Electrochemical behavior and voltammetric determination of metol using a multiwall carbon nanotubes modified electrode

Electrochemical behavior of metol, which coexists with p-benzeneiol (HQ) at a glassy-carbon electrode modified with multiwall carbon nanotubes (MWNT/GCE), is studied in the thesis. The results indicate that metol yields a well-defined peak of which two concomitant reductive peaks separate and the potential separation reaches to 178 mV, and that concomitant HQ has almost no interference with the reduction signal of metol. The values of the reductive peak current (I _pc) are found to be linearly related to metol concentration over the range of 8.0 × 10⁻² −1.0 × 10⁻⁵ M, with a detection limit of 5.0 × 10⁻⁶ M. Some common matter has no interference with the determination of metol. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 1, pp. 31–35. The text was submitted by the authors in English.     

Silver(I)-selective membrane potentiometric sensor based on two recently synthesized ionophores containing calix[4]arene

Two new PVC membrane electrodes that are highly selective to Ag(I) ions were prepared using (L₁) calyx[4]arene (L₂) as two suitable neutral carriers. The silver(I) ion selective electrodes exhibit a good response for silver ion over a wide concentration range of 1.0 × 10⁻¹ to 4.2 × 10⁻⁶ M (L₁) and 1.0 × 10⁻¹ to 6.5 × 10⁻⁶ M (L₂) with a Nernstian slope of 60 mV per decade (L₁) and 58 mV per decade (L₂) at 25°C, and was found to be very selective, precise, and usable within the pH range 4.0–8.0. They have a response time of <15 s and can be used for at least 3 months without any measurable divergence in potential. The proposed sensors show a fairly good discriminating ability towards Ag⁺ ion in comparison to some hard and soft metal ions. The electrodes were used as indicator electrodes in the potentiometric titration of silver ion and in the determination of Ag⁺ in photographic emulsion and radiographic and photographic films. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 7, pp. 862–868. The article is published in the original.     

Voltammetric determination of sorbite at a mechanically renewed copper electrode

Potentials and currents of D-sorbitol oxidation peaks as a function of polarization conditions for a copper electrode in situ renewed by mechanically cutting a 0.5-μm surface layer are studied by direct-current cyclic voltammetry. Oxidation peaks of sorbite emerge in cyclic voltammograms recorded in alkaline supporting electrolytes (0.05–0.10 M KOH and NaOH solutions) upon scanning the potential to the anodic region (E _p = 0.50–0.58 V) and in the reverse direction (E _p = 0.60–0.62 V). The shape and parameters of these peaks depend on the concentration of KOH, because of the different copper oxides involved in the oxidation of sorbite formed at the electrode surface. The regeneration of the electrode surface is the necessary condition for good reproducibility of the peak parameters. The signals obtained on the surface of the unrenewed electrodes are almost halved and less reproducible. The calibration graph of the current of the sorbite oxidation peak as a function of its concentration is linear in the range from 5 × 10⁻⁴ to 1 × 10⁻² M.     

Kinetics of nickel electroplating from methanesulfonate electrolyte

The overpotential of nickel ion electroreduction on the nickel and mercury electrodes is shown to increase in the following sequence of anions: ClO₄⁻,CH₃SO₃⁻, SO₄²⁻. On the nickel electrode, the overpotential of nickel evolution decreases as the pH^v increases from 1.5 to 4. This is associated with the increase in pH^s as the result of a parallel reaction of hydrogen evolution. It is shown that in contrast to mercury, the Tafel plots of the nickel electrode demonstrate a bend corresponding to the change in their slope from −0.044 to −0.132 V. This is accompanied by the lowering down of the reaction order in nickel ions from 2 to 1. A mechanism of nickel ion electroreduction that includes two parallel routes is proposed and substantiated by a model. In the low overpotential range, the predominant process is the electroreduction of nickel hydroxocomplexes, which is characterized by the strong dependence of the reaction rate on the potential and the concentration of electroactive species. For high overpotentials, the predominant process is the direct discharge of nickel aquacomplexes the rate of which depends weaker on the potential and the concentration of electroactive species.     

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 synthesis of poly(<Emphasis Type="Italic">O</Emphasis>-tolidine): Electrochemical and spectroscopic characterization

Polymerization of (O-tolidine), as ring substituted derivative of benzidine was achieved electro-chemically in organic solution containing Bu₄NBF₄ (0.2 M) in dichloromethane as supporting electrolyte. The film was obtained by electropolymerization in solution containing monomer in various ratio ((5 × 10⁻⁴)–(5 × 10⁻²) M). This polymer was characterized by cycling voltammetry, impedance measurement, UV-visible and FT-IR spectroscopy.     

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.     

Adsorptive Voltammetric Studies on the Cerium(III)–Alizarin Complexon Complex at a Carbon Paste Electrode

A novel procedure was developed for the determination of trace cerium on the basis of anodic adsorption voltammetry of the Ce(III)–alizarin complexon (ALC) complex at a carbon paste electrode (CPE). The procedure is convenient to determine cerium individually in the presence of other rare earths because there is a 100 mV difference between the peak potentials of Ce(III)–ALC and other rare earth(III)–ALC complexes in a supporting electrolyte of 0.08 M HAc–NaAc and 0.012 M potassium biphthalate (pH 4.7) when performing linear-scanning from −0.2 to 0.8 V (vs. SCE) at 100 mV/s. The second-order derivative peak currents are directly proportional to the Ce(III) concentration over a range of 6.0 × 10⁻⁹–3.0 × 10⁻⁷ M. The detection limit is as low as 2.0 × 10⁻⁹ M (S/N = 3) for a 120 s preconcentration. An RSD of 3.5% was obtained for 15 determinations of Ce(III) at a concentration of 4.0 × 10⁻⁸ M on the same CPE surface. The method was applied successfully to the determination of cerium in samples of rare earth nodular graphite cast iron.     

Electrochemical evaluation and determination of antiretroviral drug fosamprenavir using boron-doped diamond and glassy carbon electrodes

Fosamprenavir is a pro-drug of the antiretroviral protease inhibitor amprenavir and is oxidizable at solid electrodes. The anodic oxidation behavior of fosamprenavir was investigated using cyclic and linear sweep voltammetry at boron-doped diamond and glassy carbon electrodes. In cyclic voltammetry, depending on pH values, fosamprenavir showed one sharp irreversible oxidation peak or wave depending on the working electrode. The mechanism of the oxidation process was discussed. The voltammetric study of some model compounds allowed elucidation of the possible oxidation mechanism of fosamprenavir. The aim of this study was to determine fosamprenavir levels in pharmaceutical formulations and biological samples by means of electrochemical methods. Using the sharp oxidation response, two voltammetric methods were described for the determination of fosamprenavir by differential pulse and square-wave voltammetry at the boron-doped diamond and glassy carbon electrodes. These two voltammetric techniques are 0.1 M H₂SO₄ and phosphate buffer at pH 2.0 which allow quantitation over a 4 × 10⁻⁶ to 8 × 10⁻⁵ M range using boron-doped diamond and a 1 × 10⁻⁵ to 1 × 10⁻⁴ M range using glassy carbon electrodes, respectively, in supporting electrolyte. All necessary validation parameters were investigated and calculated. These methods were successfully applied for the analysis of fosamprenavir pharmaceutical dosage forms, human serum and urine samples. The standard addition method was used in biological media using boron-doped diamond electrode. No electroactive interferences from the tablet excipients or endogenous substances from biological material were found. The results were statistically compared with those obtained through an established HPLC-UV technique; no significant differences were found between the voltammetric and HPLC methods.     

Electrochemical behaviors of metol on ionic-liquid-modified carbon paste electrode and its analytical application

The electrochemical behaviors of metol on an ionic liquid N-butylpyridinium hexafluorophosphate modified carbon paste electrode (IL-CPE) were studied in this paper. The results indicated that a pair of well-defined quasi-reversible redox peaks of metol appeared with the decrease of overpotential and the increase of redox peak current, which was the characteristics of electrocatalytic oxidation. The electrocatalytic mechanism was discussed and the electrochemical parameters were calculated with results of the charge-transfer coefficient (α) as 0.45, the electrode reaction rate constant (k _s) as 4.02 × 10⁻³ s⁻¹, and the diffusion coefficient (D) as 6.35 × 10⁻⁵ cm²/s. Under the optimal conditions, the anodic peak current was linear with the metol concentration in the range of 5.0 × 10⁻⁶ ∼ 1.0 × 10⁻³ mol/L (n = 11, γ = 0.994) and the detection limit was estimated as 2.33 × 10⁻⁶ mol/L (3σ). The proposed method was successfully applied to determination of metol content in synthetic samples and photographic solutions.     

Direct electrochemistry of myoglobin in a room temperature ionic liquid aqueous solution and its catalysis to H<Subscript>2</Subscript>O<Subscript>2</Subscript>

Using 1-ethyl-2-methylimidazolium trifluoroacetate (EMImTfa) as the supporting electrolyte, a couple of well-defined and reversible redox peaks of Myb could be observed at the basal plane graphite (BPG) electrode through direct electron transfer between the protein and the BPG electrode, whose anodic and cathodic peak potentials were at −0.098 V and −0.144 V vs. Ag | AgCl, respectively. Both anodic and cathodic peak currents increased linearly with the potential scan rates. Compared with the supporting electrolyte of phosphate buffer solution, EMImTfa played an important role for the direct electron transfer between Myb and the BPG electrode. Further investigation suggested that Myb was adsorbed tightly on the surface of the BPG electrode in the presence of EMImTfa to form a stable, approximate monolayer Myb film. Myb adsorbed on the BPG electrode surface could retain its biological activity and showed a remarkable electrocatalytic activity for the reduction of H₂O₂ in an EMImTfa aqueous solution. Based on these, a third-generation biosensor could be constructed to directly detect the concentration of H₂O₂ in EMImTfa aqueous solution with a limit of detection of 3.24 × 10⁻⁸ M. Published in Russian in Elektrokhimiya, 2008, Vol. 44, No. 3, pp. 363–368. The text was submitted by the authors in English.     

Electrochemical determination of uric acid using a mesoporous SiO2-modified electrode

A mesoporous SiO₂-modified carbon paste electrode for the determination of uric acid is described. Owing to the regular and specific mesoporous channels, numerous active sites and a large surface area, the mesoporous SiO₂-modified electrode greatly increases the oxidation peak current of uric acid. Based on this, a highly sensitive, rapid and convenient electrochemical method was developed for the determination of uric acid after optimizing the experimental parameters (supporting electrolyte, content of mesoporous SiO₂, accumulation potential and time). The linear range is from 2.5 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹, and the limit of detection is estimated to be 8.0 × 10⁻⁸ mol L⁻¹. The relative standard deviation for 10 mesoporous SiO₂-modified electrodes is 5.8%. The method was used to determine uric acid in human serum samples. Correspondence: Kangbing Wu, Department of Chemistry, Huazhong University of Science and Technology, Wuhan 430074, P.R. China     

Aqueous solutions of colloidal nickel: Radiation-chemical preparation, absorption spectra, and properties

Radiation-chemical reduction of Ni²⁺ ions in aqueous solutions of Ni(ClO₄)₂ containing sodium formate or isopropyl alcohol was studied, γ-Irradiation of deaerated solutions in the presence of polyethyleneimine, polyacrylate, or polyvinyl sulfate gives stable metal sols containing spherical particles 2–4 nm in diameter. The optical absorption spectra of nickel nanoparticles exhibit a band with a maximum at 215±5 nm (ε₂₁₅=4.7·10³ L mol⁻¹ cm⁻¹) and a shoulder at 350 nm. A mechanism for the radiation-chemical reduction of Ni²⁺ ions by hydrated electrons and organic radicals (CO₂- radical anions in the case of HCOONa and Me₂C·OH radicals in the case of PrⁱOH). The redox potentials of the Ni²⁺/Ni⁰ and Ni⁺/Ni⁰ pairs (Ni⁰ is a nickel atom) are approximately −2.2 and −1.7 V, respectively. The nanoparticles are readily oxidized by O₂, H₂O₂, and other oxidants. The reactions of these species with silver ions yield relatively stable nanoaggregates containing both nickel and silver in addition to silver nanoparticles. Published inIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 10, pp. 1733–1739, October, 2000.     

Preconcentration of the lipid-lowering drug lovastatin at a hanging mercury drop electrode surface

Adsorption and reduction of lovastatin were investigated by cyclic and square-wave voltammetry on a hanging mercury drop electrode in aqueous solutions over a wide pH range (4–9). The electroreduction of lovastatin proceeds via a surface EC mechanism in the whole pH range investigated. Using adsorptive stripping voltammetry, the drug yielded a well-defined voltammetric response in Britton-Robinson buffer, pH 6 at −1.49 V which can be used to determine trace amount of lovastatin. The linear concentration range of application was 1.0 × 10⁻⁸–1.0 × 10⁻⁷ M by using an accumulation potential of −0.5 V and a 90 s pre-concentration time. The method has been successfully applied for the determination of lovastatin in a spiked human serum sample.     

Electroanalytical Method of Acid Blue 120 and its Supramolecular System with Cyclodextrins

In this paper, a simple, rapid, sensitive and accurate electroanalytical method of Acid Blue 120 (AB120) has been established by polarography. In a supporting electrolyte of 0.01 mol l⁻¹ Na₂HPO₄–KH₂PO₄ (pH 7.04) solution, a sensitive first derivative reduction peak (ip′) of AB120 was found by Linear Sweep Voltammetry (LSV). The peak potential is −820 mV (versus SCE). The peak current (ip′) is proportional to the concentration over the range 2.0 × 10⁻⁷–5.0 × 10⁻⁵ mol l⁻¹ (r=0.9961–0.9991) and the limit of detection (LOD) is 1.0 × 10⁻⁷ mol l⁻¹. The recovery of AB120 varied from 95.3 to 103.0% and the relative standard deviation (RSD) was 2.2% (n=8). The method has been expected to determination of wastewater in dye industry. In addition, the supramolecular system of AB120 with cyclodextrins has been studied. It can form 1:1 inclusion complex with six CDs. The inclusion constants were calculated and the inclusion ability of different kinds of CDs was compared. Furthermore, the inclusion mechanism was also preliminarily discussed, which provided some valuable information for further application of AB120 and CDs.     

Determination of hemoglobin at a novel NH2/ITO ion implantation modified electrode

A novel electrode was prepared by implanting NH₂ ⁺ into an ITO film (NH₂/ITO). Gold nanoparticles were deposited on the surface of NH₂/ITO electrode. The NH₂/ITO and Au/NH₂/ITO electrodes were used to determine hemoglobin (Hb) immobilized on the electrodes surfaces. The relationship of the reductive peak current value of Hb among different electrodes was: Hb/ITO:Hb/Au/ITO:Hb/NH₂/ITO:Hb/Au/NH₂/ITO=1:1.5:2:4. The linkage between the –NH₂ implanted into ITO film and the –COOH of Hb was recognized to be the reason for the increase of active Hb coverage on NH₂/ITO electrode compared with the ITO electrode. Increase of active Hb coverage on Au/NH₂/ITO compared with Au/ITO was attributed to the different amount of gold nanoparticles deposited. The determination of Hb at an Au/NH₂/ITO electrode was optimized. Calibration curve was obtained over the range of 1.0 × 10⁻⁸ – 1.0 × 10⁻⁶ mol · L⁻¹ with a detection limit of 1.0 × 10⁻⁸ mol · L⁻¹. Results showed that the novel NH₂/ITO and Au/NH₂/ITO electrodes exhibited good stability, reproducibility besides better electrochemical performance. Correspondence: Jing Bo Hu, Department of Chemistry, Beijing Normal University, Beijing 100875, China     

Impedance study of adsorption of iodide ions at Cd(0001) and Bi(111) electrode from various solutions with constant ionic strength

Adsorption of iodide ions at the Bi(111) and Cd(0001) electrodes from the aqueous solutions with constant ionic strength 0.1x M KI + 0.1(1−x) M KF and 0.1x M KI + 0.033(1−x) M K₂SO₄ has been studied by impedance spectroscopy. It was found that, to a first approximation, the classical Frumkin–Melik–Gaikazyan equivalent circuit with the slow diffusion-like and adsorption steps can be applied for fitting the experimental impedance data for iodide ions adsorption on Bi(111) and Cd(0001) from aqueous solutions with constant ionic strength. The modified Grafov–Damaskin circuit can be used in the region of electrode potentials, where parallel faradic processes (electroreduction of protons, oxygen traces) are probable. The more complicated Ershler equivalent circuit, taking into account the slow diffusion-like, adsorption and charge transfer steps, is not applicable for characterization of the adsorption process of I⁻ at Bi(111) and Cd(0001) electrodes.     

Catalytic Determination of Vanadium Based on the Bromate Oxidative-C oupling Reaction of Metol with Phloroglucinol

 A selective, sensitive and simple catalytic method is developed for the determination of vanadium in natural and highly polluted waste waters. The method is based on the catalytic effect of V^V and/or V^IV on the bromate oxidative-coupling reaction of metol with phloroglucinol (PG). The reaction is followed spectrophotometrically by tracing the oxidation product at 464 nm after 10 minutes of mixing the reagents. The optimum reaction conditions are metol (8.0×10⁻³ M), PG (4.0×10⁻³ M) and bromate (2×10⁻² M) at 35°C and in presence of an activator-b uffer mixture of 5×10⁻² M of each of citric and monochloroacetic acids (pH 2.40). Following the recommended procedure, vanadium can be determined with a linear calibration graph up to 8.0 ng mL⁻¹ and a detection limit, based on the 3s_b criterion, of 0.1 ng mL⁻¹. Spectrophotometric determination of as little as 1.0 ng mL⁻¹ of V^V or V^IV in aqueous solutions gave an average recovery of 98% with relative standard deviations of ?1.8% (n = 5). The proposed method was directly applied to the determination of vanadium in Nile river water and highly polluted industrial wastes. Statistical treatments of analytical results could not detect any systematic error and showed the high accuracy and precision of the developed method. Received November 25, 1999. Revision March 10, 2000.     

Lithium-ion conduction in elastomeric binder in Li-ion batteries

This paper describes two kinds of elastomeric binders which are styrene–butadiene (ST–BD) copolymer and 2-ethylhexyl acrylate–acrylonitrile (2EHA–AN) copolymer for electrode materials of rechargeable Li-ion batteries. These elastomeric binders were swollen by electrolyte solution (EC/DEC=1/2, 1 M LiPF₆), and 2EHA–AN copolymer retained larger amount of electrolyte solution than ST–BD copolymer. The Li-ionic conduction behavior was investigated for both copolymer films swollen by electrolyte solution. The Li-ion conductivity of ST–BD copolymer was 9.45 × 10⁻⁸ S·cm⁻¹ and that of 2EHA–AN copolymer was 1.25 × 10⁻⁵ S·cm⁻¹ at room temperature, and the corresponding amounts of activation energy were 0.31 and 0.26 eV, respectively. Because the observed activation energy in elastomeric binder was different from that in the bulk of electrolyte solution (0.09 eV), Li-ion conduction of the bulk of elastomeric binder swollen by electrolyte was affected by the polymer structure of binders. Electrochemical performance of cathode material, LiCoO₂, was investigated with three kinds of binders: ST–BD copolymer, 2EHA–AN copolymer, and poly(vinylidene fluoride). The initial charge–discharge capacity of the LiCoO₂ electrode with 2EHA–AN copolymer showed highest capacity, suggesting that Li⁺-ion conduction inside of the elastomeric binder contributes to the enhancement of charging and discharging capacity. This result indicates that elastomeric binder with sufficient Li-ionic conductivity can be an attractive candidate for improving cathode of lithium-ion battery.     

Voltammetric determination of salbutamol on a glassy carbon electrode coated with a nanomaterial thin film

A rapid and convenient electrochemical method is described for the determination of salbutamol based on multi-carbon nanotubes (MWNT) film coated glassy carbon electrode (GCE). The electrochemical behavior of salbutamol at this modified electrode was studied by square wave voltammetry, which indicated that the oxidation peak potential of salbutamol shifted on 40 mV to less positive potential and the peak current increased 4.5 fold, in contrast to that at a bare electrode. Various experimental parameters such as pH value of supporting electrolyte, the amount of modifier, and accumulation time were optimized. Under optimal measurement conditions, there is a good linear relationship between the peak current (I _pa) and salbutamol concentration in the range from 8.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M, and the detection limit is 2.0 × 10⁻⁷ M (S/N = 3) at 2 min accumulation. The method has been successfully employed to detect salbutamol in pharmaceutical formulations.