Voltammetric determination of linuron at a carbon-paste electrode modified with sepiolite

The determination of linuron by differential pulse voltammetry with a carbon-paste electrode modified with 20% w/w sepiolite has been studied. The linuron is preconcentrated under open-circuit conditions at pH 2.0. With 0.01M potassium nitrate at pH 1.7 in the measurement cell, a sweep rate of 30 mV/sec and a pulse amplitude of 100 mV, an oxidation wave with a peak potential of 1.2 V is obtained. Under these conditions, determination limits of 75 ng/ml have been obtained, with a relative error of +2.8% and a relative standard deviation of 8.0%. The method has been applied to the direct determination of linuron in river water with no previous separation of the pesticide. Determination in sea-water is not possible, as chloride interferes at high concentration.     

Voltammetric determination of N-alkylated anilines by adsorption/extraction at a carbon-paste electrode

The voltammetric behaviour of aniline and some of its N-alkylated derivatives at a carbon-paste (Nujol/graphite) electrode is examined. The N-alkylated anilines, in contrast to aniline, are accumulated at the electrode by a combined adsorption/extraction process. The organic bases are determined in the rane 10^?7?2×10^?6 M after a 2-min preconcentration period by differential pulse voltammetry; N,N-dimethylaniline can be determined in the presence of excess of aniline (10^?5 M) by using medium-exchange. The voltammetric method is used for evaluation of the distribution ratios of the organic bases in the Nujol/aqueous buffer system. The enhancement of the voltammetric signal of each base is correlated with the respective distribution ratio.     

Voltammetric and potentiometric study of cysteine at cobalt(II) phthalocyanine modified carbon-paste electrode

Cyclic voltammetry and potentiometry were used to investigate the electrochemical behavior of cysteine at a chemically modified electrode prepared by incorporating cobalt(II) phthalocyanine [Co(II)Pc] into carbon paste matrix. The modified electrode showed high electrocatalytic activity toward cysteine; the overpotential for the oxidation of cysteine was decreased by more than 100 mV, and the corresponding peak current increased significantly. The electrocatalytic process was highly dependent on the pH of the supporting electrolyte. The peak currents decreased when the pH was raised to 6 and totally disappeared at pH≥ 7, resulting from the autocatalytic oxidation of cysteine by Co(II)Pc at the electrode surface. Therefore, at pH values of 6 to 8, the modified electrode was used as a potentiometric sensor for quantitative measurement of cysteine in the presence of oxygen in air saturated solutions. In fact, the Co(II)Pc/Co(I)Pc couple acts as a suitable mediator for indirect oxidation of cysteine by dissolved oxygen at approximately neutral pH values. Under the optimized conditions, the potentiometric response of the modified electrode was linear against the concentration of cysteine in the range of 0.6 μM to 2 mM. The limit of detection was found to be 0.5 μM. The potentiometric response time was ≤15 s. The electrode showed long term stability; the standard deviation of the slope obtained after repeated calibration during a period of two months was 2.8% (n = 7). Application of the electrode in a recovery experiment for the determination of cysteine added to a synthetic serum sample is described.     

Voltammetric determination of thiocytosine based on its electrocatalytic oxidation on the surface of carbon-paste electrode modified with cobalt Schiff base complexes

The electrochemical oxidation of thiocytosine on the surface of carbon-paste electrode modified with Schiff base (salophen derivatives) complexes of cobalt is studied. The effect of the substituents in the structure of salophen on the catalytic property of the modified electrode is investigated by using cyclic and differential pulse voltammetry. Cobalt (II)-5-nitrosalophen, because of its electrophilic functional groups, leads to a considerable enhancement in the catalytic activity, sensitivity (peak current), and a marked increase in the anodic potential of the modified electrode. The differential pulse voltammetry is applied as a very sensitive method for the detection of thiocytosine. The linear dynamic range was between 1 × 10⁻³ to 4 × 10⁻⁶ M with a slope of 0.0168 μA/μM, and the detection limit was 1 × 10⁻⁶ M. The modified electrode is successfully applied for the voltammetric detection of thiocytosine in human synthetic serum sample and also pharmaceutical preparations. A linear range from 1 × 10⁻³ to 1 × 10⁻⁵ M with a slope of 0.0175 μA/μM is resulted for the standard addition of thiocytosine spiked to the buffered human serum, which is differing from the curve in buffer solution about 4%. The electrode has a very good reproducibility (relative standard deviation for the slope of the calibration curve is less than 3.5% based on six determinations in a month), high stability in its voltammetric response and low detection limit for thiocytosine, and high electrochemical sensitivity with respect to other biological thiols such as cysteine.     

Voltammetric sensing of sugar by an electrode covered with wheat germ agglutinin/chitin film

The binding between wheat germ agglutinin (WGA) and N-acetylglucosamine at the electrode covered with chitin film was investigated with voltammetry. Chitin, β-1,4-poly-N-acetylglucosamine, is one of the biolpolymers which have a high biocompatibility. WGA is immobilized to the surface of chitin film by the affinity of WGA to N-acetylglucosamine residue of chitin. To investigate the binding event of WGA on the chitin modified electrode, N-acetylglucosamine labeled with an electroactive compound was prepared. The binding causes the changes in the electrode response of labeled sugar. The peak current of labeled sugar decreased due to the specific binding with WGA on the chitin film modified at the electrode. N-Acetylglucosamine was successfully determined by using the competitive reaction with labeled sugar to WGA on the chitin film electrode.     

盐酸环丙沙星在导电碳黑糊电极上的伏安行为及其检测应用

本文研究了盐酸环丙沙星(Cipro)在导电碳黑糊电极上的伏安行为,结果表明:B-R缓冲溶液(p H 4.0)中,该药物分子发生2质子、2电子转移的不可逆电化学氧化,过程受吸附控制,电子转移系数α为0.37,表面反应标准速率常数k's为0.95 s-1。实验研究了支持电解质种类、p H值、离子强度、富集电位与时间等对其伏安响应的影响,并据结果对检测条件进行优化,进而建立了Cipro的电化学检测方法。优化条件下,Cipro的氧化峰电流(ipa)与其浓度(C0)在2.5×10-7~6.0×10-5mol·L-1范围内呈良好的线性关系,检测限为9.5×10-8mol·L-1(S/N=3)。将该方法应用于测定淡水渔业水样中痕量Cipro,得到了较满意的结果(平均回收率102.73%)。     

Electrooxidation of oxalic acid at a carbon-paste electrode with deposited palladium nanoparticles

Palladium nanoparticles deposited on the surface of a carbon-paste electrode exhibit electrocatalytic activity in the oxidation of oxalic acid. The surface topography of modified carbon-paste electrodes (CPEs) and the shapes of the metal crystallite particles were studied with atomic-force microscopy. These data were correlated with the voltammetric parameters of the electrooxidation of palladium and oxalic acid. As the dispersity of palladium nanoparticles electrodeposited on the CPE surface increased and their size diminished, the peak current of the catalytic oxidation of oxalic acid decreased, whereas the increment of this current increased as compared to the limiting current of metal oxidation. Evidently, this was due to an increase in the catalytic activity of the metal. The use of CPE modified with palladium nanoparticles instead of CPE containing palladium macrocrystals lowered the detection limit for the analyte by one order of magnitude (down to 2 × 10^?5 M).     

Potentiometric determination of monohydrogen arsenate by zeolite-modified carbon-paste electrode

A purge-and-trap method was developed for sensitive and fast determination of trace MTBE in aqueous samples. The sample solutions were added with 10% (w/w) sodium sulfate and adjusted to pH 4 by acetic acid and sodium acetate buffer solution to improve the purge efficiency before the analysis. A CP-4010 purge-and-trap injector (PTI) was used to purge MTBE from water and cool it in the cold-trap kept at m 75°C, then the cooled trap was flash heated to release the analytes onto a HP-1 capillary column and detected by gas chromatography-flame ionization detector (GC-FID). A good linear response was obtained and the detection limit was 0.1 µg L m 1 . This method has been successfully applied to the determination of MTBE in several Chinese river samples.     

Electrocatalytic oxidation of captopril using a carbon-paste electrode modified with copper-cobalt hexacyanoferrate

A carbon-paste electrode was modified with copper-cobalt hexacyanoferrate by consecutive potential cycling. The kinetic parameters were calculated for the electroactive species. The resulting electrode exhibited electrocatalytic activity towards the oxidation of captopril. The kinetics of the electrocatalytic reaction was studied. A linear relationship was observed between anodic current and the concentration of captopril in the range of 5.0 × 10^?6–3.1 × 10^?5 μM with a detection limit of 4.2 μM (S/N = 3). The modified electrode was used in the analysis of captopril tablets successfully.     

Preconcentration and voltammetric measurement of mercury with a crown-ether modified carbon-paste electrode

Carbon-paste electrodes modified with crown-ethers were constructed by mixing the crown-ethers into a graphite powder/Nujol oil matrix. The electrodes so formed were able to bind mercuric ions chemically, and gave greater voltammetric response to mercury than that of ordinary carbon-paste electrodes. The response was characterized with respect to paste composition, crown-ether, preconcentration period, mercury concentration, reproducibility, possible interferences, and other variables. Best results were obtained with 18-crown-6 and an acetate buffer (pH 4.0). The electrode gave good linearity for 1 x 10(-5)-6 x 10(-6)M mercury, a detection limit of 2 x 10(-6)M, and a relative standard deviation of 11%. The investigation may lead to a new class of modified (complexing) electrodes, with different patterns of reactivity.     

Cyclic voltammetry of Tifluadom at a C18-modified carbon-paste electrode

Tifluadom, N-[5-(2-fluorophenyl)-2,3-dihydro-methyl-1H-1,4-benzodiazepine]-2-4-methyl-3-thiophene carboxamide, was determined by using a carbon-paste electrode modified with C₁₈ μBondapak. Adsorption on the electrode served as a preconcentration step which improved the limit of detection. Preconcentration for 5 min (open circuit) gave a linear range of 2.2×10^?7 M?4.5×10^?6 M with a detection limit of 1.3×10^?7 M (%C₁₈=25, w/w) for Tifluadom in Britton-Robinson buffer pH 6. The determination of Tifluadom added to urine required no preliminary treatment; the detection limit was 1.3×10^?6 M.     

Voltammetric analysis using a self-renewable non-mercury electrode

Galinstan is a new kind of electrode material and the galinstan electrode is a promising alternative to the commonly used mercury electrodes. The eutectic mixture of gallium, indium and tin is liquid at room temperature (m.p. −19°C) and its voltammetric behaviour is similar to that of mercury. The potential windows of use were determined for different pH values and are similar to those obtained with conventional mercury electrodes. Furthermore, the high hydrogen overpotential, which is characteristic for mercury, can be observed when galinstan is used as electrode material. Galinstan can be employed as a liquid electrode in the voltammetric analysis of different metal ions, such as lead and cadmium, in different supporting electrolytes. Our results indicate that the non-toxic liquid alloy galinstan could therefore become immensely important in electrochemical research as a potential surrogate material for mercury.     

Voltammetric method based on an ion-pairing reaction for the determination of trace amount of iodide at carbon-paste electrodes.

Carbon-paste electrodes (CPEs) were studied to elucidate the cathodic stripping voltammetric determination of iodine. At an accumulation potential of 1.0 V (vs. SCE), iodide was preconcentrated on CPEs via an ion-pairing reaction, followed by oxidation to iodine. Then a linear scan voltammogram was recorded after 10 s of quiescent time by a cathodic potential scan from 0.7 V to 0.1 V at a scan rate of 100 mV s(-1). A cathodic peak current was obtained at about 0.38 V. Various experiment parameters such as the acidity, chloride concentration, accumulation potential, accumulation time, concentration of cetyltrimethylammonium bromide (CTAB) and scan rate, were optimized to analyze the iodide by employing linear-scan stripping voltammetry. Under the optimal conditions, calibration curves were obtained over a wide concentration range of the iodide ion from 8 x 10(-9) mol L(-1) to 5 x 10(-6) mol L(-1) with a detection limit of 2 x 10(-9) mol L(-1) at an accumulation time of 3 min. The effect of interfering species was evaluated and the procedure was applied to an iodide analysis in table salt, with good results.     

Voltammetric behavior of some fluorinated quinolone antibacterial agents and their differential pulse voltammetric determination in drug formulations and urine samples using a β-cyclodextrin-modified carbon-paste electrode

A voltammetric determination of ofloxacin (OF), norfloxacin (NF), gatifloxacin (GF), and lomefloxacin (LF) at a β-cyclodextrin-modified carbon-paste electrode (CDMCPE) is described. A large increase in the peak currents was observed in cyclic voltammetry (CV) and differential pulse voltammetry (DPV) of OF, NF, GF, and LF at CDMCPE compared with a bare carbon-paste electrode (CPE). These increases in the peak currents were attributed to the complex formation of the quinone group of the drugs with β-cyclodextrin. CV studies indicate that the process is irreversible and adsorption-controlled. The experimental parameters which influence the peak current responses of OF, NF, GF, and LF were studied. The reduction peak currents of OF, NF, GF, and LF change linearly over the common concentration range from 3.2 × 10⁻⁸ to 2 × 10⁻⁵ M, with a common correlation coefficient and limit of detection of 0.9995 and 2.4 × 10⁻⁸ M, respectively, in pH 4.0 Britton-Robinson (BR) buffer at an accumulation time of 160 s. The interference of metal ions in the peak current response was also studied. The modified electrode exhibited good sensitivity and stability. The proposed method was applied to the determination of OF, NF, GF, and LF in both commercially available drugs and spiked human urine samples. The text was submitted by the authors in English.     

Voltammetric detection of paraquat pesticide on a phthalocyanine-based pyrolitic graphite electrode

This work describes the application of an ordinary pyrolitic graphite electrode modified by metallophthalocyanine allied to square wave voltammetry for the study of the electrochemical behavior of the herbicide paraquat and the development of a method for its analytical determination in natural water samples. Preliminary experiments indicated that the best responses, considering the intensities of the current and voltammetric profile for the paraquat reduction process, were obtained when the electrode modified by cobalt phthalocyanine was employed, which had a better catalytic activity as a result of this modification compared with that for an unmodified electrode and electrodes modified by iron, manganese and the acid form of the phthalocyanines. Studies of the concentration of cobalt phthalocyanine and the adsorption time showed that 1.0 × 10⁻⁴ mol L⁻¹ cobalt phthalocyanine with an adsorption time of 10 min was sufficient to obtain reliability and stability of modification for employment in the development of the electroanalytical procedure for paraquat determination in natural water samples. The variation in pH of a 0.10 mol L⁻¹ Britton–Robinson buffer solution and the square wave parameters indicated that the best conditions to reduce paraquat were pH 7.0, a frequency of 100 s⁻¹, a scan increment of 2 mV and a square wave amplitude of 50 mV. Under such conditions, the variation of paraquat concentrations from 5.00 × 10⁻⁷ to 2.91 × 10⁻⁵ mol L⁻¹ showed a linear relation, with detection and quantification limits of 26.53 and 88.23 μg L⁻¹; those values were lower than the maximum limits for drinking water permitted by the Brazilian Environmental Council (100 μg L⁻¹), indicating that the method could be employed to analyze paraquat in drinking water samples.