Oxidation pathways and kinetics of morphine in acidic and neutral media on the aluminum electrode covered by metallic palladium and modified by Prussian blue

This paper describes the use of an aluminum electrode covered by metallic palladium and modified by Prussian blue prepared by a simple and rapid electroless method for the electro-oxidation of morphine. Two different pathways for electro-oxidation of morphine at various pH ranges were suggested. Also, some thermodynamic and kinetic parameters such as the number of electrons involved in the rate determining step, n _α , transfer coefficient α, and the total electrons (n) involved in morphine oxidation at the time scale of the cyclic voltammetric technique, the catalytic rate constant of the electrochemical process k, and diffusion coefficient of morphine D were determined. The mean values obtained are 0.5, 0.5, 1, 26.8 M^-1 s^-1 and 3.1 × 10⁻⁵ cm² s⁻¹, respectively.     

Electrocatalytic reduction of hydrogen peroxide at Prussian blue modified electrodes: a RDE study

Electrocatalytic reduction of hydrogen peroxide at Prussian blue modified electrode has been studied with rotating disk electrode in pH 5.5 and 7.3 solutions. It has been shown that the electrocatalytic cathodic reduction obeys Koutecky–Levich relationship at electrode potentials ranging from 0.1 to −0.4 V vs. Ag/AgCl for low concentrations of peroxide not exceeding 0.3 mM. Within this potential window, the calculated kinetic cathodic current ranges within the limits of 2.15–6.09 and 1.00–3.60 mA cm⁻² mM⁻¹ for pH 5.5 and 7.3, respectively. For pH 5.5 and 7.3 solutions, a linear slope of the dependence of kinetic current on electrode potential of −10.8 and −2.89 mA cm⁻² mM⁻¹ V⁻¹, respectively, has been obtained. At a higher concentration of peroxide, exceeding 0.6 mM, deviations from Koutecky–Levich relationship have been observed. These deviations appear more expressed at higher potentials and higher solution pH. The results obtained have been interpreted within the frame of two-step reaction mechanism, including (1) dissociative adsorption of hydrogen peroxide with the formation of OH radicals and (2) one-electron reduction of these radicals to OH⁻ anions. At a higher concentration of peroxide, and especially at a higher pH, the second process becomes rate limiting.     

Tetracyanoquinodimethanide adsorbed on a silica gel modified with titanium oxide for electrocatalytic oxidation of hydrazine

The electrocatalytical oxidation of hydrazine at low potential using tetracyanoquinodimethanide adsorbed on silica modified with titanium oxide was investigated by cyclic voltammetry and amperometry. The modified electrode was prepared modifying a carbon paste electrode employing lithium tetracyanoquinodimethanide adsorbed onto silica gel modified with titanium oxide. This electrode showed an excellent catalytic activity and stability for hydrazine oxidation. With this modified electrode, the oxidation potential of hydrazine was shifted toward less positive value, presenting a peak current much higher than those observed on a bare GC electrode. The linear response range, sensitivity and detection limit were, respectively, 2 up to 100 μmol l⁻¹, 0.36 μA l μmol⁻¹, and 0.60 μmol l⁻¹. The repeatability of the modified electrode evaluated in term of relative standard deviation was 4.2% for 10 measurements of 100 μmol l⁻¹ hydrazine solution. The number of electrons involved in hydrazine oxidation (4), the heterogenous electron transfer rate constant (1.08 × 10³ mol⁻¹ l s⁻¹), and diffusion coefficient (5.9 × 10⁻⁶ cm² s⁻¹) were evaluated with a rotating disk electrode.     

Caffeic acid modified glassy carbon electrode for electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH)

A new modified electrode was prepared by electrodeposition of caffeic acid (CFA) at the surface of an activated glassy carbon electrode. Cyclic voltammetry was used to investigate the redox properties of this electrode at various solution pH values and at various scan rates. The pH dependence of the electrode response was found to be 58.5 mV/pH, which is very close to the expected Nernstian value. The electrode was also employed to study electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH), using cyclic voltammetry, chronoamperometry and rotating disk voltammetry as diagnostic techniques. It was found that the modified electrode exhibits potent and persistent electrocatalytic properties toward NADH oxidation in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of about 450 mV compared to the process at an unmodified electrode. The electrocatalytic current increases linearly with NADH concentration in the range tested from 0.05 to 1.0 mM. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized CFA were calculated as 11.2 s⁻¹ and 0.43, respectively. The heterogeneous rate constant for oxidation of NADH at the CFA-modified electrode surface was also determined and found to be about 3 × 10³ M⁻¹ s⁻¹. Finally, the diffusion coefficient of NADH was calculated as 3.24 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometric results. Received: 6 January 1999 / Accepted: 11 May 1999     

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     

Evaluation of antioxidants using oxidation reaction rate constants

An evaluation method for the capacity of antioxidants to protect drugs against oxidation is presented. As a new viewpoint, to determine the priority of the competitive oxidations between the antioxidant and the protected drug, and to compare the drug-protection capacity of antioxidants, it is important to determine their oxidation rate constants using chemical kinetics instead of standard oxidation (or reduction) potentials. Sodium sulfite, sodium bisulfite and sodium pyrosulfite were used as models for the determination of oxidation reaction rate constants in aqueous solutions. In the experiments, sufficient air was continually infused into the solution to keep the concentration of dissolved oxygen constant. The residual concentrations of the antioxidants were determined by iodimetry, and the concentration of dissolved oxygen by oxygen electrode. The data were fitted by linear regressions to obtain the reaction rate constants. It was found that the degradation of sodium sulfite, sodium bisulfite or sodium pyrosulfite obeyed pseudo zero-order kinetics in the buffer solutions. Because of the ionization equilibrium, these three antioxidants have the same ion form in solutions at a definite pH value and therefore their apparent rate constants were essentially the same. The average apparent rate constants of the three antioxidants at 25°C are (1.34 ± 0.03) × 10⁻³ at pH 6.8, (1.20 ± 0.02) × 10⁻³ at pH 4.0 and (6.58 ± 0.02) × 10⁻³ mol·L⁻¹·h⁻¹ at pH 9.2, respectively. Translated from Acta Chimica Sinica, 2006, 64(6): 496–500 (in Chinese)     

Raman spectroscopic investigation of the antimalarial agent mefloquine

The antimalarial agent mefloquine was investigated using Fourier transform near-infrared (FT NIR) Raman and FT IR spectroscopy. The IR and Raman spectra were calculated with the help of density functional theory (DFT) and a very good agreement with the experimental spectra was achieved. These DFT calculations were applied to unambiguously assign the prominent features in the experimental vibrational spectra. The calculation of the potential energy distribution (PED) and the atomic displacements provide further valuable insight into the molecular vibrations. The most prominent NIR Raman bands at 1,363 cm⁻¹ and 1,434 cm⁻¹ are due to C=C stretching (in the quinoline part of mefloquine) and CH₂ wagging vibrations, while the most intense IR peaks at 1,314 cm⁻¹; 1,147 cm⁻¹; and 1,109 cm⁻¹ mainly consist of ring breathings and δCH (quinoline); C–F stretchings; and asymmetric ring breathings, C–O stretching as well as CH₂ twisting/rockings located at the piperidine moiety. Since the active agent (mefloquine) is usually present in very low concentrations within the biological samples, UV resonance Raman spectra of physiological solutions of mefloquine were recorded. By employing the detailed non-resonant mode assignment it was also possible to unambiguously identify the resonantly enhanced modes at 1,619 cm⁻¹, 1,603 cm⁻¹ and 1,586 cm⁻¹ in the UV Raman spectra as high symmetric C=C stretching vibrations in the quinoline part of mefloquine. These spectroscopic results are important for the interpretation of upcoming in vitro and in vivo mefloquine target interaction experiments.     

Voltammetric determination of <Emphasis Type="Italic">D</Emphasis>-penicillamine based on its homogeneous electrocatalytic oxidation with potassium iodide at the surface of glassy carbon electrode

The electrooxidation of D-penicillamine (D-PA) has been studied in the presence of potassium iodide in various buffered aqueous solutions (4.00 ≤ pH ≤ 9.00) at the surface of glassy carbon electrode using cyclic voltammetry, differential pulse voltammetry and chronoamperometry. It has been found that under optimum pH (pH 5.00) in cyclic voltammetry, the electrooxidation of D-PA in the presence of potassium iodide as a homogeneous mediator occurred at a potential about 220 mV less positive than that in absence of potassium iodide at the surface of glassy carbon electrode. The homogeneous electrocatalytic oxidation current wave of D-penicillamine was linearly dependent on the D-PA concentration and a linear calibration curve was obtained in the ranges 3.0 × 10⁻⁵−1.5 × 10⁻³ M and 9.0 × 10⁻⁶−1.2 × 10⁻⁴ M of D-PA with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods, respectively. The detection limits (2σ) were determined as 3.0 × 10⁻⁵ and 3.5 × 10⁻⁶ M with CV and DPV, respectively. This method was also used for voltammetric determination of D-PA in pharmaceutical preparation by standard addition method.     

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.     

Electrochemical investigations of the reaction mechanism and kinetics between NADH and riboflavin immobilised on amorphous zirconium phosphate

Electrochemical investigations of the reaction mechanism and kinetics between riboflavin immobilised on zirconium phosphate (ZPRib) in carbon paste and NADH showed results yielding reliable information about aspects on the mechanism of the electron transfer reaction between the flavin and NADH. The formal potential (E°′) of the adsorbed riboflavin was −220 mV versus SCE at pH 7.0. A shift about 250 mV towards a more positive potential compared with its value in solution was assigned to the interaction between the basic nitrogen of riboflavin and the acid groups of ZP. The invariance of the E°′ with the pH of the contacting solution and the effect of different buffer constituents were attributed to the protection effect of ZP over the riboflavin. The electrocatalytic oxidation of NADH at the electrode was investigated using cyclic voltammetry and rotating disk electrode methodology using a potential of −50 mV versus SCE. The heterogeneous electron transfer rate constant, k _obs, was 816 M⁻¹ s⁻¹ and the Michaelis-Menten constant, K _M, was 1.8 mM (confirming a charge transfer complex intermediate in the reaction) for an electrode with a riboflavin coverage of 6.8 × 10⁻¹⁰ mol cm⁻². This drastic increase in the reaction rate between NADH and the immobilised riboflavin was assigned to the shift of the E°′. A surprising effect with addition of calcium or magnesium ion to the solution was also observed. The E°′ was shifted to −150 mV versus SCE and the reaction rate for NADH oxidation increased drastically. Received: 22 February 1999 / Accepted: 10 March 1999     

Fabrication of TiO2/Ti nanotube electrode and the photoelectrochemical behaviors in NaCl solutions

Titanium oxide nanotube electrodes were successfully prepared by anodic oxidation on pure Ti sheets in 0.5 wt.% NH₄F + 1 wt.% (NH₄)₂SO₄ + 90 wt.% glycerol mixed solutions. Nanotubes with diameter 40–60 nm and length 7.4 μm were observed by field emission scanning electron microscope. The electrochemical and photoelectrochemical characteristics of TiO₂ nanotube electrode were investigated using linear polarization and electrochemical impedance spectroscopy techniques. The open-circuit potential dropped markedly under irradiation and with the increase of Cl⁻ concentrations. A saturated photocurrent of approximately 1.3 mA cm⁻² was observed under 10-W low-mercury lamp irradiation in 0.1 M NaCl solution, which was much higher than film electrode. Meanwhile, the highest photocurrent in NaCl solution implied that the photogenerated holes preferred to combine with Cl⁻. Thus, a significant synergetic effect on active chlorine production was observed in photoelectrocatalytic processes. Furthermore, the generation efficiency for active chlorine was about two times that using TiO₂/Ti film electrode by sol–gel method. Finally, the effects of initial pH and Cl⁻ concentration were also discussed.     

Electrochemical oxidation of salicylic acid at well-aligned multiwalled carbon nanotube electrode and its detection

In this paper, an electrochemical sensor for sensitive and convenient determination of salicylic acid (SA) was constructed using well-aligned multiwalled carbon nanotubes as electrode material. Compared to the glassy carbon electrode, the electro-oxidation of SA significantly enhanced at the multiwalled carbon nanotube (MWCNT) electrode. The MWCNT electrode shows a sensitivity of 59.25 μA mM⁻¹, a low detection limit of 0.8 × 10⁻⁶ M and a good response linear range with SA concentration from 2.0 × 10⁻⁶ to 3.0 × 10⁻³ M. In addition, acetylsalicylic acid was determined indirectly after hydrolysis to SA and acetic acid, which simplified the detection process. The mechanism of electrochemical oxidation of SA at the MWCNT electrode is also discussed.     

Electrochemical oxidation of methylthiomethyleneisoquinolinium chloride — the first water soluble alkylthiomethylene substituted ammonium salt

Electrochemical oxidation of methylthiomethyleneisoquinolinium chloride (MTMIQ), the first alkylthiomethyl substituted ammonium salt, which is fully miscible with water has been investigated by voltammetric (SWV) method using glassy carbon electrode. On the electrode, MTMIQ undergoes oxidation at the potential near Ep = 0.07V (vs. Ag/AgCl/3 M KCl). The influence of the pH of buffers, amplitude, frequency, step potential on the received signal was studied. The best results were obtained with a citrate buffer at a pH of 5. The oxidation peak current used for MTMIQ voltammetric determination was in the range of 2–8×10⁻⁵ mol L⁻¹, LOD = 3.7×10⁻⁶, LOQ = 1.2×10⁻⁵. The product of the oxidation was accumulated at the working electrode and was investigated by spectroscopic method. Mechanistic pathways of the oxidation have been proposed.      

Specific adsorption of iodide-ion at liquid (Ga-In)-electrode of eutectic composition from aqueous solutions with constant ionic strength

The adsorption behavior of iodide-ion at liquid (Ga-In)-electrode in aqueous electrolyte solutions at 305 K is studied by the electrochemical impedance spectroscopy, differential capacitance, and cyclic voltammetry. The equivalent circuit describing the experimental data in the presence and in the absence of the I⁻ ion is a series connection of a resistance and a capacitance that is frequency-independent over a ∼500 Hz to 100 kHz range. The experimental data were obtained by the mixed electrolyte method in electrolyte solutions acidified with HClO₄ down to pH 3, with excess of surface-inactive ion ClO₄⁻ and constant ionic strength (0.1 M). The analysis resulted in the determination of the charge σ₁ of the I⁻ ion specifically adsorbed at the liquid (Ga-In)-electrode at the adsorbate maximal concentration: σ_I = 7.73 μC/cm² in the case of analysis at σ = const (at the zero charge potential) or σ_I = 7.50 μC/cm² in the case of analysis at E = const. These values are characteristic of rather strong specific adsorption. The values of σ_I of the studied anion were used in the calculations of different isotherms with the purpose of the adsorption parameters determination. The obtained results were compared with literature data determined on other metals in the presence of specifically adsorbable I⁻ ion.     

Chronoamperometric study of proton transfer/electron transfer in solid state electrochemistry of organic dyes

Chronoamperometric data for proton-assisted reduction/oxidation of a series of dyes, namely, alizarin, purpurin, luteolin, morin, and indigo are compared with the predictions of the model of Lovric and Scholz for redox conductive microcrystals assuming electron and proton hopping in mutually perpendicular directions. Upon the attachment of solid dye microparticles to paraffin-impregnated graphite electrodes in contact with aqueous electrolytes in the pH range between 4 and 7, an excellent agreement between theory and experimental data was obtained. The diffusion coefficients of electrons and protons across dye microcrystals were estimated as 5×10⁻⁷ cm² s⁻¹ and 1×10⁻¹⁰ cm² s⁻¹, respectively.     

Neutral thallium clusters in aqueous solutions. Pulse radiolysis study

The formation of neutral Tl₂ (λ_max = 390 nm) and Tl₄ (λ_max = 360 nm) clusters in dilute aqueous solutions of Tl₂SO₄ containing formate ions was found by pulse radiolysis. The rate constants for the recombination of Tl⁰ atoms and Tl₂ clusters are equal to 1.5·10¹⁰ L mol⁻¹ s⁻¹ and 1.0·10¹⁰Lmol⁻¹ s⁻¹ (±30%), respectively, and the extinction coefficient of Tl₂ at 390 nm is −6.0·10³ L mol⁻¹ cm⁻¹ Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2367–2369, December, 1999.     

Quantitative analysis of methyl green using surface-enhanced resonance Raman scattering

Surface-enhanced resonance Raman scattering (SERRS) spectra of aqueous solutions of the triphenylmethane dye methyl green have been obtained for the first time by use of citrate-reduced silver colloids and a laser excitation wavelength of 632.8 nm. Given the highly fluorescent nature of the analyte, which precluded collection of normal Raman spectra of the dye in solution and powdered state, it was highly encouraging that SERRS spectra showed no fluorescence due to quenching by the silver sol. The pH conditions for SERRS were optimised over the pH range 0.5–10 and the biggest enhancement for SERRS of this charged dye was found to be at pH 2.02, thus this condition was used for quantitative analysis. SERRS was found to be highly sensitive and enabled quantitative determination of methyl green over the range 10⁻⁹ to 10⁻⁷ mol dm⁻³. Good fits to correlation coefficients were obtained over this range using the areas under the vibrational bands at 1615 and 737 cm⁻¹. Finally, a limit of detection of 83 ppb was calculated, demonstrating the sensitivity of the technique.     

Synthesis,characterization and thermal stability of complex <Emphasis Type="Italic">cis</Emphasis>-[Co(bipy)<Subscript>2</Subscript>(CN)<Subscript>2</Subscript>] and its interaction with NO<Subscript>2</Subscript> gas

cis-[Co(bipy)₂(CN)₂] complex was successfully prepared from the reaction of [Co(CN)₂] with 2,2′-bipyridine in ethanol 96%. The product of complex synthesized was light brown crystal with percent yield of 75.86%. The IR spectrum of this complex showed strong characteristics vibration band at 1471.9 and 1442.75 cm⁻¹ for C=N stretching, 1664.57 cm⁻¹ for C=C aromatic, 1600.9 cm⁻¹ 1 for HC=CH aromatic, 653.87 cm⁻¹ for pyridine ring on 2,2′-bipyridine ligand, 3109.25 and 3080.32 cm⁻¹ for C-H of aromatic ring. The thermogram of DTA-TG analysis showed 4 endothermic peaks at temperature of 53.4, 323.2, 444.2, and 526.4°C, where the bond breaking of the complex occurred at different temperatures. The result of moment magnet measurement of the complex showed that it was a high spin complex with moment magnet value of 3.68 BM, so it is a paramagnetic complex.     

The effect of heat-treatment on the physico-chemical properties of silica aerogel prepared by sub-critical drying technique

Synthesis of transparent and crack-free monoliths of silica aerogel by sub-critical drying technique is reported in the present article. Silane ageing with 50% tetraethylorthosilicate:ethanol followed by solvent exchange using ethanol was adopted. The effect of heat-treatment on the textural and physical characteristics of silica aerogel was evaluated. The chosen composition resulted in a high surface area silica aerogel of 1,000 m² g⁻¹ and a pore volume of 1.4 cm³ g⁻¹ at room temperature. The aerogel heat-treated at 900 °C possessed a surface area of 450 m² g⁻¹ with a pore volume of 0.4 cm³ g⁻¹. The decrease in surface area and pore volume was associated with the sintering process. The present technique seems advantageous in preserving the high surface area of the material at high temperatures. The XRD studies showed that the amorphous nature of aerogel matrix was retained till 1,400 °C, beyond which it crystallized to phase pure crystoballite.