Electrochemical characterization and analytical application of arsenopyrite mineral in non-aqueous solutions by voltammetry and potentiometry

Based on the cyclic voltammetric method, in the present study we have employed carbon paste for arsenopyrite mineral characterization in non-aqueous solution. Arsenopyrite yields well-defined cyclic voltammetric responses with well-defined oxidation (in the potential region from −0.7 to 0.7 V, versus Ag/AgCl) and reduction (from −1.0 to 0.8 V) peaks using this electrode. In addition, arsenopyrite mineral was studied as a new indicator electrode for the potentiometric titrations of acids (benzoic, anthranilic and salicylic acids) and bases (N,N′-diphenylguanidine, tributylamine and collidine) in acetonitrile and propionitrile. Potassium hydroxide, tetrabutylammonium hydroxide (TBAH) and perchloric acid proved to be very suitable titrating agents for these titrations.The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid concentrations in the range 0.1-0.001 M, with a Nernstian slope of 38.5 mV in acetonitrile and 44.6 mV in propionitrile. The electrode showed a relatively fast response time and can be used without any time limit or without considerable divergence in potentials. The response time of the electrode was less than 10 s in both solvents. The standard deviation of the determination of the investigated acids and bases was less than 0.6% from those obtained with a glass electrode.The advantages of the electrodes are long-term stability, fast response and reproducibility, while the sensors are easy to prepare and are of low cost.     

Flow injection analysis of sulfite ion with a potentiometric titanium phosphate-epoxy based membrane sensor

A novel pH sensor suitable for use in both aqueous and non-aqueous mediums is reported. The sensor is derived from polymer modified electrode obtained from electrochemical polymerisation of aniline in dry acetonitrile containing 0.5 M tetraphenyl borate at 2.0 V versus Ag/AgCl. The light yellow colour polymer modified electrode obtained under the present experimental condition has been characterised by scanning electron microscopy (SEM). The pH sensing of polymer modified electrode in both aqueous and non-aqueous mediums is examined and reported. As the typical examples, we used weak acid (acetic acid) and weak base (ammonium hydroxide) as analytes. The acetic acid is analysed in both aqueous and dry acetonitrile whereas ammonium hydroxide is analysed only in aqueous medium. The analysis in aqueous medium is conducted in 1 mM Tris-HCl buffer pH 7.0 and also in 0.1 M KCl. The slope of pH sensing is calculated from the data recorded in typical buffers and found to be approximately 86 mV per pH. The application of polymer modified electrode for the construction of urea biosensor is described based on immobilised urease within poly vinyl alcohol (PVA) matrix and also within organically modified sol-gel glass on the surface of polymer-modified electrode. The new urea sensor has shown maximum response of 160 mV at 25 degrees C with a lowest detection limit of 20 muM. The performance of new pH sensor and urea sensor has been studied and reported in this communication.     

Electrochemical studies and square wave adsorptive stripping voltammetry of the antidepressant fluoxetine

The electrochemical reduction of the antidepressant drug fluoxetine was investigated by cyclic, linear sweep, differential pulse and square wave voltammetry using a hanging mercury drop electrode in alkaline buffer solution in water and in a water/acetonitrile mixed solvent. Cyclic voltammograms in aqueous solution showed very strong adsorption of fluoxetine on the electrode with formation of a compact film. The effect of addition of different percentages of acetonitrile on the voltammetric response was evaluated. It is shown that acetonitrile protects the electrode surface, thus preventing the adsorption of fluoxetine as a compact film, although reduction occurs at more negative potentials. Adsorption was used to accumulate the drug onto the electrode surface. The adsorbed species were measured voltammetrically by reduction at -1.3 V in an aqueous 0.05 M Ringer buffer, pH 12, 20% acetonitrile v/v. Linear calibration graphs were obtained in the range 0.52-5.2 M. The quantification of fluoxetine in pharmacological formulations existing in the market was performed using adsorptive square wave cathodic stripping voltammetry. and compared with data from UV spectrophotometry. The method is simple and not time-consuming. A comparative high performance liquid chromatography assay with UV detection was performed. Recovery data for both methods are reported.     

A deuterium–palladium electrode as a new sensor in non-aqueous solutions: potentiometric titration of weak acids in N,N-dimethylformamide and N-methylpyrrolidone

A deuterium–palladium electrode was employed as a new indicator electrode for the titration of weak acids in N,N-dimethylformamide and methylpyrrolidone. The investigated electrode showed a linear dynamic response for p-toluenesulfonic acid in the concentration range from 0.1 to 0.001 M, with a nernstian slope of 78.0 mV in N,N-dimethylformamide and of 64 mV per decade in N-methylpyrrolidone. Sodium methylate, potassium hydroxide, and tetrabutylammonium hydroxide proved to be very suitable titrating agents for these titrations. The potential in the course of the titration and at the titration end point was rapidly established. The response time was less than 10–11 s, and the lifetime of the electrode is long. The experimental results obtained for the proposed electrochemical sensor and a conventional glass electrode were in good agreement. The advantages of the electrode are log-term stability, fast response, reproducibility, and easy preparation.     

Enhanced conductivity of a glassy carbon electrode modified with a graphene-doped film of layered double hydroxides for selectively sensing of dopamine

A strategy is presented for doping graphene into layered double hydroxide films (LDHs) as a means of improving charge transport of the LDH film in a modified glassy carbon electrode. This result in an enhanced electrocatalytic current for dopamine (DA) and a good separation of the potentials of DA, uric acid and ascorbic acid. Under selected conditions, the square wave voltammetric response of the electrode to DA is linear in the concentration range from 1.0 to 199???M even in the presence of 0.1?mM ascorbic acid, and the detection limit is 0.3???M at a signal-to-noise ratio of 3. The method was applied to the determination of DA in pharmaceutical injections with satisfactory results.

New cesium ion-selective electrodes based on anilino-(1,3-dioxo-2-indanylidene) acetonitrile derivatives

Cesium ion-selective PVC membrane electrodes based on anilino-(1,3-dioxo-2-indanylidene) acetonitrile derivatives as a novel class of neutral ionophores were examined. The ionophores were p-methoxyanilino-(1,3-dioxo-2-indanylidene) acetonitrile, p-methylanilino-(1,3-dioxo-2-indanylidene) acetonitrile and p-N,N-dimethylanilino-(1,3-dioxo-2-indanylidene) acetonitrile. The anilino-(1,3-dioxo-2-indanylidene) acetonitrile proved to work well with cesium, the corresponding electrodes display a response to this ion. The most favourable ionophore was p-methoxyanilino-(1,3-dioxo-2-indanylidene) acetonitrile, especially when the secondary ion exchanger potassium tetrakis (4-chlorophenyl) borate was incorporated in 2-nitrophenyl octyl ether for ion-selective electrode membrane construction. The response function was linear within the concentration range 10(-1)-2.5x10(-5) mol l(-1) and the slope was 52 mV decade(-1). The detection limit remained at 6.3x10(-6) mol(-1). The selectivity and response time of the electrode was studied and it was found that the electrode exhibited good selectivity for cesium over alkali, alkaline earth and some transition metal ions. The electrode response was stable over a wide pH range. The lifetime of the electrode was about 1 month.     

A Sensitive Simultaneous Determination of Epinephrine and Piroxicam Using a Glassy Carbon Electrode Modified with a Nickel Hydroxide Nanoparticles/Multiwalled Carbon Nanotubes Composite

The electrooxidation of epinephrine (EPI) and piroxicam (PRX) has been investigated by application of nickel hydroxide nanoparticles/multiwalled carbon nanotubes composite electrode (MWCNTs‐NHNPs/GCE) using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry (CA) methods. The modified electrode showed suitable electrochemical responses for EPI and PRX determination. Under the optimum conditions the electrode provides a linear response versus EPI and PRX concentrations in the range of 1–220 µM and 0.7–75 µM, respectively using the DPV method. Linear responses versus EPI and PRX concentrations in the range of 1–1000 µM and 1–800 µM, respectively, were obtained using the CA method. The modified electrode was used for determination of EPI and PRX in human urine with satisfactory results.     

Electrochemical Oxidation of Epinephrine and Uric Acid at a Layered Double Hydroxide Film Modified Glassy Carbon Electrode and Its Application

Increasing attention has been paid to layered double hydroxide (LDH) film modified electrode attributing to its desirable properties for fabrication of electrochemical sensor. In this paper, the Zn‐Al LDH film modified glassy carbon electrode was characterized by electrochemical methods. The enhanced electrocatalytic currents and well‐separated potentials for epinephrine (EP) and uric acid (UA) were observed at the as‐prepared electrode. Under selected condition, the differential pulse voltammetry response of the modified electrode to EP (or UA) shows a linear concentration range of 0.5 μM to 0.3 mM (or 2 μM to 0.4 mM) in the presence of 10.0 μM UA (or 20.0 μM EP). At a signal‐to‐noise ratio of 3, the calculated limits of detection are 0.13 μM and 0.66 μM, respectively. The proposed method has been performed to successfully detect EP and UA in analysis of real samples, such as in EP injection solution and human urine samples.     

Single step modification of copper electrode for the highly sensitive and selective non-enzymatic determination of glucose

A non-enzymatic sensor was developed for the determination of glucose in alkaline medium by anodisation of copper in sodium potassium tartrate solution. The morphology of the modified copper electrode was studied by scanning electron microscopy, and its electrochemical behavior by cyclic voltammetry and electrochemical impedance spectroscopy. The electrode enables direct electrocatalytic oxidation of glucose on a CuO/Cu electrode at 0.7 V in 0.1 M sodium hydroxide. At this potential, the sensor is highly selective to glucose even in the presence of ascorbic acid, uric acid, or dopamine which are common interfering species. The sensor displays a sensitivity of 761.9 μA mM^?1 cm^?2, a linear detection range from 2 μM to 20 mM, a response time of <1 s, and a detection limit of 1 μM (S/N = 3). It was tested for determination of glucose level in blood serum.     

Flow injection,amperometric determination of ethanol in wines after solid-phase extraction

Ethanol in wines was determined by flow injection analysis with an amperometric detector using an oxidized nickel wire. Solid-phase extraction with a strong anion exchanger was used to remove interferences such as organic acids from the matrix, and the residue of the extraction was injected directly into the FIA system. The recoveries of ethanol from wines spiked with standards ranged from 101% to 103%. The response of the nickel electrode to ethanol is dependent on the applied potential and the pH of the carrier. The optimal conditions for the detection of ethanol were an applied potential of +0.60 V (vs. Ag/AgCl) in a carrier of 100 mM sodium hydroxide solution. The electrode exhibited a linear response from 10⁻⁵ to 10⁻³ M, with a detection limit of 1 × 10⁻⁶ M. The method was demonstrated by the determination of ethanol in wines.     

Rate Study of the Alkline Hydrolysis of Nicotinamide Using an Ammonia Gas-Sensing Electrode

Abstract

The hydrolysis of nicotinamide in alkaline solutions was studied. An ammonia gas-sensing electrode was used to follow the formation of ammonia. A technique making use of simulated reactions has been developed to calibrate the electrode under dynamic conditions overcoming problems arising because of the relatively slow response of the sensor. A general expression has been derived for the pseudo first-order rate constant valid over the concentration range 0.005 to 0.10 M nicotinamide, 0.1 to 0.5 M hydroxide and the temperature range 22° to 31° C, under constant ionic strength (0.5 M NaOH + NaC1O₄).     

Anthill Earth as a Gold Occurrence Indicator,and Gold Determination by Solid Sampling Flame Atomic Absorption Spectrometry

With the aim of avoiding cumbersome sample treatment, we present a device for the introduction of solid soil samples into AAS-flames for gold determination, as well as the proposition of earth from anthills as a gold occurrence indicator. A previous ground sample of anthill earth (0.50 mg) was weighed directly into a small recipient of polyethylene which was then connected to a sampling boronsilicate glass chamber. The sample was carried by an airflow (5 L min⁻¹) to a quartz cell positioned between the burner top and the optical beam. The generated atomic vapor produced a transient signal which was totally integrated in three seconds. The performance was compared with conventional flame atomic absorption spectrometry after proper sample digestion. No significant differences were observed between both procedures (mean deviation ±1.90%), and a LOQ of 0.03 μg Au was achieved using the proposed method. The anthill earth was found to be very suitable for indication of gold occurrence in soils and related materials.     

A New and Cost Effective Approach for Simultaneous Voltammetric Analysis of two Related Benzimidazole Drugs and their Determination in Biological Fluids

A polymerized film of eriochrome black T (EBT) was prepared on the surface of pencil graphite electrode in alkaline solution by cyclic voltammetry. The redox response of the poly (EBT) film at the electrode appeared in a couple of redox peak in 0.1 M sodium hydroxide. The poly (EBT) film‐coated electrode exhibited excellent electrocatalytic activity towards the oxidation of rabeprazole sodium (RAB sodium) and domperidone (DOM) in Britton‐Robinson buffer (pH 4.0). The polymer film modified electrode conspicuously enhanced the redox currents of the cited mixture and could sensitively and separately determine them. Both cyclic voltammetry (CV) and square wave adsorptive stripping voltammetric (SWAdSV) methods were utilized to determine this mixture. The linearity of CV ranged from 4.1‐120 μM and 5.2‐90 μM for RAB sodium and DOM, respectively while SWAdSV was 7.5‐80×10⁻⁷M and 5–70×10⁻⁷M for RAB sodium and DOM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of RAB sodium and DOM binary mixture in synthetic mixtures and biological fluids.     

Simultaneous determination of amoxicillin and ampicillin in eggs by reversed-phase high-performance liquid chromatography with fluorescence detection using pre-column derivatization

A sensitive and robust method is presented for the simultaneous determination of amoxicillin (AMO) and ampicillin (AMP) in eggs by reversed-phase high-performance liquid chromatography with fluorescence detection (RP-HPLC-FLD). This method used a simple liquid-liquid extraction of the samples with acetonitrile and dichloromethane as precipitation of proteins and extraction solvent. AMO and AMP reacted with salicylaldehyde to form fluorescent derivatives, which were then analyzed with RP-HPLC-FLD. Separation was carried out on an Athena C18 column with a mobile phase consisting of 0.01 M potassium dihydrogen phosphate, adjusted to pH 5.5 by 2M potassium hydroxide and acetonitrile. The detector response was linear over the tested concentration range from 5.0 to 800 ng/mL for AMO and AMP. The recovery values ranged from 78.4 to 88.7% for AMO and from 77.6 to 82.0% for AMP. The limits of detection were 1.2 for AMO and 0.4 μg/kg for AMP. The limits of quantification were 3.9 for AMO and 1.5 μg/kg for AMP. The corresponding intra-day and inter-day variation (relative standard deviation) were found to be less than 9.6 and 14.8%, respectively.     

Electrochemical Behavior and Determination of Hyoscine‐N‐Butylbromide from Pharmaceutical Preparations

The electrooxidation of hyoscine N‐butylbromide (HBB) was investigated by rotating disk electrode voltammetry, cyclic voltammetry and controlled potential coulometry in 0.1 M HNO₃ and in 0.1 M tetrabutylammonium perchlorate (TBAP) solutions of acetonitrile at a platinum (Pt) electrode. Based on the results obtained, it is suggested that a bromide ion of HBB was oxidized in one reversible step in aqueous solutions and in two reversible steps in acetonitrile. A differential pulse voltammetric (DPV) method at a Pt electrode was developed for the determination of HBB in the concentration range of 1.0 × 10^?6‐1.0 × 10^?3 M. The procedure was applied to the determination of HBB in its formulations as well as its recovery from blood serum and urine samples.     

Complex formation of univalent cations in acetonitrile with other solvents. Study of potassium ion complex formation with a cation-sensitive glass electrode

The formation constants of the complexes formed between potassium ion and other solvents in acetonitrile have been obtained potentiometrically by using a univalent cation-sensitive glass electrode. The formation constants of the mono-complexed potassium ions were 1.5 with dimethylformamide, 2.0 with dimethylacetamide, 2.4 with N-methylpyrrolidone, 2.8 with N-methylformamide, 2.9 with dimethyl sulfoxide, and 6.9 with hexamethylphosphoramide. Within the concentration ranges of the solvents studied (0.35–0.7 M for different solvents), only up to 1:2 K⁺: solvent species were detected, in contrast to the results for other univalent cations. Rubidium ion could not be studied effectively because of unsatisfactory response of the glass electrode. The complex formation constants of various univalent cations in acetonitrile with other solvents are summarized and discussed in detail.     

Electrochemical detection of cyanogenic glycosides after enzymatic post-column cleavage

Crude and partly purified extracts from Helix pomatia and linamarase from cassava were immobilized on columns packed with porous glass or silica and used as post-column reactors in the high-performance liquid chromatography of cyanogenic glycosides. Sodium hydroxide (2 M) was added to the flowstream after the enzyme-reactor resulting in the formation of cyanide, which was then detected at a silver electrode by an amperometric measurement at 0 V with reference to a silver-silver chloride electrode. The selective detection of cyanide allows measurements in a complex matrix. The response is linear and the detection limit is in the low picomole range.     

Electrochemical Determination of Hydrogen Peroxide Using a Glassy Carbon Electrode Modified with Three-Dimensional Copper Hydroxide Nanosupercages and Electrochemically Reduced Graphene Oxide

Three-dimensional copper hydroxide nanosupercages and electrochemically reduced graphene oxide were used to modify the glassy carbon electrode for the selective determination of hydrogen peroxide. The morphology and electrochemistry properties of copper hydroxide nanosupercage/electrochemically reduced graphene oxide/glassy carbon electrode were characterized using transmission electron microscopy, scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectra, Raman spectra, cyclic voltammetry, and electrochemical impedance spectroscopy. The resulting copper hydroxide nanosupercage/electrochemically reduced graphene oxide/glassy carbon electrode showed favorable performance for the electrocatalytic reduction of hydrogen peroxide. The amperometric current–time curve of the electrochemical sensor exhibited a wide linear range from 0.5 to 1030?µM with a limit of detection of 0.23?µM at a signal-to-noise ratio of three. Moreover, the sensor provided favorable selectivity, reproducibility, and stability and was used for the determination of H₂O₂ in tap water.     

Horseradish peroxidase-based organic-phase enzyme electrode

An organic-phase enzyme electrode (OPEE) based on horseradish peroxidase (HRP) immobilized within Nafion on spectroscopic graphite was investigated in acetonitrile. The amperometric electrode response to hydrogen peroxide and cumene hydroperoxide present was found to be the result of the reduction of oxygen, produced upon enzymatic decomposition of both hydroperoxides (i.e., by the catalase-like activity of HRP). The electrode response was found to depend linearly on the hydroperoxide concentration up to 700 M within the range of potentials from –200 to –400 mV (versus Ag|AgCl). Detection limits of approximately 45 M for H₂O₂ and 100 M for cumene hydroperoxide were determined under the selected experimental conditions. Nernstian dependence (the open circuit voltage of HRP-based electrode versus logarithm of H₂O₂ concentration) was obtained between 0.2 and 2.0 mM, with a slope of approximately 23 mV per logarithmic unit, suggesting a catalase-like, two-electron disproportionation of the substrate in acetonitrile.     

Preparation of cobalt hydroxide film modified electrode and its analytical application

Cobalt hydroxide film modified electrode was prepared by depositing cobalt hydroxide on glassy carbon electrode (GCE) surface in an alkaline aqueous solution and then characterized by cyclic voltammetry. The electrochemical behavior of resorcin on the film modified electrode was investigated. The results show that cobalt hydroxide films in alkaline solutions have good electrocatalytical activity towards the oxidation of resorcin. The recovery of resorcin from sample ranged from 95.2 to 103.4% and the oxidation peak currents were directly proportional to the resorcin concentration from 5.0 × 10⁻⁶ to 1.05 × 10⁻⁴ M with correlation coefficient of 0.9986. A detection limit of 1.0 × 10⁻⁷ M for resorcin was estimated. Various factors affecting the electrocatalytical activity of cobalt hydroxide film were investigated in detail. Real water samples were analyzed and satisfactory results were obtained.