High sensitive determination of trace amount of cobalt by catalytic adsorptive stripping voltammetry

A very sensitive and selective catalytic adsorptive cathodic stripping procedure for trace measurements of cobalt is presented. The method is based on adsorptive accumulation of cobalt-CCA (calcon carboxylic acid) complex onto a hanging mercury drop electrode followed by reduction of the adsorbed species by voltammetric scan using differential pulse modulation. The reduction current is enhanced catalytically by nitrite. The effect of various parameters such as pH, concentration of CCA, concentration of nitrite, accumulation potential and accumulation time on the selectivity and sensitivity were studied. The optimum condition for the analysis of cobalt, include pH 5.2 (Acetate buffer), 2.1 μM clacon carboxylic acid, 0.032 M sodium nitrite and an accumulation potential of 0.05 V (versus Ag/AgCl). Under these optimum conditions and for an accumulation time of 60 s, the measured peak current at −0.480 V is proportional to the concentration of cobalt over the entire concentration range tested 0.003–2.0 ng ml⁻¹ with a detection limit of 1 pg ml⁻¹ for an accumulation time of 60 s and 2.0–10.0 ng ml⁻¹ for an accumulation time of 40 s. The relative standard deviations for ten replicate measurement of 0.5 ng ml⁻¹ of cobalt were 3.1%. The main advantage of this new system is the microtrace Co(II) determination by ASV. The method was applied to determination of cobalt in a water sample and some analytical grade salts with satisfactory results. Published in Elektrokhimiya in Russian, 2009, Vol. 45, No. 2, pp. 221–228. The article is published in the original.     

The new composites,polyacetylene-carbon nanotubes: Electrochemical properties

Polyacetylene- and carbon-nanotube-based composite materials are prepared by the method of polymerization filling for the first time. It is shown that the acetylene polymerization mainly occurs at catalytic centers attached to the carbon nanotubes. It follows from TEM data that in the case of single-wall nanotubes the polyacetylene fibriles are wound up onto the nanotubes. In the case of multi-wall nanotubes, polyacetylene can form separate bodies that are connected to the multi-wall nanotubes. The specific electrochemical capacity of the novel composite materials is nearly twice as large as compared with that of the composite prepared by mechanical mixing; it is by two orders of magnitude larger than the pure polyacetylene capacity. The reversibility of the Li⁺ intercalation-deintercalation electrode reaction appears significantly improved at the polyacetylene-carbon nanotubes composites.     

Interference from Trace Copper in Electrochemical Investigations Employing Carboxylic Acid Terminated Thiol Modified Gold Electrodes

Unexpectedly, electrochemistry at variable chain length carboxylic acid terminated alkylthiol self‐assembled monolayers (SAMs) on gold electrodes gives rise to a Faradaic process in buffered aqueous electrolyte solution. In particular, the three‐carbon chain length, 3‐mercaptopropionic acid (MPA), exhibits a chemically reversible process with a mid‐point potential of 175 mV vs. Ag/AgCl under conditions of cyclic voltammetry. This process is associated with the presence of trace (parts per billion) amounts of copper(II) ions present in the chemical reagents used to prepare the aqueous electrolyte and also from the gold electrode itself. The carboxylic acid moiety on the SAM concentrates Cu²⁺ ions by coordination and this surface confined layer is then reduced. Methods to minimize the interference of Cu²⁺ ions at carboxylic acid terminated SAM are discussed and caution with respect to the interpretation of protein electrochemistry is recommended when using carboxylic acid functionalized SAMs to provide biocompatible electrochemical transduction surfaces, unless a metal free environment can be obtained.     

Adsorptive Stripping Voltammetric Detection of Tea Polyphenols at Multiwalled Carbon Nanotubes‐Chitosan Composite Electrode

This study reports the catalytic oxidation and detection of tea polyphenols (TPs) at glassy‐carbon electrode modified with multiwalled carbon nanotubes‐chitosan (MWCNTs‐CS) film. The adsorption of TPs at the surface of the MWCNTs through π–π conjugation prevents the aggregation of nanotubes and induces a stable MWCNTs suspension in water over 30 days. Based on the adsorptive accumulation of polyphenols at MWCNTs, TPs is sensitively and selectively detected by adsorptive stripping voltammetry. The accumulation conditions and pH effect on the adsorptive stripping detection were examined. The linear range was found to be 100 to 1000 mg L^?1 with a detection limit of 10 mg L^?1 (S/N=3) for 2.5 min accumulation. Additionally, the MWCNTs‐CS electrode is easily renewed by applying positive potential to remove the adsorbed TPs. This method was successfully applied to determine TPs in commercially available teas with satisfied result compared with that of conventional spectrometric analysis.     

Electrochemical Preparation and Characterization of Neodymium Hexacyanoferrate and Its Application

A novel route for the fabrication of neodymium hexacyanoferrate (NdHCF) modified glassy carbon electrodes (GCE) was proposed. The morphological characterization of NdHCF was examined by scanning electron microscopy (SEM) and Fourier transform infrared spectra (FTIR). The performances of the NdHCF/GCE were characterized with cyclic voltammetry and differential pulse voltammograms (DPV). The modified electrode showed excellent electrocatalytic effect and high stability toward the electrochemical oxidation of dopamine (DA) in phosphate buffer solution (pH 5.5) with a diminution of the anodic overpotential of 155 mV. The anodic peak currents increased linearly with the concentration of DA from 5.0×10^?7 to 6.0×10^?4 M with a detection limit of 1.0×10^?8 M (S/N=3). The most important is that the modified electrode could be used for the determination of DA in the presence of an ascorbic acid concentration as large as 100‐fold that of DA. The proposed method was used to determine DA in DA‐hydrochloride injection and showed excellent sensitivity and recovery. The ease of fabrication, high stability, and low cost of the modified electrode are the promising features of the proposed sensor.     

Metallophthalocyanine Based Carbon Paste Electrodes for the Determination of 2′,3′‐Dideoxyinosine

Novel electrochemical sensors based on carbon paste impregnated with metallopthalocyanine (MPc, M=Co, Fe) complexes, have been constructed for the assay of anti‐HIV drug 2′,3′‐dideoxyinosine (didanosine, DDI). Both modified electrodes showed electrocatalytic activity towards the oxidation of dideoxyinosine in phosphate buffer pH 7.4 with a working concentration range of 10^?6–10^?4 mol/L and a detection limit of 10^?7 mol/L magnitude order. The sensor proved to be highly reliable for the assay of the purity of DDI ‐ raw material as well as for the uniformity content test of Videx tablets.     

Adsorptive Stripping Voltammetric Determination of Nanomolar Concentration of Cerium(III) at a Carbon Paste Electrode Modified by N′‐[(2‐Hydroxyphenyl)Methylidene]‐2‐Furohydrazide

The aim of this work was to obtain an adsorptive stripping voltammetric method for the Ce(III) determination at a carbon paste electrode, chemically modified with N'‐[(2‐hydroxyphenyl)methylidene]‐2‐furohydrazide (NHMF). The electroanalytical procedure comprised two steps: the Ce(III) chemical accumulation at ?200 mV followed by the electrochemical detection of the Ce(III)/NHMF complex, using anodic stripping voltammetry. The factors, influencing the adsorptive stripping performance, were optimized including the modifier quantity in the paste, the electrolyte concentrations, the solution pH and the accumulation potential or time. The resulting electrode demonstrated a linear response over a wide range of Ce(III) concentration (5.0–90 nmol dm^?3). The detection limit was found to be 0.8 nmol dm^?3 on the basis of a signal to noise ratio of 3. The precision for six determinations of 10 and 55 nmol dm^?3 Ce(III) was 5.6% and 2.1% (relative standard deviation), respectively. Application of the procedure to the determination of cerium in phosphate rock and wastewater samples gave good results.     

Electroanalytical Determination of Cadmium(II) and Lead(II) Using an Antimony Nanoparticle Modified Boron‐Doped Diamond Electrode

We report the simultaneous electroanalytical determination of Pb²⁺ and Cd²⁺ by linear sweep anodic stripping voltammetry (LSASV) using an antimony nanoparticle modified boron doped diamond (Sb‐BDD) electrode. Sb deposition was performed in situ with the analytes, from a solution of 1 mg L^?1 SbCl₃ in 0.1 M HCl (pH 1). Pb²⁺ inhibited the detection of Cd²⁺ during simultaneous additions at the bare BDD electrode, whereas in the presence of antimony, both peaks were readily discernable and quantifiable over the linear range 50–500 μg L^?1.     

Novel Sensitive Voltammetric Detection of Trace Gallium(III) with Presence of Catechol Using Mercury Film Silver Based Electrode

A new adsorptive stripping voltammetric method for the determination of trace gallium(III) based on the adsorption of gallium(III)‐catechol complex on the cyclic renewable mercury film silver based electrode (Hg(Ag)FE) is presented. The effects of various factors such as: preconcentration potential and time, pulse height, step potential and supporting electrolyte composition are optimized. The calibration graph is linear from 2 nM (0.14 μg L^?1) to 100 nM (6.97 μg L^?1) for a preconcentration time of 30 s, with correlation coefficient of 0.9993. For a Hg(Ag)FE with a surface area of 9.7 mm² the detection limit for a preconcentration time of 90 s is as low as 7 ng L^?1. The repeatability of the method at a concentration level of the analyte as low as 0.05 μg L^?1, expressed as RSD is 3.6% (n=5). The proposed method was successfully applied by studying the natural samples and simultaneous recovery of Ga(III) from spiked water and sediment samples.     

A study of adsorptive stripping voltammetric behavior of ofloxacine antibiotic in the presence of Fe(III) and its determination in tablets and biological fluids

Square-wave voltammetry was used to explore the adsorption property of ofloxacine complex with iron ion on the hanging mercury drop electrode (HMDE). By employing the adsorptive stripping voltammetric approach, a sensitive electroanalytical method for the quantitative analysis of ofloxacine antibiotic was achieved. A well-developed voltammetric peak was obtained in pH 7.5 Britton–Robinson buffer (B–R buffer) at ?1400 mV. The cyclic voltammetric studies indicated that the reduction process was irreversible and primarily controlled by adsorption. An investigations of the variation of adsorptive voltammetric peak current with supporting electrolyte, pH, accumulation time, accumulation potential, ion concentration, scan rate, pulse amplitude, SW frequency, working electrode area and convection rate has resulted in the recognition of optimal experimental conditions for ofloxacine analysis. The studied electroanalytical signal showed a linear response for ofloxacine in the concentration range 5 × 10^?7 to 1.7 × 10^?6 mol l^?1 (r = 0.999). A limit of detection of 1.1 × 10^?8 mol l^?1 (3.98 ppb) with relative standard deviation of 1.21 RSD% and mean recovery of 99.6% were obtained. Possible interferences by several substances usually present in pharmaceutical formulation were also evaluated. The analytical quantification of ofloxacine in commercially available pharmaceutical formulation was performed and compared with data from HPLC technique.