Simple spectrophotometric method for the determination of sulfur dioxide by its decolorizing effect on the peroxovanadate complex

A simple, rapid, and economical spectrophotometric method is developed for the determination of sulfur dioxide in sugar and air samples. The developed method is based on a red-brown peroxovanadate complex (λ_max = 470 nm) produced in 2 M sulfuric acid when ammonium metavanadate is treated with hydrogen peroxide. Under fixed concentrations of hydrogen peroxide and ammonium metavanadate, when sodium metabisulfite (Na₂S₂O₅ = 2SO₂) is added, it preferentially reacts with hydrogen peroxide producing sulfuric acid, and the unreacted hydrogen peroxide then reacts with ammonium metavanadate; therefore, the concentration of sulfur dioxide is directly proportional to a decrease in the concentration of the peroxovanadate complex. The stoichiometric ratio between hydrogen peroxide and ammonium metavanadate as well as the stability constant of the complex are determined by the modified Job’s method and the respective values are found to be 1: 1 and 2.5 × 10⁴ mol⁻¹ L, respectively. The system obeys Lambert-Beer’s law in the concentration range 3.57–64.26 ppm of sulfur dioxide. The molar absorptivity, correlation coefficient, and Sandell’s sensitivity values are found to be 0.649 × 10³ L mol⁻¹ cm⁻¹, 0.9908, and 0.1972 μg cm⁻², respectively. The method is applied to the determination of sulfur dioxide present in commercial sugars and air samples. The results obtained are reproducible with a standard deviation of 0.02–0.05. For method validation, sulfur dioxide is also determined separately following the AOAC method for an air sample and the ICUMSA method for commercial sugars. The results obtained by the developed and official methods are in good agreement. The text was submitted by the authors in English.     

Flow injection determination of hydrogen peroxide by means of a solid-state-peroxyoxalate chemiluminescence reactor

A solid-state reactor for detection of hydrogen peroxide in aqueous samples by peroxyoxalate chemiluminescence is described. Bis(2,4,6-trichlorophenyl)oxalate in solid form is packed into a bed reactor, which eliminates mixing problems and facilitates the instrumental development. Perylene is added as a sensitizer to a water/acetonitrile (20:80) carrier stream into which the samples (200–600 μl) are injected. Detection limits of 6 × 10^?9 M H₂O₂ (0.2 μg l^?1) are obtained with both a commercial and a home-made luminescence detector. Calibration graphs are linear up to 10^?5 M. The r.s.d. for 2 × 10^?7 M (6.7 μg^?1) hydrogen peroxide (n = 10) is 2.8%. Sample throughput is ca. 120 h^?1.     

Electrocatalytic Reduction of H2O2 and Oxygen on the Surface of Thionin Incorporated onto MWCNTs Modified Glassy Carbon Electrode: Application to Glucose Detection

A new H₂O₂ enzymeless sensor has been fabricated by incorporation of thionin onto multiwall carbon nanotubes (MWCNTs) modified glassy carbon electrode. First 50 μL of acetone solution containing dispersed MWCNTs was pipetted onto the surface of GC electrode, then, after solvent evaporations, the MWCNTs modified GC electrode was immersed into an aqueous solution of thionin (electroless deposition) for a short period of time <5–50 s. The adsorbed thin film of thionin was found to facilitate the reduction of hydrogen peroxide in the absence of peroxidase enzyme. Also the modified electrode shows excellent catalytic activity for oxygen reduction at reduced overpotential. The rotating modified electrode shows excellent analytical performance for amperometric determination of hydrogen peroxide, at reduced overpotentials. Typical calibration at ?0.3 V vs. reference electrode, Ag/AgCl/3 M KCl, shows a detection limit of 0.38 μM, a sensitivity of 11.5 nA/μM and a liner range from 20 μM to 3.0 mM of hydrogen peroxide. The glucose biosensor was fabricated by covering a thin film of sol–gel composite containing glucose oxides on the surface of thionin/MWCNTs modified GC electrode. The biosensor can be used successfully for selective detection of glucose based on the decreasing of cathodic peak current of oxygen. The detection limit, sensitivity and liner calibration rang were 1 μM, 18.3 μA/mM and 10 μM–6.0 mM, respectively. In addition biosensor can reach 90% of steady currents in about 3.0 s and interference effect of the electroactive existing species (ascorbic acid–uric acid and acetaminophen) is eliminated. The usefulness of biosensor for direct glucose quantification in human blood serum matrix is also discussed. This sensor can be used as an amperometric detector for monitoring oxidase based biosensors.     

Grape tissue-based electrochemical sensor for the determination of hydrogen peroxide

The use of grape tissue as a source of catalase for the determination of hydrogen peroxide is reported. A slice of grape tissue attached to the membrane of a Clark-type oxgen sensor was used to monitor the oxidation of hydrogen peroxide by catalase. At the steady state, the sensor responds linearly to hydrogen peroxide in the concentration range 1 × 10^?5–5 × 10^?4 M. The response time (T₉₀) was of the order of 1 min for this sensor. No interference was observed from ethanol, amino acids, glucose and lactic acid. The long-term stability of the grape tissue sensor was much better than previously reported immobilized enzyme and liver tissue-based hydrogen peroxide sensors.     

Effect of the hydrogen peroxide concentration in stereospecific oxidation of alkanes by models of non-heme oxygenases

The biomimetic oxidation of alkanes (cyclohexane, adamantane, cis-1,2-dimethylcyclohexane) with hydrogen peroxide catalyzed by Fe(II) complexes containing tetradentate nitrogen ligands (M = [Fe(bpmen)(MeCN)₂](ClO₄)₂ (bispicolyl-1,2-dimethylethylenediamine), [Fe(bpen)(MeCN)₂](ClO₄)₂ (bispicolylethylenediamine), and [Fe(tpcaH)(MeCN)₂]₂(ClO₄)₄ (tripyridylcarboxamide) is studied. The effects of the hydrogen peroxide concentration on the alcohol/ketone (A/K) ratio and on the regioselectivity of oxidation (3/2) are discovered. Rather high stereospecificity (RC = 96–99%) persisting at high hydrogen peroxide concentrations is hardly consistent with the participation of the HO^. radical, inferred from the rather low regioselectivity and low A/K ratio observed under these conditions. The molecular mechanism of oxygen transfer from hydrogen peroxide, which was earlier proved reliably for low concentrations of hydrogen peroxide ([H₂O₂]/[M] ? 10), can be applied to high peroxide concentrations ([H₂O₂]/[M] > 10) if a new ferryl species containing two equivalents of the oxidant is assumed to be involved in the process. This assumption is confirmed by the direct stereospecific formation of alkyl hydroperoxide from alkane at a high concentration of hydrogen peroxide.     

Eine thermometrisch-kinetische methode zur bestimmung von kupfer und cyanid mit hilfe der kupfer-katalysierten zersetzung von wasserstoffperoxid

The copper-catalyzed decomposition of hydrogen peroxide is retarded by cyanide. The oxidation of cyanide by hydrogen peroxide is likewise catalyzed by copper ions. The decomposition reaction of H₂O₂; can be followed thermometrically. Therefore, at a known copper concentration an unknown amount of cyanide can be determined from the retardation time; and an unknown amount of copper can be determined by adding a known amount of cyanide. Moreover, after the end of the retardation time, unknown copper concentrations can also be determined from the slope of the temperature curve (tana). Copper was determined in the range 4–40 μg, and cyanide in the ranges 3–30, 6–60 and 8–80 μg.     

Determination of cholesterol with a microporous membrane chemiluminescence cell with cholesterol oxidase in solution

A reagent solution, containing cholesterol oxidase buffered at pH 7, is contained in a pressurized reservoir and forced through a microporous membrane at 2–5 μl min^?1 into a stream flowing at 2–10 ml min^?1 which contains injected slugs of cholesterol as the analyte. The hydrogen peroxide produced then reacts with luminol in pH 9.0 Tris buffer, catalyzed by horseradish peroxidase, to produce chemiluminescence, the intensity of which is related to the cholesterol concentration. The working range is 0.4–40 mg dl^?1; precision is 1–4% over this range. The detection limit is 0.2 mg dl^?1 or 5 μM. Sample throughput is 60 h^?1, and only 0.01 unit of enzyme is consumed per sample. Blood serum samples may be analyzed for either free or total cholesterol by using standard addition and pre-treatment with Somogyi reagents for removal of reducing species.     

Mechanistic studies of the photorearrangement of o-nitrobenzyl esters

α-Methyl-o-nitrobenzyl isobutyrate and copolymers containing α-methyl-o-nitrobenzyl acrylate were irradiated in dilute solution with 20 ns flashes of 347 nm light or with 60 ps flashes of 355 nm light. The formation of the absorption spectrum of a nitronic acid with τ = 5 ns was observed in each case. A transient absorption spectrum similar to that of the nitronic acid was formed with τ ⩽ 0.5 ns. This spectrum is assumed to belong either to the triplet biradical formed upon intramolecular triplet state hydrogen abstraction or to nitronic acid formed very rapidly by intramolecular singlet state hydrogen abstraction, the latter mechanism being operative in parallel with the triplet mechanism.In 60vol.%EtOH-40vol.%H₂O the nitronate anion was formed by dissociation of the nitronic acid with τ = 3 μs as indicated by a build-up of a new absorption band with λ_max = 420 nm and by an increase in electrical conductivity. The conversion into the end products (nitroso compound and carboxylic acid) occurred at a much faster rate from the nitronic acid than from the nitronate anion: τ = 80 μs (CH₂Cl₂), τ = 360 μs (60vol.%EtOH-40.vol.%H₂O, 10⁻⁴ M H₂SO₄) and τ = 10 ms (60vol.%EtOH-40vol.%H₂O, no H₂SO₄). Conversion of the nitronate anion into another transient was inferred from the partial decrease in the electrical conductivity (τ ≈ 15 μs).Irradiation of the copolymers gave the same results as for the low molecular weight model compound, indicating that there is no polymer effect with respect to the kinetics or the mechanism of the photorearrangement. This conclusion was substantiated by the quantum yields for acid formation (0.24 – 0.25) measured with both the model compound and the copolymers.     

Homogeneous, unmodified gold nanoparticle-based colorimetric assay of hydrogen peroxide

An unmodified gold nanoparticle-based colorimetric assay system in homogeneous format has been developed using hydrogen peroxide (H₂O₂) as a model analyte. H₂O₂ is added to o-phenylenediamine/horseradish peroxidase solution, and allowed to react for 10 min. Then, unmodified gold nanoparticles that serve as “reaction indicators” are added to the reaction solution. The resulting mixture color changes dramatically from red to blue. The reason is that azoaniline, a horseradish peroxidase-catalyzed oxidation product, induces the nanoparticle aggregation. Using this approach, H₂O₂ can be semiquantitatively determined over the concentration range of ∼4 orders of magnitude by the naked eye. If the observed peak intensity at 420 nm is used for the construction of the calibration plot, hydrogen peroxide can be accurately determined down to concentration levels of 1.3 × 10⁻⁶ M. Compared with the conventional electrochemical protocol, this sensing system offers several important advantages: (1) ability to be monitored by the naked eye, (2) avoiding the need of surface modification of electrodes or gold nanoparticles and (3) detection in homogeneous solution. It is worthy of note that this efficient and convenient strategy is also suitable for the detection of other species, such as glucose and cholesterol.     

Utility of Derivative Spectrophotometry in the Determination of Carbochromen Hydrochloride and Dipyridamole in the Presence of Their Oxidative Degradation Products

Abstract

Direct spectrophotometric methods for the determination of carbochromen hydrochloride and dipyridamole, each in the presence of its oxidative degradation products, are presented. the methods are based on the first derivative (D₁) and second derivative (D₂) spectrophotometric measurement (absolute trough, U) at 336 nm and (Peak-trough, Y) at 309–342 nm for carbochromen hydrochloride and at 240–260 nm(U) and 246–268 nm(Y) for dipyridamole. Plots of D₁ or D₂ versus concentration were linear over the concentration range of 8.00–16.00 μg/ml for carbochromen hydrochloride and 4.00–12.00 μg/ml for dipyridamole. Oxidative degradation of these drugs has been optimized with respect to hydrogen peroxide concentration. Determining the intact in coexistence with its oxidative degradation product, the proposed derivative spectrophotometric methods proved to be of high potential in correcting the systematic error appearing in the results of the A_max method due to the latter. Assaying the commercial tablets, the proposed method gave results of high accuracy and reproducibility.     

Pt Nanoparticle‐modified Carbon Fiber Microelectrode for Selective Electrochemical Sensing of Hydrogen Peroxide

We report a simple approach to the production of carbon fiber‐based amperometric microbiosensors for selective detection of hydrogen peroxide (H₂O₂), which was achieved by electrometallization of carbon fiber microelectrodes (CFMs) by electrodeposition of Pt nanoparticles. The Pt‐carbon hybrid sensing interface provided a sensitivity of 7711±587 μA ? mM^?1 ? cm^?2, a detection limit of 0.53±0.16 μM (S/N=3), a linear range of 0.8 μM–8.6 mM, and a response time of <2 sec. The morphologies of the Pt nanoparticle‐modified CFMs were characterized by scanning electron microscopy. To achieve selectivity, permseletive layers, polyphenylenediamine (PPD) and Nafion, were deposited resulting in exclusion of the anionic and cationic interferents, ascorbic acid and dopamine, respectively, at their physiologically relevant concentrations. The resultant sensors displayed a sensitivity to hydrogen peroxide of 1381±72 μA ? mM^?1 ? cm^?2, and a detection limit of 0.86±0.19 μM (S/N=3). This simple and rapid metallization method converts carbon fiber microelectrodes, which are readily accessible, to microscale Pt electrodes in 2 min, providing a platform for oxidase‐based amperometric biosensors with improved spatial resolution over more commonly used platinum electrode array microprobes.     

Synthesis of PtxSn/MWCNTs and their Application in Non‐enzymatic Glucose and Hydrogen Peroxide Sensors

Pt_xSn/MWCNTs (x=1, 2, 3) nanocomposites were synthesized by chemical reduction. Comparing all of the materials, the results revealed that the best material was Pt₃Sn/MWCNTs. The sensor based on Pt₃Sn/MWCNTs exhibited excellent catalytic activities towards glucose and hydrogen peroxide. Sensing of glucose had a double‐linear range: one was between 50 μM and 550 μM, the other was between 1.35 mM and 16.35 mM. These were due to the fact that more and more intermediate species were adsorbed onto the electrode surface with increasing concentration of glucose, which limited the following glucose oxidation. Meanwhile, the sensor also had a linear response range between 0.05 mM and 18.95 mM for hydrogen peroxide. Furthermore, the glucose and hydrogen peroxide sensors exhibited excellent selectivity, stability, and reproducibility. Thus the sensors had potential utilities in the detection of glucose and hydrogen peroxide.     

Spectrofluorimetric determination of hydrogen peroxide based on the catalytic effect of peroxidase-like manganese tetrakis(sulphophenyl) porphyrin on the oxidation of homovanillic acid

Traces of hydrogen peroxide (8.5 × 10^?8–2.5 × 10^?6 mol/l) and, indirectly, glucose (3–44 × 10^?6 mol/l) can be determined by the fluorescence reaction between homovanillic acid and hydrogen peroxide. Mn-TPPS₄ is found to have very similar catalytic properties to horse peroxidase.     

Spectrophotometric determination of tantalum with 4,5-dibromo-o-nitrophenylfluorone

Micro amounts of tantalum can be determined directly by spectrophotometry with 4,5-dibromo-o-nitrophenylfluorone, citric acid, hydrogen peroxide and Triton X-100 in 0.5–5 mol l^?1 sulphuric acid. The apparent molar absorptivity of tantalum at 530 nm is 1.84 × 10⁵ l mol^?1 cm^?1. Beer's law is obeyed for 0–10 μg of tantalum in 25 ml of solution at 530 nm and a large amount of niobium and most foreign ions can be tolerated. Results obtained by applying the proposed method to niobium oxide, ferroniobium, nickel-base alloy and a mineral are satisfactory. The synthesis of the complexing agent is described.     

Determination of l-amino acids and alcohols with oxidase enzymes and a tubular iodide-selective electrode

L-Amino acids and alcohols (10^-2–10^-4 M) can be determined in a flow system by using a special tubular iodide-selective electrode, after reaction with L-atnino acid oxidase and alcohol oxidase, respectively. The hydrogen peroxide produced by both enzymatic reactions reacts with iodide in the presence of a molybdate catalyst, and the change in iodide concentration is monitored. The ranges of concentration that can be determined can be varied by adjusting parameters such as iodide concentration and buffer pH.     

Study of the mimetic peroxidase-catalysed chemiluminescence reaction

The chemiluminescence behaviour of the reaction in which the Mn-TPPS₄ complex the mimetic enzyme of peroxidase [manganese tetrakis(sulphophenyl)porphine] acts as a catalyst for the oxidation of luminol by hydrogen peroxide was studied. The reaction product luminesces at 427 nm. Trace amounts of hydrogen peroxide and glucose can be determined with detection limits of 5.5 × 10^?9 and 2.7 × 10^?9 M, respectively. The characteristics of Mn-TPPS₄ were compared with those of horseradish peroxidase.     

Enhancing a Novel Robust Multicomposite Materials Platform for Glucose Biosensors

An effective, stable enzymatic glucose biosensor was fabricated on a glassy carbon electrode (GCE) surface using simple multicomposite materials (MCM): a solution of prepared poly(diallyldimethylammonium chloride)‐capped gold nanoparticles‐nickel ferrite particles‐carbon nanotubes‐chitosan (PDDA‐AuNPs‐NiFe₂O₄‐CNTs‐CHIT), electropolymerization of poly(o‐phenylenediamine) (PoPD) and immobilization of glucose oxidase (GOx). Biocompatibility and synergy of the MCM enhanced the immobilization and the reaction of GOx and as well as the electron transfer from an oxidation reaction of hydrogen peroxide in the system. The NiFe₂O₄ was synthesized by co‐precipitation and calcined at 700 °C. Characterization was carried out by field emission scanning electron microscopy (FE‐SEM), energy‐dispersive X‐ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and X‐ray diffraction (XRD) which presented both tetrahedral and octahedral metal stretching with a cubic NiFe₂O₄ crystal phase. The GOx/PoPD/MCM/GCE yielded a 0.77 s^?1 charge transfer rate constant (K_s), a 2.28×10^?6 cm² s^?1 diffusion coefficient value (D), a 0.21 mm² electroactive surface area (A_e) and a 1.93×10^?8 mol cm^?2 surface concentration ( ) as determined by cyclic voltammetry. The modified electrode showed a durable operation time (n=97, more than 50 % I), repeatability (%RSD=0.38, n=10), reproducibility (%RSD=1.60, n=10), high sensitivity (853.07 μA mM^?1 cm^?2), selectivity without effects of electroactive species (aspirin, uric acid, caffeine, cholesterol, ascorbic acid and dopamine) and two linear ranges from 0.5 to 10 μM (R²=0.998) and 10 to 15,000 μM (R²=0.991) with a low detection limit (0.35 μM, S/N=3). Its Michaelis‐Menten constant (K_m) was calculated as 93.51 μM with 46.30 μA maximum current (I_max). This proposed simple method was successfully applied for glucose determination in human blood samples.     

Formation of PPy/Ta2O5/Ta structure by electropolymerization

Electropolymerization of pyrrole on tantalum (Ta) electrodes was carried out in buffer solutions (0.04 M phosphoric acid, 0.04 M acetic acid, 0.04 M boric acid and 0.2 M sodium hydroxide) containing 0.1 M sodium ptoluenesulfonate (TsONa) under galvanostatic conditions and it was found that a polypyrrole (PPy) and a tantalum oxide (Ta₂O₅) layer are formed on a Ta electrode by an electrochemical oxidation process. The conditions of this simultaneous formation were studied in respect to current density (i_d), pyrrole concentration ([Py]), pH and the amount of electricity. Under certain conditions ([Py] = 0.25 M, pH = 1.8, i_d = 10–20 mA cm^?2, the amount of electricity = 1 C), 6–8 μm thick PPy films were efficiently formed on homogeneous 30–50 nm thick Ta₂O₅ layers. The PPy film showed a high electrical conductivity (110 s cm^?1), adhered well and covered the Ta₂O₅ layer. The resulting PPy/Ta₂O₅/Ta system is therefore proved to have excellent properties as a capacitor.     

Direct Electrochemistry of Hemoglobin Immobilized on Carbon‐Coated Iron Nanoparticles for Amperometric Detection of Hydrogen Peroxide

A novel method for preparation of hydrogen peroxide biosensor was presented based on immobilization of hemoglobin (Hb) on carbon‐coated iron nanoparticles (CIN). CIN was firstly dispersed in a chitosan solution and cast onto a glassy carbon electrode to form a CIN/chitosan composite film modified electrode. Hb was then immobilized onto the composite film with the cross‐linking of glutaraldehyde. The immobilized Hb displayed a pair of stable and quasireversible redox peaks and excellent electrocatalytic reduction of hydrogen peroxide (H₂O₂), which leading to an unmediated biosensor for H₂O₂. The electrocatalytic response exhibited a linear dependence on H₂O₂ concentration in a wide range from 3.1 μM to 4.0 mM with a detection limit of 1.2 μM (S/N=3). The designed biosensor exhibited acceptable stability, long‐term life and good reproducibility.     

Kinetic flow-injection determination of hydrogen peroxide by use of iron(III)-catalyzed coloration and its application to the determination of biological substances.

A kinetic flow-injection (FI) method is described for the determination of hydrogen peroxide. This method is based on an iron(III)-catalyzed oxidative coupling of 4-aminoantipyrine with N,N-dimethylaniline by hydrogen peroxide. By measuring the change in the absorbance of the dye formed at 560 nm, 1 x 10(-6) - 6 x 10(-4) M hydrogen peroxide could be determined with a sampling rate of 15 h(-1). The relative standard deviation (n = 30) was 0.8% for 5 x 10(-5) M hydrogen peroxide. There was little interference of the co-existing ions and compounds. After introducing some immobilized enzyme reactors to the FI system, the proposed method allowed the determination of glucose and uric acid ranging from 1 x 10(-6) to 6 x 10(-4) M with relative standard deviations of below 2%. The applicability of the method was demonstrated by determining these substances in serum samples.