Photocatalytic water purification: Destruction of resorcinol by irradiated titania

Ethanedial, butanoic acid anhydride and trihydroxybenzenes have been determined as intermediates of resorcinol photocatalytic oxidation. The photoreaction rate shows a 1st order dependence on resorcinol concentration. The calculated reaction rate constant is 2.1×10⁻⁸ mol dm⁻³ s⁻¹, which is similar to the rate constant of phenol photocatalytic oxidation on TiO₂ reported beore.     

Antioxidant activity of chalcones: The chemiluminescence determination of the reactivity and the quantum chemical calculation of the energies and structures of reagents and intermediates

Six antioxidants from the class of chalcones (ArOH), compounds from which flavonoids are obtained in nature, were studied. The antiradical activity of chalcones and a number of related compounds was determined by a chemiluminescence method using the scavenging of peroxide radicals ROO^· + ArOH → ROOH + OAr^· (with the rate constant k ₇) in a model reaction of diphenylmethane (RH) oxidation. The structures and energies of the reagents and intermediates were determined by semiempirical quantum chemical (PM3, PM6) calculations. 3,4-Dihydroxychalcone and caffeic acid, which have a catechol structure, that is, two neighboring OH groups in phenyl ring B, exhibited high antioxidant activity (k ₇ ≈ 10⁷ l mol⁻¹ s⁻¹); this is consistent with the lowest bond strengths D(ArO-H) of 79.2 and 76.6 kcal/mol, respectively. The abstraction of a hydrogen atom by the ROO^· radical is the main reaction path of these compounds; however, the low stoichiometric coefficients of inhibition (f = 0.3–0.7) suggest a contribution of secondary and/or side reactions of ArOH and OAr^·. In the other chalcones, the ArO-H bond is stronger (D(ArO-H) = 83–88 kcal/mol) and the antioxidant activity is lower (k ₇ = 10⁴–10⁵ l mol⁻¹ s⁻¹).     

Flow-injection analysis for the determination of total inorganic carbon and total organic carbon in water using the H2O2–luminol–uranine chemiluminescent reaction

In the presence of carbonate and uranine, the chemiluminescent intensity from the reaction of luminol with hydrogen peroxide was dramatically enhanced in a basic medium. Based on this fact and coupled with the technique of flow-injection analysis, a highly sensitive method was developed for the determination of carbonate with a wide linear range. The method provided the determination of carbonate with a wide linear range of 1.0 × 10⁻¹⁰–5.0 × 10⁻⁶ mol L⁻¹ and a low detection limit (S/N = 3) of carbonate of 1.2 × 10⁻¹¹ mol L⁻¹. The average relative standard deviation for 1.0 × 10⁻⁹–9.0 × 10⁻⁷ mol L⁻¹ of carbonate was 3.7% (n = 11). Combined with the wet oxidation of potassium persulfate, the method was applied to the simultaneous determination of total inorganic carbon (TIC) and total organic carbon (TOC) in water. The linear ranges for TIC and TOC were 1.2 × 10⁻⁶–6.0 × 10⁻² mg L⁻¹ and 0.08–30 mg L⁻¹ carbon, respectively. Recoveries of 97.4–106.4% for TIC and 96.0–98.5% for TOC were obtained by adding 5 or 50 mg L⁻¹ of carbon to the water samples. The relative standard deviations (RSDs) were 2.6–4.8% for TIC and 4.6–6.6% for TOC (n = 5). The mechanism of the chemiluminescent reaction was also explored and a reasonable explanation about chemical energy transfer from luminol to uranine was proposed. Figure Chemiluminescence profiles in batch system. 1, Injection of 100 μL of K₂CO₃ into 1.0 mL luminol-1.0 mL H₂O₂ solution; 2-3 and 4-5, Injection in sequence of 100 μL of K₂CO₃ and 100 μL of uranine into 1.0 ml luminol-1.0 mL H₂O₂ solution; C_luminol = 1.0 × 10⁻⁷ mol/L, C_H2O2 = 1.0 × 10⁻⁵ mol/L, C_uranine = 1.0 × 10⁻⁵ mol/L, C_K2CO3 = 1.0 × 10⁻⁷ mol/L except for 4-5 where C_K2CO3 = 1.0 × 10⁻⁴ mol/L     

Sorption-Catalytic Determination of Imazapyr on a Copper-Containing Sorbent

 Sorption of copper on filter-paper with chemically attached hexamethylenediamino-groups (HMDA-filter) allows to obtain the sorbent (Cu/HMDA-filter) stable in respect to desorption of copper. A nitrogen-containing herbicide imazapyr (imaz) is retained on Cu/HMDA-filters at pH 5.5–7.0 forming a relatively stable complex. Imazapyr is determined directly on the sorbent by its activating effect in the oxidation of hydroquinone with H₂O₂ catalyzed by Cu(II) with the formation of a product absorbing at 490 nm. The copper ions serve both to preconcentrate imazapyr and to catalyze the indicator reaction. The use of 1-μL sample aliquots pipetted onto the Cu/HMDA-filters allows to determine 1 × 10⁻³–0.03 μmol of imazapyr, whereas preconcentration of the analyte by pumping of its solution through the same sorbent expands the linear range to 1 × 10⁻⁴–1 × 10⁻¹ μmol of imazapyr. When the indicator reaction is carried out in solution, the range of activating action of imazapyr is narrower (0.06–0.1 μmol a for a solution volume of 10 mL). The determination is selective: 5–100-fold amounts of amines, aminoacids, carboxylic acid derivatives and other model compounds do not interfere. Soil extracts and carrot juice samples spiked with imazapyr have been analyzed. Received January 10, 2000. Revision July 28, 2000.     

Another Approach to the Modified Zhuravlev Equation on the Basis of the Oxidation of V2O4 and V6O13

It has been found that the modified Zhuravlev equation, [(1−α)^−1/3−1]²=ktⁿ, which describes the kinetics of oxidation of V₂O₄ and V₆O₁₃ in the temperature range 820–900 K and in the oxygen pressure range 1.0–20 kPa, can be derived via the assumption that the changes in the observed activation energy result from the changing contributions of the two diffusion processes controlling the reaction rate. The values of the observed activation energy are in the range 160–175 kJ mol⁻¹ for V₂O₄ and 188–201 kJ mol⁻¹ for V₆O₁₃ in the scope of the experimental oxygen pressures and temperatures and conversion degrees of 0.1–0.9. This revised version was published online in August 2006 with corrections to the Cover Date.     

Oxygen ionic and electronic transport in Gd<Subscript>2−x</Subscript>Ca<Subscript>x</Subscript>Ti<Subscript>2</Subscript>O<Subscript>7</Subscript><Subscript>−δ</Subscript> pyrochlores

Oxygen ion transference numbers for Gd_2−xCa_xTi₂O_{7 −δ} (x=0.10–0.14) pyrochlore ceramics were determined at 973–1223 K by the modified e.m.f. and faradaic efficiency techniques, taking into account electrode polarization, and from the results on oxygen permeation. The ion transference numbers vary in the range 0.95–0.98 in air, increasing when the temperature or oxygen partial pressure decreases. The activation energies for the ionic and p-type electronic transport in air are 74–77 and 87–91 kJ/mol, respectively. The p-type conductivity and oxygen permeability of Gd₂Ti₂O₇-based pyrochlores can be adequately described by relationships common for other solid electrolytes. At temperatures below 1273 K under a gradient of 10%H₂+90%N₂/air, average ion transference numbers for doped gadolinium titanate are not less than 0.97. Thermal expansion coefficients for Gd_2−xCa_xTi₂O_{7 −δ} ceramics, calculated from dilatometric data in air, are in the range (10.4–10.6)×10⁻⁶ K⁻¹ at 400–1300 K.     

Kinetics and mechanism of oxidation of l-Cysteine by Corey’s reagent

The kinetics of oxidation of l-Cysteine by pyridinium chlorochromate (PCC) was studied at 0.1–0.3 mol dm⁻³ HClO₄ in the range 25–40 °C. The reaction exhibits first order dependence with respect to PCC and fractional order in cysteine. The increase in the oxidation rate with acidity suggests the involvement of a protonated chromium(VI) species in the rate-determining step. Cysteic acid is identified as the product of oxidation. A suitable mechanism involving the formation of a complex is proposed. The activation parameters of the rate-determining step are computed using the linear least squares method and the values of E _a and ΔS ^# are found to be 46.0 ± 2.0 KJ mol⁻¹ and −38.0 ± 3.2 JK⁻¹ mol⁻¹ respectively.     

Interrelation between the chemical structure and antioxidant properties of N-substituted amides of salicylic acid

The kinetics of the initiated oxidation of a model lipid (methyl oleate has been investigated in the presence of a group of new “hybrid” structures, namely, N-substituted amides of salicylic acid whose structure contains an amide residue conjugated with, or separated by a bridging fragment (three methylene groups) from, an N-phenolic substituent. The compounds also differ in the degree of screening of the OH groups. The process was initiated by thermal decomposition of azobisisobutyronitrile at 60°C (initiation rate of w _i = 4.2 × 10⁻⁸ mol L⁻¹ s⁻¹) or by UV irradiation (γ = 313−365 nm, w _i = 0.6 × 10⁻⁸ mol L⁻¹ s⁻¹). The compounds examined exhibit antiradical activity owing to the presence of the phenolic hydroxyl groups. N-substituted salicylamides efficiently inhibit the overall methyl oleate oxidation process and are comparable in activity with dibunolum and α-tocopherol or are superior to them. The structures in which the residues of salicylamide and sterically hindered phenol are separated by the bridging fragment are particularly efficient. The advantages of the salicylamides absorbing at 300–365 nm manifest themselves in UV-initiated oxidation. The peroxidase activity of the N-substituted salicyl acid derivatives is determined by the structure of the amide moiety. The compounds examined here are new, promising, effective antioxidants, whose particular structural fragments act via different mechanisms in oxidation.     

Features of the initiated oxidation mechanism of n-heptadecane at low concentrations of oxygen

The oxidation of n-heptadecane was studied at various partial pressures of oxygen in an oxygen-argon mixture from 100 to 10% and various initiation rates W _i in the range (1–5)·10⁻⁶ mol L⁻¹ s⁻¹ at 413 K. The kinetic curves of oxygen uptake and hydroperoxide buildup were obtained under the indicated conditions. The observed features of n-heptadecane oxidation at low concentrations of oxygen may qualitatively be explained and quantitatively described if the oxidation scheme takes into account the cross termination of alkyl and peroxyl radicals R · + RO₂ · \underrightarrow k₅ \underrightarrow {k_5 } ROOR along with the square termination of peroxyl radicals. A method for determination of the corresponding kinetic parameter by the dependence of the initial oxidation rate on the partial oxygen pressure was proposed. A method for identification of the key reactions and determination of the kinetic parameters by the kinetics of oxygen uptake at lowered oxygen concentrations was developed. The kinetic model of the process was obtained, which quantitatively describes the kinetic curves of oxygen uptake and hydroperoxide buildup at the initial steps of initiated oxidation of n-heptadecane.     

Electrocatalytic oxidation of pyridoxine (vitamin B<Subscript>6</Subscript>) on aluminum electrode modified by metallic palladium particles/iron (III) hexacyanoferrate (II) film

The electrocatalytic activity of a Prussian blue (PB) film on the aluminum electrode by taking advantage of the metallic palladium characteristic as an electron-transfer bridge (PB/Pd–Al) for electrooxidation of 2-methyl-3-hydroxy-4,5-bis (hydroxyl–methyl) pyridine (pyridoxine) is described. The catalytic activity of PB was explored in terms of Fe^III [Fe^III (CN)₆]/Fe^III [Fe^II (CN)₆]¹⁻ system. The best mediated oxidation of pyridoxine (PN) on the PB/Pd–Al-modified electrode was achieved in 0.5 M KNO₃ + 0.2 M potassium acetate of pH 6 at scan rate of 20 mV s⁻¹. The mechanism and kinetics of the catalytic oxidation reaction of PN were monitored by cyclic voltammetry and chronoamperometry. The results were explained using the theory of electrocatalytic reactions at chemically modified electrodes. The charge transfer-rate limiting reaction step is found to be a one-electron abstraction, whereas a two-electron charge transfer reaction is the overall oxidation reaction of PN by forming pyridoxal. The value of α, k, and D are 0.5, 1.2 × 10² M⁻¹ s⁻¹, and 1.4 × 10⁻⁵ cm² s⁻¹, respectively. Further examination of the modified electrodes shows that the modifying layers (PB) on the Pd–Al substrate have reproducible behavior and a high level of stability after posing it in the electrolyte or Pyridoxine solutions for a long time.     

Determination of rutin using a CeO2 nanoparticle-modified electrode

CeO₂ nanoparticles approximately 12 nm in size were synthesized and subsequently characterized by XRD, TEM and UV-vis spectroscopy. Then, a gold electrode modified with CeO₂ nanoparticles was constructed and characterized by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The modified electrode demonstrated strong catalytic effects with high stability towards electrochemical oxidation of rutin. The anodic peak currents (measured by differential pulse voltammetry) increased linearly with the concentration of rutin in the range of 5.0 × 10⁻⁷–5.0 × 10⁻⁴ mol · L⁻¹. The detection limit (S/N = 3) was 2.0 × 10⁻⁷ mol · L⁻¹. The relative standard deviation (RSD) of 8 successive scans was 3.7% for 5.0 × 10⁻⁶ mol · L⁻¹ rutin. The method showed excellent sensitivity and stability, and the determination of rutin in tablets was satisfactory.     

Determination of human serum albumin using aurothiomalate as electroactive label

A new electroactive label has been used to monitor immunoassays in the determination of human serum albumin (HSA) using glassy-carbon electrodes as supports for the immunological reactions. The label was a gold(I) complex, sodium aurothiomalate, which was bound to rabbit IgG anti-human serum albumin (anti-HSA-Au). The HSA was adsorbed on the electrode surface and the immunological reaction with gold-labelled anti-HSA was then performed for one hour by non-competitive or competitive procedures. The gold(I) bound to the anti-HSA was electrodeposited in 0.1 mol L⁻¹ HCl at −1.00 V for 5 min then oxidised in 0.1 mol L⁻¹ H₂SO₄ solution at +1.40 V for 1 min. Silver electrodeposition at −0.14 V for 1 min followed by anodic stripping voltammetry were then performed in aqueous 1.0 mol L⁻¹ NH₃–2.0×10⁻⁴ mol L⁻¹ AgNO₃. For both non-competitive and competitive formats, calibration plots in the ranges 5.0×10⁻¹⁰ to 1.0×10⁻⁸ mol L⁻¹ and 1.0×10⁻¹⁰ to 1.0×10⁻⁹ mol L⁻¹ HSA, respectively, with estimated detection limits of 1.5×10⁻¹⁰ mol L⁻¹ (10 ng mL⁻¹) and 1.0×10⁻¹⁰ mol L⁻¹ (7 ng mL⁻¹), respectively, were obtained. Levels of HSA in two healthy volunteer urine samples were also evaluated, using both immunoassay formats.     

Preconcentration and Voltammetric Determination of the Antihypertensive Doxazosin on a C8 Modified Carbon Paste Electrode

 An electrochemical study of the doxazosin oxidative process at carbon paste electrodes using different voltammetric techniques has been carried out. The process is irreversible and controlled by adsorption, giving rise to an oxidation wave around 1.0 V in citric acid-citrate buffer (pH 3.0). A mechanism based on the oxidation of the amine group is postulated. Two methods based on adsorptive stripping (AdS) of doxazosin at the C₈-modified carbon paste electrode (C₈-MCPE), before its voltammetric determination, are studied, using differential pulse voltammetry (DPV) and square wave voltammetry (SWV) as redissolution techniques. By means of AdS-DPV and C₈-MCPE, doxazosin can be determined over the 1.0 × 10⁻⁹ to 3.0 × 10⁻⁸ mol L⁻¹ range with a variation coefficient of 2.2% (2.0 × 10⁻⁸ mol L⁻¹) and a limit of detection of 7.4 ×10⁻¹⁰ mol L⁻¹. If AdS-SWV is used, a linear range from 1.0 × 10⁻⁹ to 4.0 × 10⁻⁸ mol L⁻¹ is obtained, the variation coefficient being 2.8% (2.0 × 10⁻⁸ mol L⁻¹, and the limit of detection reached 7.7 × 10⁻¹⁰ mol L⁻¹. The AdS-DPV procedure was applied to the determination of doxazosin in urine and formulations. Received March 13, 1999. Revision December 23, 1999.     

A novel kinetic-spectrophotometric method for determination of nitrites in water

A simple, selective and sensitive kinetic method for the determination of nitrite in water was developed. The method is based on the catalytic effect of nitrite on the oxidation of methylene blue (MB) with bromate in a sulfuric acid medium. During the oxidation process, absorbance of the reaction mixture decreases with the increasing time, inversely proportional to the nitrite concentration. The reaction rate was monitored spectrophotometrically at λ = 666 nm within 30 s of mixing. Linear calibration graph was obtained in the range of 0.005–0.5 μg mL⁻¹ with a relative standard deviation of 2.09 % for six measurements at 0.5 μg mL⁻¹. The detection limit was found to be 0.0015 μg mL⁻¹. The effect of different factors such as acidity, time, bromate concentration, MB concentration, ionic strength, and order of reactants additions is reported. Interference of the most common foreign ions was also investigated. The optimum experimental conditions were: 0.38 mol L⁻¹ H₂SO₄, 5 × 10.4 mol L⁻¹ KBrO₃, 1.25 × 10.5 mol L⁻¹ MB, 0.3 mol L⁻¹ sodium nitrate, and 25°C. The proposed method was conveniently applied for the determination of nitrite in spiked drinking water samples.     

Study on mechanism for oxidation of <Emphasis Type="Italic">N,N</Emphasis>-dimethylhydroxylamine by nitrous acid

The oxidation of N,N-dimethylhydroxylamine (DMHAN) by nitrous acid is investigated in perchloric acid and nitric acid medium, respectively. The effects of H⁺, DMHAN, ionic strength and temperature on the reaction are studied. The rate equation in perchloric acid medium has been determined to be −d[HNO₂]/dt = k[DMHAN][HNO₂], where k = 12.8 ± 1.0 (mol/L)⁻¹ min⁻¹ when the temperature is 18.5 °C and the ionic strength is 0.73 mol/L with an activation energy about 41.5 kJ mol⁻¹. The reaction becomes complicated when it is performed in nitric acid medium. When the molarity of HNO₃ is higher than 1.0 mol/L, nitrous acid will be produced via the reaction between nitric acid and DMHAN. The reaction products are analyzed and the reaction mechanism is discussed in this paper.     

The kinetics of the oxidation of cyclopentanone with decaneperoxysulfonic acid

The kinetics of the oxidation of cyclopentanone with decaneperoxysulfonic acid at 291–323 K in CCl₄ has been studied. The reaction is not autocatalytic, and its rate increases linearly with increases in the concentrations of each of the reagents. The addition of CF₃COOH does not affect the reaction rate. The observed results are explained within a scheme which is a special case of the well-known Baeyer-Villiger reaction mechanism established for peroxycarboxylic acids. The effective rate constant of the process has been determined: logk (L mol⁻¹ s⁻¹)=(7.6±1.7) — (42.1±9.6)/θ, where θ=2.30RT kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1827–1829, October, 1993.     

Catecholase biomimetic catalytic activity of copper(II) complexes with 2-methyl-3-amino-(3H)-quinazolin-4-one

The oxidation of catechol by molecular oxygen in the presence of a catalytic amount of copper(II) complex with 2-methyl-3-amino-(3H)quinazoline-4-one (MAQ) and various anions (Cl⁻, Br⁻, ClO ₄ ⁻ , SCN⁻, NO ₃ ⁻ and SO ₄ ^– ) was studied. The catecholase biomimetic catalytic activity of the copper(II) complexes has been determined spectrophotometrically by monitoring the oxidative transformation of catechol to the corresponding light absorbing o-quinone (Q). The rate of the catalytic oxidation reaction was investigated and correlated with the catalyst structure, time, concentration of catalyst and substrate and finally solvent effects. Addition of pyridine or Et₃N showed a dramatic effect on the rate of oxidation reaction. Kinetic investigations demonstrate that the rate of oxidation reaction has a first order dependence with respect to the catalyst and catechol concentration and obeying Michaelis–Menten Kinetics. It was shown that the catalytic activity depends on the coordination environment of the catalyst created by the nature of counter anions bound to copper(II) ion in the complex molecule and follows the order: Cl⁻ > NO ₃ ⁻ > Br⁻ > SO ₄ ^– > SCN⁻ > ClO ₄ ⁻ . To further elucidate the catalytic activity of the complexes, their electrochemical properties were investigated and the catecholase mimetic activity has been correlated with the redox potential of the Cu²⁺/Cu⁺ couple in the complexes.     

Microstructure of the LaSrCuO<Subscript>4 − δ</Subscript>|Ce<Subscript>0.9</Subscript>Gd<Subscript>0.1</Subscript>O<Subscript>2 − δ</Subscript> interface and its reversibility with respect to oxygen

The conditions of formation of electrode/electrolyte interfaces LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ} are optimized. It is shown that electrode layers formed by the screen printing method have better developed surfaces and are more uniform and strong as compared with thick film layers applied by a brush. Symmetric LaSrCuO_{4 − δ}|Ce_0.9Gd_0.1O_{2 − δ}|LaSrCuO_{4 − δ} cells with porous electrodes are studied by impedance spectroscopy and cyclic voltammetry in the temperature range of 773–1173 K at the oxygen partial pressure of (28–2.1) × 10⁴ Pa. The oxygen process is shown to be limited by the charge transfer across the electrode/electrolyte interface. The exchange currents are calculated in the temperature range of 773–1173 K to amount from 1 × 10⁻³ to 3.5 × 10⁻² A/cm², which points to the high reversibility of the electrode/electrolyte interface with respect to oxygen.     

Electrochemistry and electrocatalytic activity of polypyrrole/ferrocyanide films on a glassy carbon electrode

Functionalized polypyrrole films were prepared by incorporation of Fe(CN)₆ ³⁻ as doping anion during the electropolymerization of pyrrole at a glassy carbon electrode from aqueous solution. The electrochemical behavior of the Fe(CN)₆ ³⁻/Fe(CN)₆ ⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry. An obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole film with ferrocyanide incorporated was demonstrated by oxidation of ascorbic acid at the optimized pH of 4 in a glycine buffer. The catalytic effect for mediated oxidation of ascorbic acid was 300 mV and the bimolecular rate constant determined for surface coverage of 4.5 × 10⁻⁸ M cm⁻² using rotating disk electrode voltammetry was 86 M⁻¹ s⁻¹. Furthermore, the catalytic oxidation current was linearly dependent on ascorbic acid concentration in the range 5 × 10⁻⁴–1.6 × 10⁻² M with a correlation coefficient of 0.996. The plot of i _p versus v ^1/2 confirms the diffusion nature of the peak current i _p. Received: 12 April 1999 / Accepted: 25 May 1999     

Formations of Active Species and By-Products in Water by Pulsed High-Voltage Discharge

Formations of active species and by-products are different from bubbling different gases in a pulsed high-voltage discharge reactor. The identification of all the products and the formation rate determination of active species are quite important as the process is applied to wastewater disposal. Serials of measurements were conducted to do the identifications and determinations in this paper. Amounts of · OH all increased but that of H₂O₂ all decreased by bubbling gas. The · OH formation rate was 3.49 × 10⁻⁷, 3.56 × 10⁻⁷, 3.21 × 10⁻⁷ and 1.94 × 10⁻⁷ mol l⁻¹ s⁻¹ with bubbling nitrogen, argon, air and oxygen respectively, but it was 1.61 × 10⁻⁷ mol s⁻¹ l⁻¹ without bubbling. Without any bubbling, the H₂O₂ formation rate was up to 6.53 × 10⁻⁶ mol l⁻¹ s⁻¹, while it was 9.97 × 10⁻⁷, 1.663 × 10⁻⁷, 1.73 × 10⁻⁶ and 3.14 × 10⁻⁶ mol l⁻¹ s⁻¹ with bubbling nitrogen, argon, air and oxygen, respectively. NO₂⁻ and NO₃⁻ was detected in discharged water with bubbling nitrogenous gas. Their formation made the pH decreased.