Isomer-selective carbon-paste electrodes for the determination of nitrophenol, nitroaniline, and nitrobenzoic acid by adsorption-stripping voltammetry

The preconcentration and voltammetric behavior of aromatic nitro compounds at paste electrodes made of synthetic carbon materials were studied. The relationship between the energies of adsorption of molecules on active carbons and the peak current constants was determined. Conditions for the isomer-selective determination ofo- andp-isomers of nitrophenol, nitroaniline, and nitrobenzoic acid in aqueous solutions were optimized.     

Electrocatalytic Oxidation of Cysteine and Cystine at a Carbon-Paste Electrode Modified with Ruthenium(IV) Oxide

The electrocatalytic activity of ruthenium(IV) oxide incorporated into a carbon-paste electrode was studied in the oxidation of cysteine and cystine. The oxidation potentials of the amino acids decreased and the current peaks of their oxidation increased at a modified electrode as compared to an unmodified one. Procedures for the voltammetric determination of cysteine and cystine with the use of electrodes chemically modified with ruthenium(IV) oxide were proposed.     

Determination of iron(II) with chemically-modified carbon-paste electrodes

The utility of carbon-paste electrodes modified with 2,2'-bipyridyl and Nafion for the differential pulse voltammetric determination of iron(II) in aqueous medium is demonstrated. The method is based on formation of the 2,2'-bipyridyl complex of iron(II) and its accumulation by the Nafion. The differential pulse voltammetric response of the accumulated complex is used as the analytical signal. The response was evaluated with respect to carbon-paste composition, preconcentration time, pH, iron(II) concentration and other variables. A 3-min accumulation period permits measurement of iron(II) down to 10(-8)M, and a relative standard deviation of 3.8% for 2 x 10(-6)M iron(II). Rapid and convenient chemical renewal allows use of a single modified carbon-paste electrode in multiple analytical measurements over several days. The proposed procedure was applied to the determination of iron in certified standard reference materials and trace iron in natural waters.     

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).     

Electroanalysis at modified carbon-paste electrodes containing natural ionic polysaccharides

The strong affinity of natural ionic polysaccharides for certain metal ions is exploited in the design of a new class of voltammetric sensing devices. In particular, carbon-paste electrodes containing pectic and alginic acids are used for the nonelectrolytic collection and subsequent voltammetric determination of copper and lead, respectively. Cyclic and differential pulse voltammetry are used to quantify the accumulated ions. The response is characterized with respect to modifier loading, preconcentration period, metal concentration, reproducibility, possible interferences and other variables. Titrimetric experiments illustrate the potential of polysaccharide electrodes for speciation work. Preliminary data are also given for analogous measurements of copper at heparin-modified electrodes. Detection limits are 1 mug/ml and the relative standard deviation is 4.8%.     

Screen-printed voltammetric sensor for TNT

Screen-printed carbon electrodes have been developed as disposable voltammetric sensors for 2,4,6-trinitrotoluene (TNT). Thick-film electrodes based on various conventional and modified inks have been compared for this task. The operation is based on placing the selected thick-film carbon sensor in the non-deaerated/quiescent sample and using a fast (<1 s) and sensitive square-wave voltammetric scan. Different experimental variables have been optimized to yield a detection limit of 200 ppb TNT and a wide linear range. The high selectivity, demonstrated in assays of various untreated environmental samples, is attributed to the facts that the reducible nitro group is rare in nature and that most electroactive organic compounds require higher potentials. The new single-use sensor strips should facilitate the on-site environmental screening of TNT.     

Nitro radical anion formation from nitrofuryl substituted 1,4-dihydropyridine derivatives in mixed and non-aqueous media

Three new nitrofuryl substituted 1,4-dihydropyridine derivatives were electrochemically tested in the scope of newly found compounds useful as chemotherapeutic alternative to the Chagas' disease. All the compounds were capable to produce nitro radical anions sufficiently stabilized in the time window of the cyclic voltammetric experiment. In order to quantify the stability of the nitro radical anion we have calculated the decay constant, k2. Furthermore, from the voltammetric results, some parameters of biological significance as E7(1) (indicative of in vivo nitro radical anion formation) and KO2 (thermodynamic indicator of oxygen redox cycling) have been calculated. From the comparison of E7(1), KO2 and k2 values between the studied nitrofuryl 1,4-DHP derivatives and well-known current drugs an auspicious activity for one of the studied compounds i.e. FDHP2, can be expected.     

Voltammetric identification of motor oils using an “electronic tongue”

The possibility of the voltammetric identification of motor oils using a chemometric method of the principal component analysis and voltammograms of metal ions and organic markers (Cu(II), Pb(II), o- and p-aromatic nitro compounds) on carbon paste electrodes containing analytes as binders.     

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.     

Electrochemical Study of 4‐Substituted Analogues of Megazol

The electrochemical behavior of three different megazol analogues substituted at position 4 and their comparison with the parent compound megazol in protic and aprotic media by cyclic voltammetry, Tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in both media with the reduction of the nitroimidazole group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized only in aprotic media for all these compounds. By applying cyclic voltammetric methodology we have calculated the dimerization reaction decay constants (k₂) of the corresponding nitro radical anions in aprotic media. The nitro radical anion obtained from the synthesized nitroimidazole compound having a bromine substituent in 4‐position (GC‐141) was significantly more stable than the corresponding radical formed from the compound lacking of the substituent in 4‐position, megazol.     

Factors affecting the analytical applications of zeolite-modified electrodes preconcentration of electroactive species

The influence of various experimental parameters on the electrochemical response of zeolite-modified electrodes (ZMEs) have been studied using ion-exchange voltammetry, with regard to their application as amperometric sensors. In order to cover a large range of parameters, the investigations were carried out using three zeolites (A, X and Y), selected for their different cation-exchange capacities and ion-sieving properties; four electroactive species (silver(I), copper(II), dopamine and methylviologen) chosen on the basis of their size, charge and inorganic or organic nature; and two kinds of ZMEs: the zeolite-modified carbon-paste electrode (ZMCPE) and the zeolite monograin layers coated on glassy carbon electrode (ZMGCE). Compared to the corresponding unmodified electrodes, improvements in sensitivity were observed when using ZMEs, due to analyte accumulation at the electrode's surface by ion exchange in zeolite particles. The concentration process, and the subsequent voltammetric response of ZMEs, were found to be strongly affected by the accessibility of the analytes to the ion-exchanging sites and their mobility within the zeolite framework. Good correlation was found between results obtained in ion-exchange voltammetry and conventional ion-exchange characterization. The advantage of these fundamental studies for electroanalytical purposes was exemplified for the determination of copper(II) ions, using both ZMCPE and ZMGCE, zeolites A, X and Y, and the following analytical scheme: “preconcentration/voltammetric detection”.     

Reagentless enzymatic sensors based on carbon-paste electrodes containing ruthenium mediators for the on-line determination of glycerol

A new design is proposed for biosensors based on carbon-paste electrodes enriched with ruthenium-containing acyclic compounds. Two biologically sensitive elements, a fragment of the cellular wall of Gluconobacter sp.33 containing a new enzyme PQQ-dependent glycerol dehydrogenase and a commercially available preparation of NAD-dependent glycerol dehydrogenase, are analyzed and compared. The activity of biologically sensitive preparations is studied using artificial electron carriers represented by ruthenium-containing acyclic and coordination compounds.     

An enantioselective voltammetric sensor for the recognition of propranolol stereoisomers

An enantioselective voltammetric sensor for the recognition of propranolol stereoisomers is developed based on a carbon-paste electrode modified with uracil supramolecular structures and the use of chemometric methods. The conditions for the formation of an enantioselective selector on the surface of graphite (graphene) particles and the selective registration of voltammograms of enantiomers, as well as the possibility of the recognition thereof, are studied by methods of molecular dynamics simulation and voltammetry.     

Voltammetric Study of Nitro Radical Anion Generated from Some Nitrofuran Compounds of Pharmacological Significance

The electrochemical behavior of 2‐(5‐amino‐ 1,3,4‐oxadiazolyl)‐5‐nitrofuran (NF359) and its comparison with well‐known drugs such as nifurtimox (NFX) and nitrofurazone (NFZ) in protic, mixed and aprotic media by cyclic voltammetry, tast and differential pulse polarography was studied. All the compounds were electrochemically reducible in all media being the reduction of the nitrofuran group the main voltammetric signal. The one‐electron reduction couple due to the nitro radical anion formation was visualized in mixed (for NF359 and NFZ) and aprotic media (for all compounds). By applying a cyclic voltammetric methodology we have calculated the decay constants (k₂) of the corresponding nitro radical anions in mixed and aprotic media. In mixed medium data fit well with a disproportionation reaction of the nitro radical anion but in aprotic medium fit better with a dimerization reaction. Also, considering cyclic voltammetric measurements in aprotic media we have estimated the reduction potential of the RNO₂/RNO₂^.? couple in aqueous medium, pH 7 (E¹₇ values) finding very good correlation with E¹₇ values obtained by pulse radiolysis. Furthermore we have calculated the equilibrium constants from the electron transfer from nitro radical anion to oxygen (k_O2) finding that nitro radical anion from NF359 is thermodynamically favored to react with oxygen in respect to both NFZ and NFX.     

A Sensor for the Recognition and Determination of Tryptophan Enantiomers Based on Carbon-Paste Electrode Modified by Enantiomorphic Crystals of Bromotriphenylmethane

Journal of Analytical Chemistry - We developed an enantioselective voltammetric sensor based on a carbon-paste electrode modified by enantiomorphic crystals of bromotriphenylmethane for the...     

Simultaneous determination of phenolic compounds in water by normal and derivative flow injection/cyclic votammetry

The possibilities for using fast-scan voltammetric detectors in flow injection analysis for individual and multicomponent determination of phenolic compounds are explored. A cyclic voltammetric unit for potential scans is connected to a three-electrode cell (carbon-paste working electrode, silver/silver chloride reference electrode and glassy-carbon auxiliary electrode). The resolution and hence the selectivity obtainabel from the electrode processes can be improved by derivatizing the current/potential recordings. Individual and simultaneous determinations of phenol, guaiacol and 2,4-dichlorophenol at levels lower than 1 μg ml^?1 can be achieved by judicious selection of measurement potentials, and in worst cases, by use of simultaneous equations.     

A new method for the voltammetric determination of nitrite

A voltammetric method is presented for the determination of nitrite by use of a carbon-paste electrode chemically modified with an anion-exchanger. Because it is possible to accumulate nitrite on the electrode surface, trace nitrite concentrations down to 1 ng/ml can be determined.     

Fast-scan a.c. voltammetry for better resolution of chromatographically overlapping peaks

A fast-scan phase-sensitive a.c. voltammetric detector for high-performance liquid chromatography is described. The mercury drop electrode is periodically polarized over a certain potential range and only the maximum current value of each scan is plotted. The technique makes it possible to resolve chromatographically overlapping peaks provided that the voltammetric peak potentials of the corresponding compounds differ enough. Practical examples of application of the technique to nitro compounds are reported, and limitations are discussed.     

Pulse Voltammetric Determination of Thiazolidine-4-Carboxylic Acids

Abstract

The electrochemical behaviour of thiazolidine-4-carboxylic acid (thioproline, Thz) and its derivatives 2-propyl-thiazolidine-4-carboxylic acid (PrThz) and 2-tetrahydroxybutyl-thiazolidine-4-carboxylic acid (AThz) has been studied by cyclic, normal and reverse pulse, differential pulse and Osteryoung square wave voltam-metrics. Anodic waves, resulting from the formation of adsorbed mercury thiazolidates, were obtained for all the compounds at mercury electrodes in acetate and phosphate buffers at pH from 3.8 to 8. the initially formed mercury compounds rapidly transform into the final products accumulating on the electrode surface. the oxidation products inhibit the anodic processes and distorted voltammetric curves were obtained at the covered electrodes. Various effects of these processes in various pulse voltammetric techniques were evident. Normal pulse voltammograms were least affected by the product transformations. Osteryoung square wave voltammetry permitted very fast determinations of Thz derivatives.     

Differential pulse voltammetric determination of selected nitrophenols on novel type of porous silver working electrode prepared by powder metallurgy

Nitrophenols are important environmental pollutants and their monitoring is important because of their genotoxic and ecotoxic properties. Easy electrochemical reduction of nitro groups can be used for their voltammetric determination using mercury based electrodes. However, requirements of green analytical chemistry prompted us to investigate a novel type of silver porous electrode (AgPE) prepared by powder metallurgy compatible with both “green” and “white” analytical chemistry requirements. In this paper, AgPE was for the first time successfully used for differential pulse voltammetric determination of micromolar concentrations of 2-nitrophenol (NP), 2,4-dinitrophenol (DNP), and 2,4,6-trinitrophenol (TNP) in aqueous media. The main advantage of the novel method is the possibility to use small sample volume (down to 25 μL) and to work in the presence of oxygen when using supporting electrolyte of pH 3. This advantage partially compensates the fact that the obtained sensitivity and limit of detection are not better than with the previously investigated electrodes.