Re-Examination of the Cadmium Reduction Method and Optimisation of Conditions for the Determination of Nitrate by Flow Injection Analysis

The present paper re-examines the widely used flow injection method for nitrate which is based on heterogeneous reduction of nitrate with copperized cadmium followed by spectrophotometric detection of nitrite formed. The thorough investigation presented here has shown that the reduction step is very critical as far as conversion rate, possible further reduction of nitrite and long-term stability of the reductor column is concerned. The reasons for inconsistent and irreproducible results mentioned in original papers and often obtained in routine applications could be traced back to the way the reductor material had been prepared, the chemical conditions under which reduction takes place (i.e. pH and concentration of complexing agents) and the poisoning of the cadmium surface by sample constituents. The concomitant reduction of dissolved oxygen has been identified as a potential problem causing loss of pH control (due to in-situ generation of hydroxide). De-oxygenation of the carrier solution was found to be a means to overcome this problem with the additional advantages of increasing the lifetime of the reductor column and significantly reducing the concentration of cadmium ions in the waste stream. The role of particle size and column dimension as well as sample residence time within the reductor column on reduction efficiency and sample dispersion has also been investigated.     

On the optimum conditions for the reduction of nitrate to nitrite by cadmium

The variables of direct importance in the reduction of nitrate to nitrite by a metallic reductant such as cadmium used in a reductor column are discussed with special reference to the determination of nitrate as nitrite in very dilute solutions, e.g., natural waters. As a result of these considerations the effect of flow-rate (expressed as bed-volumes min ), pH, temperature, chloride concentration and various types of reductor cadmium on the yield of nitrite is investigated. The effect of dissolved oxygen in the sample solution on pH and cadmium concentration in the reduced solution is demonstrated. At constant pH a maximum yield of nitrite is obtained at a certain flow-rate, which is explained as the result of a rapid formation and simultaneously proceeding slow reduction of nitrite. With increasing pH this maximum is shifted to lower flow-rates, and grows broader whilst the yield at maximum approaches 100%; at pH 9.5 a yield of 99.9 +/- 0.1% is obtained. The temperature has little effect on the reduction rate in the interval 20-30 degrees but at 10 degrees the reduction is noticeably slower. Chloride ions have a strongly retarding effect on the reduction rate but the yield at maximum is not affected. Electrolytically precipitated cadmium, filings of pure cadmium or amalgamated pure cadmium all give practically the same yield at maximum though some differences in reduction rate are observed. Impure cadmium or copper-cadmium and silver-cadmium, owing to the formation of galvanic cells with higher reducing power, give a high reduction rate, which also applies to nitrite, causing a poorer yield at maximum. The practical consequences of the results are thoroughly discussed.     

The formation of hydrogen peroxide in metallic reductors

The formation of hydrogen peroxide in various types of metallic reductors both in the presence and absence of oxygen has been studied. Only when oxygen is rigorously excluded is peroxide undetectable. The oxidimetric determination of iron is seriously affected by this peroxide when the test solution and reductor are open to atmospheric oxygen. Systems which are completely oxygen-free give satisfactory results.     

Supercritical water decomposition of polyethylene samples for the determination of their trace cadmium content.

A novel pretreatment method has been developed for determination of toxic metals in plastic materials by their decomposition under supercritical water conditions. Particularly, quantitative analysis of cadmium in polyethylene has been demonstrated using inductively coupled plasma atomic emission spectrometry combined with supercritical water treatment. All the cadmium in a polyethylene sample was obtained as an aqueous solution by the treatment with supercritical water containing 12.4% of hydrogen peroxide at 673 K. Although a complete recovery of the aqueous solution from the reactors has not yet been attained, we verified that the present method was effective and promising for quantitative analysis of trace amounts of hazardous metals in plastic materials.     

BIOLOGICAL EFFECTS OF NEAR-UV RADIATION 2'-ACETYLFORMANILIDE-SENSITIZED FORMATION OF HYDROGEN PEROXIDE FROM NUCLEOSIDES AND NUCLEOTIDES

Abstraet—2'-Acetylformanilide has been found to be an effective, near-ultraviolet (300–380 nm) sensitizer for the photooxidation of nucleosides and nucleotides in aqueous solution, with hydrogen peroxide being formed in high yield. The decreasing order of hydrogen peroxide formation and substrate destruction was found to be: guanosine. adenosine, thymidine, uridine and cytidine. The process was highly pH dependent, low pH being most favorable for photooxidation. Experiments using deuterium oxide and superoxide dismutase indicate that both singlet oxygen and superoxide ion can be involved in hydrogen peroxide formation.     

Electropolymerization of iron tetra(<Emphasis Type="Italic">o</Emphasis>-aminophenyl)porphyrin from aqueous solution and the electrocatalytic behavior of modified electrode

Stable electroactive iron tetra(o-aminophenyl)porphyrin (FeTAPP) films are prepared by electropolymerization from aqueous solution by cycling the electrode potential between −0.4 and 1.0 V vs Ag/AgCl at 0.1 V s⁻¹. The cyclic voltammetric response indicates that polymerization takes place after the oxidation of amino groups, and the films could be produced on glassy carbon (GC) and gold electrodes. The film growth of poly(FeTAPP) was monitored by using cyclic voltammetry and electrochemical quartz crystal microbalance. The cyclic voltammetric features of Fe(III)/Fe(II) redox couple in the film resembles that of surface confined redox species. The electrochemical response of the modified electrode was found to be dependent on the pH of the contacting solution with a negative shift of 57 mV/pH. The electrocatalytic behavior of poly(FeTAPP) film-modified electrode was investigated towards reduction of hydrogen peroxide, molecular oxygen, and chloroacetic acids (mono-, di-, and tri-). The reduction of hydrogen peroxide, molecular oxygen, and dichloroacetic acid occurred at less negative potential on poly(FeTAPP) film compared to bare GC electrode. Particularly, the overpotential of hydrogen peroxide was reduced substantially. The O₂ reduction proceeds through direct four-electron reduction mechanism.     

Adsorption of hydrogen peroxide on functionalized mesoporous silica surfaces

MCM-41 and MSU-H mesoporous silicas were successfully functionalized with hydrogen bonds forming organic moieties, which have been proven by elemental analysis. Both moieties, based on oxygen and nitrogen containing groups, were introduced with high efficiency—the amount of carbon in all cases exceeded 10 % and the elemental ratios suggest binding to the surface through two or three Si–O–Si bonds. Hydrogen peroxide adsorption was conducted in its aqueous solutions and the amount adsorbed was determined using the ferric thiocyanate method. Results are presented as a function of hydrogen peroxide concentration in aqueous solution from 5 to 30 %. Both functionalized silicas show increased adsorption capacity when compared with that of their unfunctionalized analogues. The surface modified with nitrogen-based organic moiety revealed better adsorption properties as well as higher resistance against oxidation. MSU-H silica, due to its larger pore diameter, provides more space to bind hydrogen peroxide molecules and thus was found to have higher adsorption capacity: it adsorbed up to four times more hydrogen peroxide than MCM-41.     

PHOTOGENERATION OF SUPEROXIDE ION and HYDROGEN PEROXIDE FROM TRYPTOPHAN and ITS PHOTOOXIDATION PRODUCTS: THE ROLE OF 3a-HYDROPEROXYPYRROLIDINOINDOLE*

Abstract— Mechanism of the photogeneration of hydrogen peroxide and superoxide ion from tryptophan (Trp) and its photooxidation products was investigated. Near-ultraviolet irradiation of 3a-hydro-peroxypyrrolidinoindole, an intermediate in the photooxidation of Trp, has been shown to generate hydrogen peroxide efficiently under aerobic conditions. Irradiation of N-formylkynurenine in the presence of 3α-hydroxypyrrolidinoindole also produced hydrogen peroxide. The formation of superoxide ion in both reactions has been confirmed, whereas the reaction of Trp with chemically generated singlet oxygen did not produce any detectable amount of superoxide ion.     

Electrode reactions during electrolytic preparation of nickel hydroxide

We investigate the electrode reactions involved in the electrolytic precipitation of nickel hydroxide from nickel nitrate through quantitative assessment of the reaction products using stainless steel cathode with Ti and Ni anodes. The nitrate ion is reduced at the cathode to form nitrite and ammonium ion, the later being the major product. The production of ammonium ion followed either directly via reduction of nitrate to ammonium ion or in steps via nitrate reduction to nitrite which further reduced to ammonium ion. Finally, we report XRD measurements, tap density, and the discharge capacity performed on the electrolytically precipitated Ni(OH)₂.     

Simultaneous determination of35S and14C in double-labelled organic compounds by liquid scintillation counting

An isotope analytical method for the simultaneous determination of³⁵S and¹⁴C in double-labelled organic compounds by liquid scintillation counting is described. The sample is burned in a stream of oxygen. Sulfur oxides are converted to sulfuric acid and separated from other combustion products, including carbon-14 dioxide, on a heated quartz wool column previously wetted with hydrogen peroxide. Carbon dioxide is collected from the gas stream by an absorbent suitable for liquid scintillation counting. The residual sulfuric acid is rinsed off the column with water and the aqueous solution obtained is mixed with a liquid scintillation cocktail for radioactivity measurement. The final solutions ready for counting are obtained in less than fifteen minutes, quantitative collection recovery is achieved and no cross contamination occurs.     

Flow-injection determination of nitrite and nitrate with biamperometric detection at two platinum wire electrodes

The flow-injection determination of nitrite is based on oxidation of iodide by nitrite. The triiodide formed is detected amperometrically in a flow-through cell with two teflonized graphite or platinum wire electrodes polarized with a voltage of 100 mV. More sensitive and faster response was observed with the platinum wire electrodes. The same detector is used for determination of nitrate after reduction to nitrite in a reductor column containing copperized cadmium. Detection limits under optimized conditions are 6 μg l^?1 for both nitrite- and nitrate-nitrogen. Effects of oxygen and interfering metal ions are discussed.     

Controlled potential lodometric determination of nitrate after reduction to nitrite by coppered cadmium

A controlled-potential coulometric iodometric method previously developed for the accurate determination of small amounts of nitrite has been extended for the determination of nitrate after its reduction on a coppered cadmium reductor. The conditions for quantitative reduction have been investigated with respect to type of reductor and pH. Nitrate-nitrogen in the range 0.01-100 mug ml may be determined with high accuracy in less than 10 min, including the reduction step. The method has been applied with good results to a large variety of samples such as meat products, juices and waste waters.     

Surface Modification of Boron-Doped Diamond with Microcrystalline Copper Phthalocyanine: Oxygen Reduction Catalysis

Surface modification of boron-doped diamond (BDD) with copper phthalocyanine was achieved using a simple and convenient dropcast deposition, giving rise to a microcrystalline structure. Both unmodified and modified BDD electrodes of different surface terminations (namely hydrogen and oxygen) were compared via the electrochemical reduction of oxygen in aqueous solution. A significant lowering of the cathodic overpotential by about 500 mV was observed after modification of hydrogen-terminated (hydrophobic) diamond, while no voltammetric peak was seen on modified oxidised (hydrophilic) diamond, signifying greater interaction between copper phthalocyanine and the hydrogen-terminated BDD. Oxygen reduction was found to undergo a two-electron process on the modified hydrogen-terminated diamond, which was shown to be also active for the reduction of hydrogen peroxide. The lack of a further conversion of the peroxide was attributed to its rapid diffusion away from the triple phase boundary at which the reaction is expected to exclusively occur.     

Heavy Atom Effects in Tellurapyrylium Dyes Useful in Photodynamic Therapy and Catalytic Generation of H2O2

Abstract

The large rate of intersystem crossing between singlet and triplet states of tellurapyrylium dyes leads to efficient generation of singlet oxygen in irradiated airsaturated aqueous solutions containing these dyes. One reaction of tellurapyrylium dyes with singlet oxygen and water is the formation of dihydroxy tellurane [tellurium(IV)] species. We have found that the photochemical generation of dihydroxy telluranes is reversible thermally. The tellurapyrylium dye is regenerated while a molecule of hydrogen peroxide is produced. The thermal generation of hydrogen peroxide coupled with a photochemical generation of singlet oxygen allows a catalytic cycle to be devised for the conversion of oxygen and water to hydrogen peroxide. The dihydroxy telluranes are efficient two-electron oxidizing agents and can be used as catalysts to accelerate reactions using hydrogen peroxide as a two-electron oxidizing agent. Examples of tellurapyrylium dye-mediated reactions of hydrogen peroxide include reactions of leucodyes normally oxidized by horseradish peroxidase and hydrogen peroxide. These processes lead to thermal and photochemical reactions that are potentially cytotoxic following the generation of singlet oxygen in photodynamic therapy. The regeneration of the original catalyst allows repeated treatment from a single dose.     

Comparative study of various methods of the heterogeneous reduction of nitrate ions

Working pH ranges of heterogeneous reductors of nitrate ions based on metallic cadmium, zinc, and brass as well as on cadmium-copper, cadmium-mercury, zinc-copper, and zinc-mercury galvanic couples were examined. Metallic cadmium and the cadmium-copper couple were selected as reductants; freshly prepared, these reductants provided the 100% reduction of nitrate ions to nitrite ions in the pH range from 3.4 to 9.5. A method was developed for prolonging the reducing properties of reductants based on metallic cadmium by permanently copper-plating these reductants. The relationship between the degree of reduction of nitrate ions in a cadmium reductor and the concentration of Cd²⁺ ions in the solution at the outlet of the reductor was determined.     

Silver nanoparticle assemblies supported on glassy-carbon electrodes for the electro-analytical detection of hydrogen peroxide

Electrochemical detection of hydrogen peroxide using an edge-plane pyrolytic-graphite electrode (EPPG), a glassy carbon (GC) electrode, and a silver nanoparticle-modified GC electrode is reported. It is shown, in phosphate buffer (0.05 mol L^–1, pH 7.4), that hydrogen peroxide cannot be detected directly on either the EPPG or GC electrodes. However, reduction can be facilitated by modification of the glassy-carbon surface with nanosized silver assemblies. The optimum conditions for modification of the GC electrode with silver nanoparticles were found to be deposition for 1 min at –0.5 V vs. Ag from 5 mmol L^–1 AgNO₃/0.1 mol L^–1 TBAP/MeCN, followed by stripping for 2 min at +0.5 V vs. Ag in the same solution. A wave, due to the reduction of hydrogen peroxide on the silver nanoparticles is observed at –0.68 V vs. SCE. The limit of detection for this modified nanosilver electrode was 2.0×10^–6 mol L^–1 for hydrogen peroxide in phosphate buffer (0.05 mol L^–1, pH 7.4) with a sensitivity which is five times higher than that observed at a silver macro-electrode. Also observed is a shoulder on the voltammetric wave corresponding to the reduction of oxygen, which is produced by silver-catalysed chemical decomposition of hydrogen peroxide to water and oxygen then oxygen reduction at the surface of the glassy-carbon electrode.     

Fe(III) complexes of 1,4,8,11-tetraaza[14]annulenes as catalase mimics

The development of enzyme mimics of catalase which decompose hydrogen peroxide to water and molecular oxygen according to the 2:1 stoichiometry of native catalase and in aqueous solution at pH 7 and at micromolar concentrations of the enzyme model and hydrogen peroxide is reported. For this purpose, iron(III) complexes of 1,4,8,11-tetraaza[14]annulenes are prepared by various procedures. Efficacious preparations utilize reaction of the [N4] macrocyles with FeII salts in the presence of triphenylamine, followed by gentle oxidation of the FeII complexes by molecular oxygen or by tris(4-bromophenyl)aminium hexachloroantimonate. The complexes are characterized by SQUID magnetometry and by M?ssbauer, EPR, and UV/vis spectrometry. In the solid state, the iron(III) center of the catalytically active complexes exists in the intermediate (quartet, S = 3/2) spin state. Several of these complexes decompose hydrogen peroxide in aqueous buffer solution at pH 7.2 at room temperature with turnover numbers between 40 and 80. The apparent second-order rate constant for hydrogen peroxide decomposition is in the range of 1400-2400 M(-1) s(-1), about 3 orders of magnitude lower than the value for native catalase. Besides oxygen production, a non-oxygen releasing pathway of hydrogen peroxide decomposition is unveiled.     

Precipitation of calcium oxalate from homogeneous solution by cation release

Calcium oxalate can be precipitated from homogeneous solution by oxidation of the calcium/EDTA complex with hydrogen peroxide in boiling solution, in the presence of oxalate ion, at pH 6-8. The method gives large crystals and enables calcium to be determined in the presence of lead, which remains complexed.     

PHOTOSENSITIZATION BY ANTITUMORAGENTS–6. PRODUCTION OF SUPEROXIDE RADICAL AND HYDROGEN PEROXIDE DURING ILLUMINATION OF DIAMINOANTHRACENEDIONES IN THE PRESENCE OF NADH IN AQUEOUS SOLUTIONS: AN EPR STUDY

Abstract— Photosensitizing capabilities of anthracenedione anticancer agents to oxidize NADH in aqueous solutions have been studied by EPR and spin trapping techniques. It is demonstrated that 1,4-diamino substituted anthraquinones, like mitoxantrone and ametantrone, do not photosensitize NADH oxidation while 1,5- and l,8-bis[[(diethylamino)ethyl]amino]anthraquinones do, undergoing simultaneous one-electron reduction to their semiquinone radical forms upon illumination with visible light. In aerated aqueous solutions the reaction leads to the production of superoxide ion and hydrogen peroxide.     

Chromatography-free product separation in the Mitsunobu reaction

Mitsunobu products can be isolated pure without column chromatography by first washing a solution of the crude reaction mixture in dichloromethane with 15% aqueous hydrogen peroxide followed by aqueous sodium sulfite. A final filtration through silica gel secures the pure Mitsunobu product.