Showing papers on "Polarography published in 2006"

Determination of Critical Micelle Concentration by Electrochemical Means

Abstract: Overview of electrochemical methods for critical micelle concentration (cmc) determination is given. The principles and examples of cmc determination by conductometry, potentiometry, tensammetry, polarography, voltammetry, electrophoresis, and isotachophoresis are reviewed and discussed.

Rapid Differential Pulse Polarographic Determination of Thiamethoxam in Commercial Formulations and some Real Samples

Abstract: The polarographic behavior of thiamethoxam (a neonicotinoid insecticide) was studied by direct current and differential pulse polarography. Depending on the pH thiamethoxam exhibited one or two well-defined cathodic polarographic waves. The characteristics of the electrode reaction were investigated and it was found that at pH > 5.0 the target molecule captures four electrons in the first step, and two in the second. Based on the reduction behavior of the target molecule on the mercury electrode, a differential pulse polarographic method was elaborated for the rapid determination of thiamethoxam at pH 8.0. With the optimized method, a linear response for thiamethoxam was found in the concentration range of 31.1 − 470 ng cm−3, the relative standard deviation did not exceed 1.6%, and the detection and quantitation limits were found to be 9.3 ng cm−3 and 31.1 ng cm−3, respectively. The method was applied to the determination of thiamethoxam in commercial formulations and real samples (potato and maize). The procedure is simple, fast, sensitive, and compares well with comparative spectrophotometric and chromatographic (HPLC/DAD) methods.

Electrooxidation of COad Intermediated from Methanol Oxidation on Polycrystalline Pt Electrode

Abstract: The poisonous intermediate of methanol oxidation on a Pt electrode was validated to be CO(ad) by electrochemical method. An approximate treatment to bimolecular elementary reactions on an electrode was advanced and then was applied to the stripping normal pulse voltammetry (NPV) for complex multistep multielectron transfer processes on plane electrodes to study the kinetics of completely irreversible process of CO(ad) oxidation to CO2. The kinetic parameters for this process, such as standard rate constant (k0) and anodic transfer coefficient (alpha) for this irreversible heterogeneous electron-transfer process at electrode/solution interface and apparent diffusion coefficient (D(app)) for charge-transfer process within the monolayer of CO(ad) on electrode surface, were obtained with stripping NPV method. The effect of the approximate treatment on the kinetic parameters was also analyzed.

Polarographic determination of herbicide thifensulfuron methyl/application to agrochemical pesticide, soil, and fruit juice

Abstract: A novel, sensitive, simple, fast, and fully validated differential pulse polarographic (DPP) method for the determination of trace amounts of thifensulfuron-methyl in pesticide formulation, soil, and orange juice is reported. This procedure was based on a highly sensitive peak formed due to the reduction of thifensulfuron-methyl on a dropping mercury electrode over the pH range 1.00–10.00 in Britton–Robinson buffer. The polarographic reduction exhibits only a single peak in the pH ranges pH ≥ 3.0 and pH ≤ 6.0 and pH = 10.0 located at potential values of −1.010, −1.350, and −1.610 V (vs. SCE), respectively. The single peak appeared as a maximum at pH 3.0 (−1.010 V) was well resolved and suitable to be investigated for analytical use. This peak showed quantitative increments with the additions of standard thifensulfuron-methyl solution under the optimal conditions, and the cathodic peak current was linearity proportional to the thifensulfuron-methyl concentration in the range of 2 × 10−7–5 × 10−5 M. The lim...

Development of Uric Acid Sensor Based on Molecularly Imprinted Polymer-Modified Hanging Mercury Drop Electrode

Abstract: Uric acid (UA) sensor based on molecularly imprinted polymer-modified hanging mercury drop electrode was developed for sensitive and selective analysis in aqueous and blood serum samples. The uric acid-imprinted polymer was prepared from melamine and chloranil and coated directly onto the surface of a hanging mercury drop electrode, under charge-transfer interactions at +0.4 V (vs. Ag/AgCl), in model 303A electrode system connected with a polarographic analyzer/stripping voltammeter (PAR model 264A). The binding event of uric acid was detected in the imprinted polymer layer through differential pulse, cathodic stripping voltammetry (DPCSV) at optimized operational conditions [accumulation potential +0.4 V (vs. Ag/AgCl), accumulation time 120 s, pH 7.0, scan rate 10 mV s−1, pulse amplitude 25 mV]. The limit of detection for UA was found to be 0.024 μg mL−1 (RSD=0.64%, S/N=3). Under the optimized operational conditions, the sensor was able to differentiate between uric acid and other closely structural-related compounds and interfering substances. Ascorbic acid (AA), a major interferent in UA estimation, was not adsorbed on the surface of sensor electrode. The present sensor is, therefore, UA-selective at all concentrations of AA present in human blood serum samples. The precised and accurate quantification of UA have been made in the dilute as well as concentrated regions varying within limits 0.1–4.0 and 9.8–137.0 μg mL−1, respectively.

Nanopatterned Working Electrode with Carbon Nanotubes Improving Electrochemical Sensors

Abstract: This paper deals with problem of mercury drop electrodes replacement in classical polarography by more ecological solid electrodes. The screen-printed thick-film amperometric sensor was prepared for modification by nanostructures. Carbon nanotubes were grown on a working electrode. The process of nanotubes growing was improved to create homogenous and high density carbon nanotubes layer. The modified electrode is very perspective in heavy metal detection using electrochemical methods because of improvement detection properties. We were able to determine units of mumol/L of cadmium.

On the Electrochemical Reduction of the Herbicide Picloram on Mercury Electrodes

Abstract: Polarographic and voltammetric studies on the electroreduction of picloram (4-amino-3,5,6-trichloro2-pyridinecarboxylic acid) on mercury electrodes are performed. The dissociation constant of the -COOH group was obtained from the UV-visible absorption spectra (pK = 2.2 ′ 0.03). The electrochemical studies were performed in the acidity range 2.25 M H 2 SO 4 to pH 7. In dc polarography picloram presents one or two waves of reduction depending on the medium pH. Degree of polymerization (DP) polarograms showed two or three peaks. The overall reduction process involves the uptake of four electrons. Kinetics parameters such as Tafel slopes and electrochemical reaction orders have been determined at the potentials corresponding to the foot of the waves. From the experimental reaction pathways are proposed. The reduction of the species protonated at the heterocyclic nitrogen reactions correspond to a chemical reaction in equilibrium placed after a reversible electron transfer and followed by the reductions of both species appearing in the equilibrium EC(EE) scheme. The acidic components of the buffer are involved in the rate-determining step. At pH > 2 the recombination of the carboxylate anion with the H + ion precedes the reduction process. Peak 3 must correspond to the reduction of the protonated picloram carboxylate.

Current examples of practical and fundamental applications of DC polarography

Abstract: Examples of recent applications—applied, analytical, and fundamental are given. For these types of applications, an understanding of at least the number of electrons and protons transferred and the sequences of electron transfers and chemical reactions is essential. Polarography can be used for the determination of the active component in tablets and injection solutions, where it offers speed and sensitivity of the analytical method. Electroactive species can also be determined in heterogeneous systems without pre-separation. This has been taken advantage of in the investigation of reactions of bile salts with heavy metal ions or of binding of pesticides and other toxins on lignin. Polarography can also be used in the investigation of alkaline cleavage of lignin under mild conditions. Kinetics of this cleavage was used as a tool in proving that humic acids are not natural products. Among fundamental applications belong proofs of limited delocalization in groupings >C=N–N=C<, of diprotonation of hydrazones and oximes at the electrode surface, as well as of formation of imines as intermediates in reductions of hydrazones and oximes. Simultaneous determination of benzaldehyde and its hydrazone enabled providing experimental evidence for formation of carbinolamines as reaction intermediates and interpretation of the pH dependence of the equilibrium constant of this reaction. Differences in hydration of three isomeric phthalaldehydes are discussed as well as the covalent hydration of 1,2,4- and 1,3,5-triazines and pyrimidines. The ω,ω,ω-trifluoroacetophenone oxime the C=N bond is covalently hydrated.

Electrochemical reduction behaviour of the synthetic pyrethroid insecticide cyfluthrin and its determination in formulations and environmental samples

Abstract: The polarographic behaviour of cyfluthrin (CY), an α-cynoester pyrethroid, was studied using a dropping mercury electrode and hanging mercury drop electrode in methanolic Britton–Robinson (B–R) buffer of pH 2.0–12.0 with different ionic media. The nature of the electrode process was examined, the number of electrons was evaluated, and the reduction mechanism was proposed. Quantitative determination was achieved in the concentration range of 6.0 × 10−8 to 1.15 × 10−5 mol dm−3 using a differential pulse polarographic method with a lower detection limit of 2.4 × 10−8 mol dm−3. The proposed method was successfully applied in the determination of CY in formulations, grains, soils, and spiked water samples.

Voltammetric Determination of Stability Constants of Lead Complexes with Vitamin C

Abstract: Interactions between lead and ascorbic acid were investigated by polarography and voltammetry. Applied techniques were: sampled polarography, differential pulse anodic stripping voltammetry, and square-wave voltammetry. The measurements were performed in perchlorate aqueous solutions under physiological ionic strength (0.15 mol dm^3). The electrochemical redox reaction of Pb(II) is reversible (without adsorption effects or interactions of other lead complexes e.g. hydroxides) under the following conditions: [Pb^2+] = 4x10^-7 mol dm^-3 and pH = 5.5. Ascorbic acid was gradually added from 10^-5 to 10^-1 mol dm^-3. After each increment of ascorbic acid, the signal of lead was recorded by square wave voltammetry. Stability constants were computed by applying a DeFord-Hume non-linear regression function F0 and after have performed a Levenberg-Marquardt fit with the Sigma Plot software. The apparent stability constants were calculated assuming a series of mononuclear complexes (MLn, n = 1, 2...) and yielding the following values for Pb(II)-ascorbate complexes: logβ _1=9.3p ; 0.2 and logβ _2=18.0p ; 0.1.

Minimization of Electrode Adsorption Effects: The Cadmium–Humic Acid System Studied by Phase Sensitive Alternating Current Polarography

Abstract: The use of phase-sensitive Alternating Current Polarography (ACP) is investigated for the minimization of adsorption effects in metal-ligand systems with induced metal adsorption onto the electrode. When ACP is applied to obtain information on metal complexation in bulk, some problems arise from the faradaic contribution of adsorbed species. This effect can be corrected by using the capacitive current measured in the potential region of the faradaic peak. Using this correction, ACP produces similar results to those from Reverse Pulse Polarography (RPP), a technique that minimizes electrode adsorption effects on the measured currents. The method proposed is applied to the study of the Cd–humic acid system that has been investigated by ACP, RPP and Differential Pulse Polarography (DPP).

Electroanalytical characteristics of a subnanometer thin-film mercury-carbon electrode in anodic stripping voltammetry with a linear high-rate potential sweep: Determination of Tl(I)

Abstract: With the example of the determination of Tl(I) with a mercury-carbon electrode generated in situ on a carbon fiber substrate, it is shown that an electrode with a monolayer thickness of the mercury film reduces the quantification limit of the metal as compared to the electrode with a thick mercury film (down to 10−10 M). Studying the electrooxidation kinetics of Tl (Zn, Cd, Pb) showed that the rate constant and the transport coefficient at this electrode are independent of the electrode polarization rate. It is found that the rate constant of the electrode process and the slope of the calibration plot linearly depend on the atomic radius of the metal under study.

Stripping Chronopotentiometry and Stripping Voltammetry of Mixtures of Heavy Metal Ions Producing Close Signals: The Cd(II)‐Pb(II)‐Phthalate System

Abstract: The effects of the proximity of the signals of two heavy metal ions in stripping voltammetry (SV) and constant-current stripping chronopotentiometry (SCP) is studied at mercury drop (HMDE) and mercury film (MFE) electrodes. For this purpose, the Cd(II)-Pb(II)-phthalate system is used, taking advantage of the approaching of the signals corresponding to Cd(II)-phthalate and Pb(II)-phthalate labile complexes as phthalate is added to mixtures of Cd(II) and Pb(II)-ions. The results are compared with those obtained by differential pulse polarography (DPP) and by stripping measurements on the Pb(II)-phthalate system alone, showing discrepancies in SCP data under nondepletive conditions and negligible differences in the other cases.

Electrochemical Characterization of Iron(III)–Glycine–Nitrilotriacetate Mixed Ligand Complexes and Their Stability in Aqueous Solutions

Abstract: Electrochemical characterisation of iron(III)-glycine-nitrilotriacetate (iron(III)-Gly-NTA) mixed ligand complexes and determination of their stability constants and retention time in aqueous solution (I = 0.1 mol dm-3 in NaClO4, pH = 8.0+-0.1 at 25+-1 oC) using differential pulse cathodic voltammetry (DPCV), cyclic voltammetry (CV) and direct current (d.c.) polarography with a static mercury drop electrode (SMDE), were performed. Iron(III) concentrations were varied from 5×10-6 to 6×10-4 mol dm-3, NTA total concentrations varied from 2×10-5 to 1×10-3 mol dm-3 and glycine total concentrations were 0.2, 0.02 and 0.002 mol dm-3. Iron(III) redox reaction in the mixed ligand system (with the techniques employed) was found to be the one-electron reversible process. For the total concentration ratios of 1 : 800 : 2 for iron(III), glycine and NTA, respectively, the iron(III)-Gly-NTA mixed ligand complexes are dissolved and stable (>18 hours) in the aqueous solution. The complexes were formed either by the addition of NTA into the iron(III) and glycine aqueous solution or by the addition of iron(III) to the mixture of glycine and NTA. Under these conditions, iron(III) hydrolysis was suppressed to the great extent. By fitting of experimental data following stability constants for mixed ligand complexes, not found in the literature so far, in 0.1 mol dm-3 NaClO4 aqueous solution were calculated: for iron(III) log beta1([FeGlyNTA]-) = 27.23+-0.69, log beta2([Fe(Gly)2NTA]2-) = 30.29+-0.77 ; for iron(II) log beta1([FeGlyNTA]2-) = 14.13+-0.43 and log beta2([Fe(Gly)2NTA]3-) = 18.51+-0.51.

Differential pulse polarographic study of mercury complexes with some substituted pyrimidines in binary acetonitrile-dimethylformamide mixtures

Abstract: The complex formation of Hg 2+ ion with five synthesized substituted pyrimidines in binary acetonitrile-dimethylformamide (AN-DMF) mixtures was studied by differential pulse polarography at 25°C. The stoichiometry and stability of the complexes were determined by monitoring the shift in the Hg 2+ differential pulse peak potential against the pyrimidines concentration. In all cases studied, it was found that the stability of the resulting 1:1 complex decreases drastically by increasing the amount ofdimethylformamide in the binary mixtures. The observed stability order in a given solvent mixture is discussed in terms of the solvating ability of the solvent, donor site number, and steric hindrance on the pyrimidines.