET experiments with jet-cooled ES show an unexpected broad profile of anion formation as a function of the effective quantum number (n(*)) of the excited rubidium atoms, with peaks at n(max)(*) approximately 13.5 and 16.8. The peak at n(max)(*) approximately 16.8 corresponds to an expected dipole-bound anion with an electron binding energy of 8.5 meV. It is speculated that the peak at n(max)(*) approximately 13.5 derives from the formation of a distorted C(2)H(4)SO(3)(-) ion. We suggest that quasifree electron attachment promotes the breaking of one ring bond giving a long-lived acyclic anion and term this process incomplete dissociative electron attachment. Theoretical calculations of plausible ionic structures are presented and discussed. Electron beam studies of ES reveal the presence of multiple dissociative attachment channels, with the dominant fragment, SO(2)(-), peaking at 1.3 eV and much weaker signals due to SO(3)(-), SO(-), and (ES-H)(-) peaking at 1.5, 1.7, and 0.9 eV, respectively. All of these products appear to originate from a broad temporary negative ion resonance centered at approximately 1.4 eV.     

Spectrophotometric determination of sulfide in the presence of sulfite and thiosulfate via the precipitation of bismuth(III) sulfide.

A photometric method has been developed for the determination of sulfide at 10(-5) mol dm(-3) levels, which is based on the reaction of sulfide with a given excess amount of bismuth(III) to form a precipitate of bismuth(III) sulfide and on the spectrophotometric measurement of the residual bismuth(III) at 335 nm after extracting with bismuthiol II reagent from an aqueous solution containing acetate buffer into benzene. The presence of sulfite and thiosulfate up to 0.002 mol dm(-3) did not cause any interference in the determination of sulfide, because both sulfite and thiosulfate do not produce any precipitate with bismuth(III). A linear calibration plot with a negative slope was obtained for sulfide over the range of 5.00 x 10(-7) - 3.00 x 10(-5) mol dm(-3) (16.0 - 960 ppb). An experimental calibration plot was in accord with the theoretical plot, taking into account the known excess of bismuth(III), showing that the reaction of sulfide with bismuth(III) proceeded to completion. The relative standard deviation of results from 10 replicate determinations of standard sulfide (2.00 x 10(-5) mol dm(-3)) was 0.44%. The proposed method was successfully applied to the determination of sulfide in hotspring water samples without any pretreatment.     

Simultaneous kinetic-spectrophotometric determination of sulfide and sulfite and genetic algorithim variable selection using partial least squares calibration

Simultaneous multicomponent analysis is usually carried out using multivariate calibration models, such as the partial least squares (PLS) one, that utilize the full spectrum. It has been shown by both experimental and theoretical considerations that better results can by obtained by proper selection of the spectral range to be included in calculations. A genetic algorithm (GA) is one of the most popular methods for selecting variables for PLS calibration of mixtures with almost identical spectra without loss of predictive capability. In this work, a simple and precise method for rapid and accurate simultaneous determination of sulfide and sulfite ions based on the addition reaction of these ions with new fuchsin at pH 8 and 25°C using PLS regression and GA for variable selection is proposed. The concentrations of sulfide ions varied between 0.05–2.50 and 0.15–2.00 μg/mL, respectively. A series of model solutions containing different concentrations of sulfide and sulfite were used to check the predictive ability of GA-PLS models. The root mean square error of prediction with PLS on the whole data set was 0.19 μg/mL for sulfide and 0.09 μg/mL for sulfite. After the application of GA, these values reduced to 0.04 and 0.03 μg/mL, respectively. The text was submitted by the authors in English.     

Conditions for sulfite stabilization and determination by ion chromatography

The efficiencies of three groups of potential sulfite-stabilizing compounds were found to be in the order: carbonyls > alcohols = saccharides. A mole ratio of 1:1 between formaldehyde and sulfite was sufficient for stabilizing a sulfite solution for at least 72 h. The lower stabilizing efficiencies of the alcohols and saccharides examined could be compensated by using large excesses of these compounds. For example, if a 100-fold excess of glycerol over sulfite was used, the recovery of sulfite was 96% after 72 h compared with only 40% without addition of stabilizer. During separations by ion chromatography, almost no oxidation of the sample occurs provided the sample solution is directly injected into a deaerated eluent. For formaldehyde, the peak heights were found to depend on the molar ratio of the stabilizer to sulfite as well as on the concentration of sulfite. This effect was not found for the other stabilizers tested.     

Electrochemical determination of sulfide at various carbon substrates: a comparative study.

The direct electrochemical oxidation of sodium sulfide has been examined at five different carbon-based electrode substrates (glassy carbon (GC), boron-doped diamond (BDD), edge-plane pyrollytic graphite (EPPG), basal-plane pyrollytic graphite (BPPG) and carbon nanotubes (CNT)). An electrocatalytic response is observed at both the EPPG and CNT electrode compared to that of the other three substrates. The higher capacitative charging currents obtained at the CNT electrode hinder its detection range and, as such, the EPPG electrode has been clearly shown to be the substrate of choice for the direct electrochemical detection of sulfide. The procedure was applied to the recovery of a sulfide spike in river water, with a recovery of 104%.     

Kinetic determination of selenium using the reduction of nile blue with sulfide ions

The catalytic effect of Se(IV) was first observed in the reaction of the reduction of Nile Blue with sulfide ions. Optimal conditions for the determination of selenium by this reaction were found. The dependence of the reaction rate on the concentration of Se(IV) was linear in the concentration range 0.008-0.16 μg/mL. Solvent extraction with dithiophosphoric acids was proposed for the separation of selenium from the interferents. A procedure for the extraction-kinetic determination of selenium with the detection limit 0.006 μg/mL was developed.     

Spectrophotometric determination of microamounts of mercury(II) and sulfide ions in aqueous solution

Ammonium (2′,3′-dihydroxy pyridyl-4′-azo)benzene-4-arsonate (DHP-4A) provides a simple, rapid, and sensitive spectrophotometric microdetermination of mercury(II) in aqueous solution. The magenta colored 1:2 metal to ligand complex formed has a molecular extinction coefficient 6.25 × 10⁴ liters mol^?1 cm^?1 at the maximum absorption of 535 nm in highly alkaline medium. Beer's law is obeyed up to 3.8 ppm and Sandell's sensitivity (for an absorbance of 0.001) is 0.0032 μg of mercury(II)/cm². The mercury(II) complexed with DHP-4A has also been used in microdetermination of sulfide ions using ligand exchange reaction. The optimum concentration range of sulfide ions which can reproducibly be determined is 0.16-5.05 μg/10 ml and sensitivity of sulfide ions determination (for an absorbance 0.001) is 7.3 × 10^?4 μg/cm².     

Hydrogen sulfide determination by solid surface luminescence

In the analytical system suggested, atmospheric hydrogen sulfide reacts with the surface of a filter paper treated with aqueous cadmium chloride and yields a luminescent species whose intensity can be correlated with the analyte concentration in ambient air. It was shown that the luminescent species are CdS solid particles which were formed in a well defined size. The paper luminescence was also tried on polymeric surfaces; polyethyleneoxide, polyvinyl alcohol, ethylcellulose and carboxymethylcellulose were found to give a similar luminescence signal. The system can be used on the tip of an optical fiber for an irreversible, cumulative type of analytical device for hydrogen sulfide determination. The 3s detection limit for the paper luminescence detection system was 7.8 ppb H₂S.     

Hydrogen sulfide determination by solid surface luminescence

In the analytical system suggested, atmospheric hydrogen sulfide reacts with the surface of a filter paper treated with aqueous cadmium chloride and yields a luminescent species whose intensity can be correlated with the analyte concentration in ambient air. It was shown that the luminescent species are CdS solid particles which were formed in a well defined size. The paper luminescence was also tried on polymeric surfaces; polyethyleneoxide, polyvinyl alcohol, ethylcellulose and carboxymethylcellulose were found to give a similar luminescence signal. The system can be used on the tip of an optical fiber for an irreversible, cumulative type of analytical device for hydrogen sulfide determination. The 3s detection limit for the paper luminescence detection system was 7.8 ppb H(2)S.     

Electrochemical synthesis of copper sulfide nanoparticles

Cu₂S nanoparticles were electrosynthesized by cyclic voltammetry between 0.10 and 1.50 V in the presence of polyvinylalcohol as stabilizer. The structure and nature of the resulting Cu₂S poly (vinyl alcohol) composite were characterized by transmission electron microscopy and X-ray diffraction. The results show that electrochemically synthesized Cu₂S nanoparticles are homogeneously dispersed and well separated from one another with a mean diameter of about 12 nm.     

Amperometric determination of sulfite with sulfite oxidase immobilized at a platinum electrode surface

Sulfite oxidase is immobilized on collagen membrane at the surface of a platinum electrode and catalyzes the oxidation of sulfite to sulfate with stoichiometric production of hydrogen peroxide. The hydrogen peroxide is detected amperometically at the platinum electrode at an applied potential of 700 mV. The system responds linearly to sulfite in the range 1–150 μM, with a detection limit of 0.2 μM. The enzyme retains over 95% of its activity for three weeks if stored at ?20° C when the probe is not in use.     

High performance liquid chromatographic determination of bound sulfide and sulfite and thiosulfate at their low levels in human serum by pre-column fluorescence derivatization with monobromobimane.

A sensitive and specific high performance liquid chromatographic method for the determination of sulfide, sulfite, and thiosulfate was established. Inorganic sulfur anions were converted into fluorescent derivatives with monobromobimane. The derivatives were separated on a coupled column chromatography with a reversed-phase octadecyl silica column connected with a weakly basic anion exchanger column by isocratic elution with acetic acid solution (pH 3)-acetonitrile (13:3, v/v) containing 25 mM NaClO4. The method was applied to the determination of bound sulfide and sulfite and thiosulfate in normal human serum. Thiosulfate could be determined directly by use of an ultrafiltered sample. For the determination of bound sulfide and sulfite, the pretreatment step with continuous flow gas dialysis was effective for the sample after releasing sulfide and sulfite by reduction with dithiothreitol. The limits of quantification by the present method were 0.05 microM for thiosulfate, 0.5 microM for bound sulfide, and 0.2 microM for bound sulfite.     

Electrochemical behavior and composition of bismuth sulfide coatings

Cyclic potentiodynamic voltammetry and X-ray photoelectron spectroscopy were used to study the electrochemical behavior and composition of bismuth sulfide coatings deposited by successive ionic layer adsorption and reaction on glassy carbon in a colloid solution of bismuth(III) compounds, with Na₂S as a sulfidizing agent.     

Reactions of pentaamminechloroplatinum(IV) with iodide and sulfite ions

The reactions of pentaamminechloroplatinum(IV) with potassium iodide and sodium sulfite are studied. In the presence of an equimolar amount of the iodide, the chloride ions are completely substituted without subsequent displacement of the ammonia molecules that are in the trans position to the entering iodide ions. The reaction with sodium sulfite is accompanied by the reduction of Pt(IV) to Pt(II). The reaction kinetics of pentaamminechloroplatinum(IV) with potassium iodide at 22, 40, and 50°C are studied. The rate constants and activation energy of this reaction are calculated.     

流通式化学发光传感器测定亚硫酸根

基于酸性介质中高锰酸钾氧化亚硫酸根产生化学发光,喹咛具有较强的增敏作用,设计出了流通式化学发光传感器。该传感器对亚硫酸根的线性响应范围为1.0×10-7～5.0×10-4g mL,检出限为5×10-8g mL,对1.0×10-6g mL的亚硫酸根进行11次连续测定,相对标准偏差为3.5%。从取样到测定的全部分析过程只需1min。作为分子识别和换能作用的离子交换柱,可连续使用200次以上。方法已用于啤酒中亚硫酸根的测定。