Showing papers on "Polarography published in 2001"

Polarography and Voltammetry at Mercury Electrodes

Abstract: Scope and limitations of modern polarographic and voltammetric techniques on mercury electrodes are discussed and many practical examples of their applications in practical analysis are given to demonstrate that even in the third millennium polarography and voltammetry at mercury electrodes can be very useful analytical tools, which in certain cases can successfully compete with modern spectroscopic and separation techniques.

Determination of metallothionein at the femtomole level by constant current stripping chronopotentiometry.

Abstract: Metallothionein (MT) from rabbit liver was analyzed by differential pulse polarography, cyclic voltammetry, square wave voltammetry, and chronopotentiometric stripping analysis (CPSA) with a hanging mercury drop electrode under various conditions. The highest sensitivity of the MT determination was obtained with CPSA which produced a well-developed peak H due to catalytic hydrogen evolution at highly negative potentials. The highest peak H was obtained in borate buffer close to pH 8.0. In this medium, subnanomolar concentrations of MT were detectable. In the adsorptive transfer stripping (medium exchange) experiments, determination of few femtomoles of MT in 5-microL aliquots of the analyte was possible. CPSA determination of MT in various tissues of carp (Cyprinus carpio) yielded values in agreement with the published data.

Electrochemical methods for monitoring of environmental carcinogens

Abstract: The use of modern electroanalytical techniques, namely differential pulse polarography, differential pulse voltammetry on hanging mercury drop electrode or carbon paste electrode, adsorptive stripping voltammetry and high performance liquid chromatography with electrochemical detection for the determination of trace amounts of carcinogenic N-nitroso compounds, azo compounds, heterocyclic compounds, nitrated polycyclic aromatic hydrocarbons and aromatic and heterocyclic amines is discussed. Scope and limitations of these methods are described and some practical applications based on their combination with liquid-liquid or solid phase extraction are given.

Anodic Stripping Voltammetry with a Heated Mercury Film on a Screen‐Printed Carbon Electrode

Abstract: This article introduces a new heated mercury film electrode based on a screen-printed carbon substrate. The sensor is easy-to-build and the material costs compared to its iridium-based counterpart are low. The development procedure including electrode construction, mercury deposition and thermal properties is described in detail. As first application, cadmium and lead were determined by square-wave anodic stripping voltammetry with significantly improved signal-to-noise characteristics.

Electrochemical studies and differential pulse polarographic analysis of lansoprazole in pharmaceuticals.

Abstract: The electrochemical reduction of lansoprazole was investigated by cyclic voltammetry and direct current and differential pulse polarography. The reduction potential was −1.32 V vs. Ag/AgCl with a dropping mercury electrode in a supporting electrolyte consisting of phosphate buffer (pH 9.0)–tetramethylammonium iodide (4 + 1). The reversibility of the electrode reaction and the type of limiting current were studied. The temperature coefficient and the diffusion constant were determined. A mechanism for the electrode reaction was proposed. A new simple and sensitive differential pulse polarographic method was also developed for the quantification of lansoprazole. A linear calibration graph was obtained in the range 0.04–11.35 μg ml−1. The limit of detection was 0.03 μg ml−1 and the intra- and inter-day precisions were 0.84–2.32 and 0.72–3.09%, respectively. The developed method was applied to six different commercial pharmaceutical capsule preparations containing enteric-coated granules. The relative standard deviations ranged from 1.36 to 2.85%. Recovery studies for the accuracy of the method were performed by adding a synthetic mixture to known amounts of lansoprazole and the mean recovery was 100.45%. The data obtained from commercial preparations were compared with those from a published spectrophotometric method. No difference was found statistically.

Kinetics and electrochemical studies on superoxide

Abstract: Rate constants for the reaction of superoxide O-2 with various substrates were obtained through stationary electrode polarography theory and technique. In solvent acetonitrile, the substrate and the rate constants of the reaction O-2 + AH-k2→Product, are, AH = isopropanol (k2 < 0.01 M-1 s-1); ethanol (k2 = 1.42 × 102 M-1 s-1); methanol (k2 = 1.1 × 107 M-1 s-1), H2O (k2 = 1.0 × 105 M-1 s-1). In MeCN, O-2 was found to be rather unreactive towards glucose and acetone but it reacts with fructose and sucrose catalytically. However, in DMF2, O-2reacts with glucose and fructose with k2 order of 105 M-1 s-1. The mechanism of the reaction of O-2 with the substrates (AH) is proposed as O-3 + AH k2O, AHk2k-1 k′ [O2H + AH]-, k-2→O2H + A- with k1 = 109 M-1 s-1 and k-1 = 108 -109 s-1. With these values of k-1 and k1, k′ k2(obs). The reversible E1/2 for O2 + e O-2 in various solvents: MeCN, acetone, isopropanol, methanol, H2O were obtained either directly from the reversible voltammogram or from experimental voltammograms and the rate constants obtained (as above) using stationary electrode polargraphy theory; E1/2 being -0.82 (MeCN),-0.85 (acetone),-0.72 (isopropanol);-0.66 (MeOH),-0.56 (H2O) vs SCE.

Carbon paste electrode for trace zirconium(IV) determination by adsorption voltammetry

Abstract: A very sensitive and selective procedure was developed for trace measurement of zirconium based on the cathodic adsorptive stripping voltammetry of the zirconium–alizarin red S(ARS) complex at a carbon paste electrode (CPE). The 2nd-order derivative linear scan voltammograms of the zirconium–ARS complex were recorded by a model JP-303 polarographic analyzer from 0.0 to −1.0 V (vs. SCE). Optimal analytical conditions were found to be: an acetic acid (0.1 mol l−1)–potassium biphthalate (0.08 mol l−1) buffer solution (pH 4.8) containing 4.0 × 10−6 mol l−1 ARS; accumulation potential, 0.0 V; accumulation time, 180 or 90 s; rest time, 10 s; scan rate, 250 mV s−1. The results showed that the complex can be adsorbed on the surface of the CPE, yielding one peak at −0.51 V, corresponding to the reduction of ARS in the complex at the electrode. The detection limit was found to be 1.0 × 10−10 mol l−1 (S/N = 3) for 240 s accumulation. The linear range was 2.0 × 10−10–4.0 × 10−7 mol l−1. The developed method was applied to the determination of trace zirconium in the ore samples with satisfactory results.

Studies of the Voltammetric Behavior and Determination of Diazo Reactive Dyes at Mercury Electrode

Abstract: The electrochemical reduction of two reactive dyes: Procion Red HE-3B 9 (RR120) and Procion Green HE-4BD (RG19) was investigated using cyclic voltammetry, differential pulse and DCt polarography, chronoamperometry and controlled potential electrolysis at mercury electrodes. The bis-azo groups of the RR120 dye are reduced together in one single step of four electrons. The bis-azo groups of the RG19 dye are reduced in two steps owing to the difference in the electron densities promoted by the different substituents in the benzene rings adjacent to the azo groups. The bis-monochlorotriazine reactive groups in both dyes are reduced only in acidic medium in their protonated form, leading to the reduction of the triazine groups. The reduction mechanism of both reactive dyes is discussed. Both dyes can be quantified in aqueous medium by differential pulse polarography in the concentration range of 1×10−7 mol L−1 to 1×10−5 mol L−1 by monitoring the reduction of the chromophore group or the reactive group.

Electrolysis with a dropping mercury electrode: J. Heyrovsky's contribution to electrochemistry

Abstract: History of the development of the polarographic method from the birth of Professor Jaroslav Heyrovsky in 1890 until the Nobel Prize award in 1959 is described. Background of the Nobel Laureate's family and education is given. Factors contributing to the discovery and development of the method are discussed.

Polarographic and Voltammetric Determination of Carcinogenic Nitro and Amino Derivatives of Polycyclic Aromatic Hydrocarbons

Abstract: Nitro and amino derivatives of polycyclic aromatic hydrocarbons (NPAH and APAH) are well-known chemical carcinogens which should be carefully monitored in our environment. So far, mostly HPLC methods are used for these purposes, while polarographic and voltammetric methods are used less frequently. This article deals with the use of modern polarographic and voltammetric techniques (namely differential pulse polarography, differential pulse voltammetry, adsorptive stripping voltammetry and HPLC with electrochemical detection) for the determination of selected carcinogenic nitro and amino derivatives of polycyclic aromatic hydrocarbons. Optimum conditions are described for the determination of nanomolar or subnanomolar concentrations of nitro derivatives using mercury electrodes and for the determination of submicromolar concentrations of amino derivatives using solid or carbon paste electrodes or HPLC with electrochemical detection. The selectivity of all these methods can be improved by preliminary separation using liquid–liquid or solid phase extraction.

Electrochemical behavior and voltammetric determination of the herbicide metribuzin at mercury electrodes.

Abstract: The electrochemical behavior of the herbicide metribuzin (4-amino-6-tert-butyl-4,5-dihydro-3-methylthio-1,2,4-triazin-5-one) at mercury electrodes was studied in aqueous solutions by direct current (DC) and tast polarography, differential pulse (DPV) and cyclic voltammetry (CV), and controlled-potential coulometry. The electrolysis products were separated and identified by chromatographic techniques combined with mass spectrometric detection. The reduction process in acid media includes two irreversible steps. In the first four-electron step the N-NH2 and the 1,6-azomethine bonds are reduced. The second step leads to the formation of 5-tert-butyl-2,3,4,5-tetrahydroimidazol-4-one at the mercury-pool electrode. The first reduction step combined with adsorptive accumulation of the herbicide molecule at the mercury electrode surface was used for its determination by differential pulse adsorptive stripping voltammetry (DPAdSV). Calibration curves were linear in the range 1-30 microg L(-1) with a detection limit of 0.27 microg L(-1) (1 nmol L(-1)) under the conditions used (buffer pH 4.5, Eacc = -0.45 V relative to Ag/AgCl and tacc = 10 s). Preconcentration on solid-phase extraction columns (SPE-phenyl) was used for the determination of very small amounts of metribuzin in river water samples. Recovery was approximately 97%. The reproducibility of the analytical procedure including SPE treatment and DPV determination was expressed as relative standard deviations of 2.53 and 3.66% for 2 and 6 microg L(-1) metribuzin, respectively.

Electrochemical determination of trace amounts of environmentally important dyes.

Abstract: A number of dyes exhibit genotoxic or ecotoxic properties leading to the need for sensitive and selective methods for their determination. Because of the easy reducibility of dyes, modern polarographic and voltammetric methods (differential pulse polarography on classical dropping mercury electrode, differential pulse voltammetry on hanging mercury drop electrode or adsorptive stripping voltammetry) are suitable for the determination of trace amounts of these substances in the general environment in the vicinity of production plants. The scope and limitations of these methods is reviewed and optimum conditions for recently developed methods are summarized. It is shown that the sensitivity of newly developed polarographic and voltammetric methods is sufficient even for the most demanding applications and their selectivity can be increased by their combination with preliminary separation using thin layer chromatography or liquid extraction.

The "Presodium" Hydrogen Evolution at the Dropping Mercury Electrode Catalysed by Simple Cysteine Peptides

Abstract: "Presodium" catalytic currents at dropping mercury electrode of cysteine, cysteinylglycine, γ-glutamylcysteine and reduced glutathione were systematically studied in weakly alkaline solutions. They consist in shifts to positive potentials of current due to reduction of the blank solution, and, under some conditions, also in formation of "catalytic prewaves". The two cases have been qualitatively interpreted as based on a suggested common mechanism, differing by the occurrence of weak catalyst adsorption in the prewave case. The catalytic activity depends on the catalyst itself (concentration, structure, conformation at the electrode, partial protonation/ionisation, partial hydrophobic/hydrophilic character) as well as on the solution (pH, ionic strength, nature and concentration of components).

Voltammetric determination of nitroimidazopyran drug candidate for the treatment of tuberculosis

Abstract: PA-824 (2-nitro-6-(4-trifluoromethoxy-benzyloxy)-6,7-dihydro-5H-imidazo [2,1-b][1,3]oxazine) a nitroimidazopyran drug candidate for the treatment of tuberculosis was electrochemically studied with the aims of to propose a new electroanalytical method and to investigate the potential of an electrochemical system to mimic phase I reduction metabo lism in PA-824. The electrochemical behavior was studied using tast polarography, differential pulse polarography, differential pulse voltammetry and cyclic voltammetry. PA-824 was electrochemically reducible showing one reduction peak due to the four-electronreduction of the nitro group to form the hydroxylamine derivative. A method for the determination of PA-824 by differential pulse polarography and differential pulse voltammetry at pH 7 were proposed. The differential pulse polarography method has adequate accuracy and precise with a mean recovery of 101.11% and a standard relative deviation of 2.27%. The detection limit was 2.23 × 10−7 M. Furthermore, for com...

The cyanide photoisomerization in zinc hexacyanoferrate (ii) supported on titanium dioxide-silica gel composite: a matrix effect

Abstract: Zinc hexacyanoferrates(II), ZnHCF, were supported on the surface of titanium(IV) oxide grafted on a silica gel surface, SiO2/TiO2. The adsorbed complex species SiO2/TiO2/ZnHCF was characterized by cyclic voltammetry, differential pulse polarography and Mossbauer spectroscopy. The pentacyanoferrate (II) species would also be formed by an isomerization of one of the cyano groups, which forms a Zn-CN-Fe bond. The effect of supporting electrolyte in the electrochemical behavior of cyanoferrates was investigated using modified carbon paste electrodes. The modified electrode exhibits a quasi-Nernstian response showing the involvement of alkaline metal cations in the electrochemical process. The electrode prepared with the composite ≡TiHCFZn also exhibits electrochemical behavior dependent on acetate anion concentration selectively.

Electrochemical monitoring of the thermal decomposition of arenediazonium ions with time in acidic MeOH/H2O mixtures: dediazotization rate constant.

Abstract: The rate of the spontaneous decomposition of 4-methylbenzenediazonium tetrafluoroborate, 4MBD, was determined. The effects of solvent composition (MeOH/ H2O) on its electrochemical processes and on those of the derivatized 2-naphthol-6-sulfonate-4-toluene (6S2NPT) azo dye were investigated by Differential Pulse Polarography (DPP). This electrochemical technique shows an effective sensitivity for detecting arenediazonium ions and derivatized azo dyes.

The Role of Chemical Reactions Placed Between Successive Electron Transfer in the Reduction of 2-Methylthio-4,6-di(alkylamino)-1,3,5-triazines on Mercury Electrodes

Abstract: This paper presents polarographic [direct current and differential pulse (DP)] studies of the electroreduction of the s-triazine derivatives prometryne [2-methylthio-4,6-di(isopropylamino)-1,3,5-triazine]. desmetryne [2-methylthio-4-methylamino-6-isopropylamino-1,3,5-triazine], and terbutryne [(2-methylthio-4-ethylamino-6-terbutylamino)-1,3,5-triazine] on mercury electrodes. The studies are performed in the acidity range 2.25 M H 2 SO 4 to pH 6.5. Above this last pH value, no signals were obtained. In DP polarography, two main reduction peaks were observed, accompanied by a prepeak, at less negative potentials, due to the adsorption of the herbicides on the electrode. The main peaks corresponded to two-electron irreversible reduction processes, at pH values higher than the protonation pK of the triazine ring (ca. 4). In this pH range, the protonation of the triazine ring precedes the reduction process, this reaction being also responsible for the observed decrease in limiting current. In strongly acidic media (pH < pK) two alternative reaction pathways are considered and the corresponding theoretical expressions are presented. The main conclusion in this pH range is that the herbicides suffer a cleavage of the - -SCH 3 group via two different intermediates related by a chemical reaction, whose importance depends on the herbicide.